28 Text processing library [text]

28.1 General [text.general]

This Clause describes components for dealing with text.

These components are summarized in Table 84 .

Table 84: Text library summary [tab:text.summary]

🔗		Subclause	Header
🔗	[charconv]	Primitive numeric conversions	<charconv>
🔗	[localization]	Localization library	<locale>, <clocale>
🔗	[format]	Formatting	<format>
🔗	[text.encoding]	Text encodings identification	<text_encoding>
🔗	[re]	Regular expressions library	<regex>
🔗	[text.c.strings]	Null-terminated sequence utilities	<cctype>, <cstdlib>, <cuchar>, <cwchar>, <cwctype>

28.2 Primitive numeric conversions [charconv]

28.2.1 Header <charconv> synopsis [charconv.syn]

When a function is specified with a type placeholder of integer-type, the implementation provides overloads for char and all cv-unqualified signed and unsigned integer types in lieu of integer-type.

When a function is specified with a type placeholder of floating-point-type, the implementation provides overloads for all cv-unqualified floating-point types ([basic.fundamental]) in lieu of floating-point-type.

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namespace std { // floating-point format for primitive numerical conversion enum class chars_format { scientific = unspecified, fixed = unspecified, hex = unspecified, general = fixed | scientific }; // [charconv.to.chars], primitive numerical output conversion struct to_chars_result { // freestanding char* ptr; errc ec; friend bool operator==(const to_chars_result&, const to_chars_result&) = default; constexpr explicit operator bool() const noexcept { return ec == errc{}; } }; constexpr to_chars_result to_chars(char* first, char* last, // freestanding integer-type value, int base = 10); to_chars_result to_chars(char* first, char* last, // freestanding bool value, int base = 10) = delete; to_chars_result to_chars(char* first, char* last, // freestanding-deleted floating-point-type value); to_chars_result to_chars(char* first, char* last, // freestanding-deleted floating-point-type value, chars_format fmt); to_chars_result to_chars(char* first, char* last, // freestanding-deleted floating-point-type value, chars_format fmt, int precision); // [charconv.from.chars], primitive numerical input conversion struct from_chars_result { // freestanding const char* ptr; errc ec; friend bool operator==(const from_chars_result&, const from_chars_result&) = default; constexpr explicit operator bool() const noexcept { return ec == errc{}; } }; constexpr from_chars_result from_chars(const char* first, const char* last, // freestanding integer-type& value, int base = 10); from_chars_result from_chars(const char* first, const char* last, // freestanding-deleted floating-point-type& value, chars_format fmt = chars_format::general); }

The type chars_format is a bitmask type ([bitmask.types]) with elements scientific, fixed, and hex.

The types to_chars_result and from_chars_result have the data members and special members specified above.

They have no base classes or members other than those specified.

28.2.2 Primitive numeric output conversion [charconv.to.chars]

All functions named to_chars convert value into a character string by successively filling the range [first, last), where [first, last) is required to be a valid range.

If the member ec of the return value is such that the value is equal to the value of a value-initialized errc, the conversion was successful and the member ptr is the one-past-the-end pointer of the characters written.

Otherwise, the member ec has the value errc::value_too_large, the member ptr has the value last, and the contents of the range [first, last) are unspecified.

The functions that take a floating-point value but not a precision parameter ensure that the string representation consists of the smallest number of characters such that there is at least one digit before the radix point (if present) and parsing the representation using the corresponding from_chars function recovers value exactly.

[Note 1:

This guarantee applies only if to_chars and from_chars are executed on the same implementation.

— end note]

If there are several such representations, the representation with the smallest difference from the floating-point argument value is chosen, resolving any remaining ties using rounding according to round_to_nearest ([round.style]).

The functions taking a chars_format parameter determine the conversion specifier for printf as follows: The conversion specifier is f if fmt is chars_format::fixed, e if fmt is chars_format::scientific, a (without leading "0x" in the result) if fmt is chars_format::hex, and g if fmt is chars_format::general.

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constexpr to_chars_result to_chars(char* first, char* last, integer-type value, int base = 10);

Preconditions: base has a value between 2 and 36 (inclusive).

Effects: The value of value is converted to a string of digits in the given base (with no redundant leading zeroes).

Digits in the range 10..35 (inclusive) are represented as lowercase characters a..z.

If value is less than zero, the representation starts with '-'.

Throws: Nothing.

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to_chars_result to_chars(char* first, char* last, floating-point-type value);

Effects: value is converted to a string in the style of printf in the "C" locale.

The conversion specifier is f or e, chosen according to the requirement for a shortest representation (see above); a tie is resolved in favor of f.

Throws: Nothing.

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to_chars_result to_chars(char* first, char* last, floating-point-type value, chars_format fmt);

Preconditions: fmt has the value of one of the enumerators of chars_format.

Effects: value is converted to a string in the style of printf in the "C" locale.

Throws: Nothing.

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to_chars_result to_chars(char* first, char* last, floating-point-type value,
                         chars_format fmt, int precision);

Preconditions: fmt has the value of one of the enumerators of chars_format.

Effects: value is converted to a string in the style of printf in the "C" locale with the given precision.

Throws: Nothing.

28.2.3 Primitive numeric input conversion [charconv.from.chars]

All functions named from_chars analyze the string [first, last) for a pattern, where [first, last) is required to be a valid range.

If no characters match the pattern, value is unmodified, the member ptr of the return value is first and the member ec is equal to errc::invalid_argument.

[Note 1:

If the pattern allows for an optional sign, but the string has no digit characters following the sign, no characters match the pattern.

— end note]

Otherwise, the characters matching the pattern are interpreted as a representation of a value of the type of value.

The member ptr of the return value points to the first character not matching the pattern, or has the value last if all characters match.

If the parsed value is not in the range representable by the type of value, value is unmodified and the member ec of the return value is equal to errc::result_out_of_range.

Otherwise, value is set to the parsed value, after rounding according to round_to_nearest ([round.style]), and the member ec is value-initialized.

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constexpr from_chars_result from_chars(const char* first, const char* last,
                                       integer-type& value, int base = 10);

Preconditions: base has a value between 2 and 36 (inclusive).

Effects: The pattern is the expected form of the subject sequence in the "C" locale for the given nonzero base, as described for strtol, except that no "0x" or "0X" prefix shall appear if the value of base is 16, and except that '-' is the only sign that may appear, and only if value has a signed type.

Throws: Nothing.

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from_chars_result from_chars(const char* first, const char* last, floating-point-type& value,
                             chars_format fmt = chars_format::general);

Preconditions: fmt has the value of one of the enumerators of chars_format.

Effects: The pattern is the expected form of the subject sequence in the "C" locale, as described for strtod, except that

(6.1)
the sign '+' may only appear in the exponent part;
(6.2)
if fmt has chars_format::scientific set but not chars_format::fixed, the otherwise optional exponent part shall appear;
(6.3)
if fmt has chars_format::fixed set but not chars_format::scientific, the optional exponent part shall not appear; and
(6.4)
if fmt is chars_format::hex, the prefix "0x" or "0X" is assumed.
[Example 1:
The string 0x123 is parsed to have the value 0 with remaining characters x123.
— end example]

In any case, the resulting value is one of at most two floating-point values closest to the value of the string matching the pattern.

Throws: Nothing.

See also: ISO/IEC 9899:2018, 7.22.1.3, 7.22.1.4

28.3 Localization library [localization]

28.3.1 General [localization.general]

Subclause [localization] describes components that C++ programs may use to encapsulate (and therefore be more portable when confronting) cultural differences.

The locale facility includes internationalization support for character classification and string collation, numeric, monetary, and date/time formatting and parsing, and message retrieval.

The following subclauses describe components for locales themselves, the standard facets, and facilities from the C library, as summarized in Table 85 .

Table 85: Localization library summary [tab:localization.summary]

🔗		Subclause	Header
🔗	[locales]	Locales	<locale>
🔗	[locale.categories]	Standard locale categories
🔗	[c.locales]	C library locales	<clocale>
🔗

28.3.2 Header <locale> synopsis [locale.syn]

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namespace std { // [locale], locale class locale; template<class Facet> const Facet& use_facet(const locale&); template<class Facet> bool has_facet(const locale&) noexcept; // [locale.convenience], convenience interfaces template<class charT> bool isspace (charT c, const locale& loc); template<class charT> bool isprint (charT c, const locale& loc); template<class charT> bool iscntrl (charT c, const locale& loc); template<class charT> bool isupper (charT c, const locale& loc); template<class charT> bool islower (charT c, const locale& loc); template<class charT> bool isalpha (charT c, const locale& loc); template<class charT> bool isdigit (charT c, const locale& loc); template<class charT> bool ispunct (charT c, const locale& loc); template<class charT> bool isxdigit(charT c, const locale& loc); template<class charT> bool isalnum (charT c, const locale& loc); template<class charT> bool isgraph (charT c, const locale& loc); template<class charT> bool isblank (charT c, const locale& loc); template<class charT> charT toupper(charT c, const locale& loc); template<class charT> charT tolower(charT c, const locale& loc); // [category.ctype], ctype class ctype_base; template<class charT> class ctype; template<> class ctype<char>; // specialization template<class charT> class ctype_byname; class codecvt_base; template<class internT, class externT, class stateT> class codecvt; template<class internT, class externT, class stateT> class codecvt_byname; // [category.numeric], numeric template<class charT, class InputIterator = istreambuf_iterator<charT>> class num_get; template<class charT, class OutputIterator = ostreambuf_iterator<charT>> class num_put; template<class charT> class numpunct; template<class charT> class numpunct_byname; // [category.collate], collation template<class charT> class collate; template<class charT> class collate_byname; // [category.time], date and time class time_base; template<class charT, class InputIterator = istreambuf_iterator<charT>> class time_get; template<class charT, class InputIterator = istreambuf_iterator<charT>> class time_get_byname; template<class charT, class OutputIterator = ostreambuf_iterator<charT>> class time_put; template<class charT, class OutputIterator = ostreambuf_iterator<charT>> class time_put_byname; // [category.monetary], money class money_base; template<class charT, class InputIterator = istreambuf_iterator<charT>> class money_get; template<class charT, class OutputIterator = ostreambuf_iterator<charT>> class money_put; template<class charT, bool Intl = false> class moneypunct; template<class charT, bool Intl = false> class moneypunct_byname; // [category.messages], message retrieval class messages_base; template<class charT> class messages; template<class charT> class messages_byname; }

The header <locale> defines classes and declares functions that encapsulate and manipulate the information peculiar to a locale.216

216)216)

In this subclause, the type name tm is an incomplete type that is defined in <ctime>.

28.3.3 Locales [locales]

28.3.3.1 Class locale [locale]

28.3.3.1.1 General [locale.general]

namespace std { class locale { public: // [locale.types], types // [locale.facet], class locale::facet class facet; // [locale.id], class locale::id class id; // [locale.category], type locale::category using category = int; static const category // values assigned here are for exposition only none = 0, collate = 0x010, ctype = 0x020, monetary = 0x040, numeric = 0x080, time = 0x100, messages = 0x200, all = collate | ctype | monetary | numeric | time | messages; // [locale.cons], construct/copy/destroy locale() noexcept; locale(const locale& other) noexcept; explicit locale(const char* std_name); explicit locale(const string& std_name); locale(const locale& other, const char* std_name, category); locale(const locale& other, const string& std_name, category); template<class Facet> locale(const locale& other, Facet* f); locale(const locale& other, const locale& one, category); ~locale(); // not virtual const locale& operator=(const locale& other) noexcept; // [locale.members], locale operations template<class Facet> locale combine(const locale& other) const; string name() const; text_encoding encoding() const; bool operator==(const locale& other) const; template<class charT, class traits, class Allocator> bool operator()(const basic_string<charT, traits, Allocator>& s1, const basic_string<charT, traits, Allocator>& s2) const; // [locale.statics], global locale objects static locale global(const locale&); static const locale& classic(); }; }

Class locale implements a type-safe polymorphic set of facets, indexed by facet type.

In other words, a facet has a dual role: in one sense, it's just a class interface; at the same time, it's an index into a locale's set of facets.

Access to the facets of a locale is via two function templates, use_facet<> and has_facet<>.

[Example 1:

An iostream operator<< can be implemented as:217

template<class charT, class traits> basic_ostream<charT, traits>& operator<< (basic_ostream<charT, traits>& s, Date d) { typename basic_ostream<charT, traits>::sentry cerberos(s); if (cerberos) { tm tmbuf; d.extract(tmbuf); bool failed = use_facet<time_put<charT, ostreambuf_iterator<charT, traits>>>( s.getloc()).put(s, s, s.fill(), &tmbuf, 'x').failed(); if (failed) s.setstate(s.badbit); // can throw } return s; } — end example]

In the call to use_facet<Facet>(loc), the type argument chooses a facet, making available all members of the named type.

If Facet is not present in a locale, it throws the standard exception bad_cast.

A C++ program can check if a locale implements a particular facet with the function template has_facet<Facet>().

User-defined facets may be installed in a locale, and used identically as may standard facets.

[Note 1:

All locale semantics are accessed via use_facet<> and has_facet<>, except that:

(5.1)
A member operator template operator()(const basic_string<C, T, A>&, const basic_string<C, T, A>&) is provided so that a locale can be used as a predicate argument to the standard collections, to collate strings.
(5.2)
Convenient global interfaces are provided for traditional ctype functions such as isdigit() and isspace(), so that given a locale object loc a C++ program can call isspace(c, loc).
(This eases upgrading existing extractors ([istream.formatted]).)

— end note]

Once a facet reference is obtained from a locale object by calling use_facet<>, that reference remains usable, and the results from member functions of it may be cached and re-used, as long as some locale object refers to that facet.

In successive calls to a locale facet member function on a facet object installed in the same locale, the returned result shall be identical.

A locale constructed from a name string (such as "POSIX"), or from parts of two named locales, has a name; all others do not.

Named locales may be compared for equality; an unnamed locale is equal only to (copies of) itself.

For an unnamed locale, locale::name() returns the string "*".

Whether there is one global locale object for the entire program or one global locale object per thread is implementation-defined.

Implementations should provide one global locale object per thread.

If there is a single global locale object for the entire program, implementations are not required to avoid data races on it ([res.on.data.races]).

217)217)

Note that in the call to put, the stream is implicitly converted to an ostreambuf_iterator<charT, traits>.

28.3.3.1.2 Types [locale.types]

28.3.3.1.2.1 Type locale::category [locale.category]

🔗

using category = int;

Valid category values include the locale member bitmask elements collate, ctype, monetary, numeric, time, and messages, each of which represents a single locale category.

In addition, locale member bitmask constant none is defined as zero and represents no category.

Further, the expression (X | Y), where X and Y each represent a single category, represents the union of the two categories.

locale member functions expecting a category argument require one of the category values defined above, or the union of two or more such values.

Such a category value identifies a set of locale categories.

Each locale category, in turn, identifies a set of locale facets, including at least those shown in Table 86 .

For any locale loc either constructed, or returned by locale::classic(), and any facet Facet shown in Table 86, has_facet<Facet>(loc) is true.

Each locale member function which takes a locale::category argument operates on the corresponding set of facets.

An implementation is required to provide those specializations for facet templates identified as members of a category, and for those shown in Table 87 .

Table 87: Required specializations [tab:locale.spec]

🔗	Category	Includes facets
🔗	collate	collate_byname<char>, collate_byname<wchar_t>
🔗	ctype	ctype_byname<char>, ctype_byname<wchar_t>
🔗		codecvt_byname<char, char, mbstate_t>
🔗		codecvt_byname<wchar_t, char, mbstate_t>
🔗	monetary	moneypunct_byname<char, International>
🔗		moneypunct_byname<wchar_t, International>
🔗		money_get<C, InputIterator>
🔗		money_put<C, OutputIterator>
🔗	numeric	numpunct_byname<char>, numpunct_byname<wchar_t>
🔗		num_get<C, InputIterator>, num_put<C, OutputIterator>
🔗	time	time_get<char, InputIterator>
🔗		time_get_byname<char, InputIterator>
🔗		time_get<wchar_t, InputIterator>
🔗		time_get_byname<wchar_t, InputIterator>
🔗		time_put<char, OutputIterator>
🔗		time_put_byname<char, OutputIterator>
🔗		time_put<wchar_t, OutputIterator>
🔗		time_put_byname<wchar_t, OutputIterator>
🔗	messages	messages_byname<char>, messages_byname<wchar_t>

The provided implementation of members of facets num_get<charT> and num_put<charT> calls use_facet<F>(l) only for facet F of types numpunct<charT> and ctype<charT>, and for locale l the value obtained by calling member getloc() on the ios_base& argument to these functions.

In declarations of facets, a template parameter with name InputIterator or OutputIterator indicates the set of all possible specializations on parameters that meet the Cpp17InputIterator requirements or Cpp17OutputIterator requirements, respectively ([iterator.requirements]).

A template parameter with name C represents the set of types containing char, wchar_t, and any other implementation-defined character container types ([defns.character.container]) that meet the requirements for a character on which any of the iostream components can be instantiated.

A template parameter with name International represents the set of all possible specializations on a bool parameter.

28.3.3.1.2.3 Class locale::id [locale.id]

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namespace std { class locale::id { public: id(); void operator=(const id&) = delete; id(const id&) = delete; }; }

The class locale::id provides identification of a locale facet interface, used as an index for lookup and to encapsulate initialization.

[Note 1:

Because facets are used by iostreams, potentially while static constructors are running, their initialization cannot depend on programmed static initialization.

One initialization strategy is for locale to initialize each facet's id member the first time an instance of the facet is installed into a locale.

This depends only on static storage being zero before constructors run ([basic.start.static]).

— end note]

28.3.3.1.3 Constructors and destructor [locale.cons]

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locale() noexcept;

Effects: Constructs a copy of the argument last passed to locale::global(locale&), if it has been called; else, the resulting facets have virtual function semantics identical to those of locale::classic().

[Note 1:

This constructor yields a copy of the current global locale.

It is commonly used as a default argument for function parameters of type const locale&.

— end note]

🔗

explicit locale(const char* std_name);

Effects: Constructs a locale using standard C locale names, e.g., "POSIX".

The resulting locale implements semantics defined to be associated with that name.

Throws: runtime_error if the argument is not valid, or is null.

Remarks: The set of valid string argument values is "C", "", and any implementation-defined values.

🔗

explicit locale(const string& std_name);

Effects: Equivalent to locale(std_name.c_str()).

🔗

locale(const locale& other, const char* std_name, category cats);

Preconditions: cats is a valid category value ([locale.category]).

Effects: Constructs a locale as a copy of other except for the facets identified by the category argument, which instead implement the same semantics as locale(std_name).

Throws: runtime_error if the second argument is not valid, or is null.

Remarks: The locale has a name if and only if other has a name.

🔗

locale(const locale& other, const string& std_name, category cats);

Effects: Equivalent to locale(other, std_name.c_str(), cats).

🔗

template<class Facet> locale(const locale& other, Facet* f);

Effects: Constructs a locale incorporating all facets from the first argument except that of type Facet, and installs the second argument as the remaining facet.

If f is null, the resulting object is a copy of other.

Remarks: If f is null, the resulting locale has the same name as other.

Otherwise, the resulting locale has no name.

🔗

locale(const locale& other, const locale& one, category cats);

Preconditions: cats is a valid category value.

Effects: Constructs a locale incorporating all facets from the first argument except those that implement cats, which are instead incorporated from the second argument.

Remarks: If cats is equal to locale::none, the resulting locale has a name if and only if the first argument has a name.

Otherwise, the resulting locale has a name if and only if the first two arguments both have names.

🔗

const locale& operator=(const locale& other) noexcept;

Effects: Creates a copy of other, replacing the current value.

Returns: *this.

28.3.3.1.4 Members [locale.members]

🔗

template<class Facet> locale combine(const locale& other) const;

Effects: Constructs a locale incorporating all facets from *this except for that one facet of other that is identified by Facet.

Returns: The newly created locale.

Throws: runtime_error if has_facet<Facet>(other) is false.

Remarks: The resulting locale has no name.

🔗

string name() const;

Returns: The name of *this, if it has one; otherwise, the string "*".

🔗

text_encoding encoding() const;

Mandates: CHAR_BIT == 8 is true.

Returns: A text_encoding object representing the implementation-defined encoding scheme associated with the locale *this.

28.3.3.1.5 Operators [locale.operators]

🔗

bool operator==(const locale& other) const;

Returns: true if both arguments are the same locale, or one is a copy of the other, or each has a name and the names are identical; false otherwise.

🔗

template<class charT, class traits, class Allocator>
  bool operator()(const basic_string<charT, traits, Allocator>& s1,
                  const basic_string<charT, traits, Allocator>& s2) const;

Effects: Compares two strings according to the collate<charT> facet.

Returns: use_facet<collate<charT>>(*this).compare(s1.data(), s1.data() + s1.size(), s2.data(), s2.data() + s2.size()) < 0

Remarks: This member operator template (and therefore locale itself) meets the requirements for a comparator predicate template argument ([algorithms]) applied to strings.

[Example 1:

A vector of strings v can be collated according to collation rules in locale loc simply by ([alg.sort], [vector]):

std::sort(v.begin(), v.end(), loc); — end example]

28.3.3.1.6 Static members [locale.statics]

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static locale global(const locale& loc);

Effects: Sets the global locale to its argument.

Causes future calls to the constructor locale() to return a copy of the argument.

If the argument has a name, does setlocale(LC_ALL, loc.name().c_str()); otherwise, the effect on the C locale, if any, is implementation-defined.

Returns: The previous value of locale().

Remarks: No library function other than locale::global() affects the value returned by locale().

[Note 1:

See [c.locales] for data race considerations when setlocale is invoked.

— end note]

🔗

static const locale& classic();

The "C" locale.

Returns: A locale that implements the classic "C" locale semantics, equivalent to the value locale("C").

Remarks: This locale, its facets, and their member functions, do not change with time.

28.3.3.2 locale globals [locale.global.templates]

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template<class Facet> const Facet& use_facet(const locale& loc);

Mandates: Facet is a facet class whose definition contains the public static member id as defined in [locale.facet].

Returns: A reference to the corresponding facet of loc, if present.

Throws: bad_cast if has_facet<Facet>(loc) is false.

Remarks: The reference returned remains valid at least as long as any copy of loc exists.

🔗

template<class Facet> bool has_facet(const locale& loc) noexcept;

Returns: true if the facet requested is present in loc; otherwise false.

28.3.3.3 Convenience interfaces [locale.convenience]

28.3.3.3.1 Character classification [classification]

🔗

template<class charT> bool isspace (charT c, const locale& loc);
template<class charT> bool isprint (charT c, const locale& loc);
template<class charT> bool iscntrl (charT c, const locale& loc);
template<class charT> bool isupper (charT c, const locale& loc);
template<class charT> bool islower (charT c, const locale& loc);
template<class charT> bool isalpha (charT c, const locale& loc);
template<class charT> bool isdigit (charT c, const locale& loc);
template<class charT> bool ispunct (charT c, const locale& loc);
template<class charT> bool isxdigit(charT c, const locale& loc);
template<class charT> bool isalnum (charT c, const locale& loc);
template<class charT> bool isgraph (charT c, const locale& loc);
template<class charT> bool isblank (charT c, const locale& loc);

Each of these functions isF returns the result of the expression: use_facet<ctype<charT>>(loc).is(ctype_base::F, c) where F is the ctype_base::mask value corresponding to that function ([category.ctype]).219

219)219)

When used in a loop, it is faster to cache the ctype<> facet and use it directly, or use the vector form of ctype<>::is.

28.3.3.3.2 Character conversions [conversions.character]

🔗

template<class charT> charT toupper(charT c, const locale& loc);

Returns: use_facet<ctype<charT>>(loc).toupper(c).

🔗

template<class charT> charT tolower(charT c, const locale& loc);

Returns: use_facet<ctype<charT>>(loc).tolower(c).

28.3.4 Standard locale categories [locale.categories]

28.3.4.1 General [locale.categories.general]

Each of the standard categories includes a family of facets.

Some of these implement formatting or parsing of a datum, for use by standard or users' iostream operators << and >>, as members put() and get(), respectively.

Each such member function takes an ios_base& argument whose members flags(), precision(), and width(), specify the format of the corresponding datum ([ios.base]).

Those functions which need to use other facets call its member getloc() to retrieve the locale imbued there.

Formatting facets use the character argument fill to fill out the specified width where necessary.

The put() members make no provision for error reporting.

(Any failures of the OutputIterator argument can be extracted from the returned iterator.)

The get() members take an ios_base::iostate& argument whose value they ignore, but set to ios_base::failbit in case of a parse error.

Within [locale.categories] it is unspecified whether one virtual function calls another virtual function.

28.3.4.2 The ctype category [category.ctype]

28.3.4.2.1 General [category.ctype.general]

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namespace std { class ctype_base { public: using mask = see below; // numeric values are for exposition only. static constexpr mask space = 1 << 0; static constexpr mask print = 1 << 1; static constexpr mask cntrl = 1 << 2; static constexpr mask upper = 1 << 3; static constexpr mask lower = 1 << 4; static constexpr mask alpha = 1 << 5; static constexpr mask digit = 1 << 6; static constexpr mask punct = 1 << 7; static constexpr mask xdigit = 1 << 8; static constexpr mask blank = 1 << 9; static constexpr mask alnum = alpha | digit; static constexpr mask graph = alnum | punct; }; }

The type mask is a bitmask type ([bitmask.types]).

28.3.4.2.2 Class template ctype [locale.ctype]

28.3.4.2.2.1 General [locale.ctype.general]

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namespace std { template<class charT> class ctype : public locale::facet, public ctype_base { public: using char_type = charT; explicit ctype(size_t refs = 0); bool is(mask m, charT c) const; const charT* is(const charT* low, const charT* high, mask* vec) const; const charT* scan_is(mask m, const charT* low, const charT* high) const; const charT* scan_not(mask m, const charT* low, const charT* high) const; charT toupper(charT c) const; const charT* toupper(charT* low, const charT* high) const; charT tolower(charT c) const; const charT* tolower(charT* low, const charT* high) const; charT widen(char c) const; const char* widen(const char* low, const char* high, charT* to) const; char narrow(charT c, char dfault) const; const charT* narrow(const charT* low, const charT* high, char dfault, char* to) const; static locale::id id; protected: ~ctype(); virtual bool do_is(mask m, charT c) const; virtual const charT* do_is(const charT* low, const charT* high, mask* vec) const; virtual const charT* do_scan_is(mask m, const charT* low, const charT* high) const; virtual const charT* do_scan_not(mask m, const charT* low, const charT* high) const; virtual charT do_toupper(charT) const; virtual const charT* do_toupper(charT* low, const charT* high) const; virtual charT do_tolower(charT) const; virtual const charT* do_tolower(charT* low, const charT* high) const; virtual charT do_widen(char) const; virtual const char* do_widen(const char* low, const char* high, charT* dest) const; virtual char do_narrow(charT, char dfault) const; virtual const charT* do_narrow(const charT* low, const charT* high, char dfault, char* dest) const; }; }

Class ctype encapsulates the C library <cctype> features.

istream members are required to use ctype<> for character classing during input parsing.

The specializations required in Table 86 ([locale.category]), namely ctype<char> and ctype<wchar_t>, implement character classing appropriate to the implementation's native character set.

28.3.4.2.2.2 ctype members [locale.ctype.members]

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bool         is(mask m, charT c) const;
const charT* is(const charT* low, const charT* high, mask* vec) const;

Returns: do_is(m, c) or do_is(low, high, vec).

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const charT* scan_is(mask m, const charT* low, const charT* high) const;

Returns: do_scan_is(m, low, high).

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const charT* scan_not(mask m, const charT* low, const charT* high) const;

Returns: do_scan_not(m, low, high).

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charT        toupper(charT c) const;
const charT* toupper(charT* low, const charT* high) const;

Returns: do_toupper(c) or do_toupper(low, high).

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charT        tolower(charT c) const;
const charT* tolower(charT* low, const charT* high) const;

Returns: do_tolower(c) or do_tolower(low, high).

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charT       widen(char c) const;
const char* widen(const char* low, const char* high, charT* to) const;

Returns: do_widen(c) or do_widen(low, high, to).

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char         narrow(charT c, char dfault) const;
const charT* narrow(const charT* low, const charT* high, char dfault, char* to) const;

Returns: do_narrow(c, dfault) or do_narrow(low, high, dfault, to).

28.3.4.2.2.3 ctype virtual functions [locale.ctype.virtuals]

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bool         do_is(mask m, charT c) const;
const charT* do_is(const charT* low, const charT* high, mask* vec) const;

Effects: Classifies a character or sequence of characters.

For each argument character, identifies a value M of type ctype_base::mask.

The second form identifies a value M of type ctype_base::mask for each *p where (low <= p && p < high), and places it into vec[p - low].

Returns: The first form returns the result of the expression (M & m) != 0; i.e., true if the character has the characteristics specified.

The second form returns high.

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const charT* do_scan_is(mask m, const charT* low, const charT* high) const;

Effects: Locates a character in a buffer that conforms to a classification m.

Returns: The smallest pointer p in the range [low, high) such that is(m, *p) would return true; otherwise, returns high.

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const charT* do_scan_not(mask m, const charT* low, const charT* high) const;

Effects: Locates a character in a buffer that fails to conform to a classification m.

Returns: The smallest pointer p, if any, in the range [low, high) such that is(m, *p) would return false; otherwise, returns high.

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charT        do_toupper(charT c) const;
const charT* do_toupper(charT* low, const charT* high) const;

Effects: Converts a character or characters to upper case.

The second form replaces each character *p in the range [low, high) for which a corresponding upper-case character exists, with that character.

Returns: The first form returns the corresponding upper-case character if it is known to exist, or its argument if not.

The second form returns high.

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charT        do_tolower(charT c) const;
const charT* do_tolower(charT* low, const charT* high) const;

Effects: Converts a character or characters to lower case.

The second form replaces each character *p in the range [low, high) and for which a corresponding lower-case character exists, with that character.

Returns: The first form returns the corresponding lower-case character if it is known to exist, or its argument if not.

The second form returns high.

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charT        do_widen(char c) const;
const char*  do_widen(const char* low, const char* high, charT* dest) const;

Effects: Applies the simplest reasonable transformation from a char value or sequence of char values to the corresponding charT value or values.220

The only characters for which unique transformations are required are those in the basic character set ([lex.charset]).

For any named ctype category with a ctype<char> facet ctc and valid ctype_base::mask value M, (ctc.is(M, c) || !is(M, do_widen(c)) ) is true.221

The second form transforms each character *p in the range [low, high), placing the result in dest[p - low].

Returns: The first form returns the transformed value.

The second form returns high.

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char         do_narrow(charT c, char dfault) const;
const charT* do_narrow(const charT* low, const charT* high, char dfault, char* dest) const;

Effects: Applies the simplest reasonable transformation from a charT value or sequence of charT values to the corresponding char value or values.

For any character c in the basic character set ([lex.charset]) the transformation is such that do_widen(do_narrow(c, 0)) == c

For any named ctype category with a ctype<char> facet ctc however, and ctype_base::mask value M, (is(M, c) || !ctc.is(M, do_narrow(c, dfault)) ) is true (unless do_narrow returns dfault).

In addition, for any digit character c, the expression (do_narrow(c, dfault) - '0') evaluates to the digit value of the character.

The second form transforms each character *p in the range [low, high), placing the result (or dfault if no simple transformation is readily available) in dest[p - low].

Returns: The first form returns the transformed value; or dfault if no mapping is readily available.

The second form returns high.

220)220)

The parameter c of do_widen is intended to accept values derived from character-literals for conversion to the locale's encoding.

221)221)

In other words, the transformed character is not a member of any character classification that c is not also a member of.

28.3.4.2.3 Class template ctype_byname [locale.ctype.byname]

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namespace std { template<class charT> class ctype_byname : public ctype<charT> { public: using mask = typename ctype<charT>::mask; explicit ctype_byname(const char*, size_t refs = 0); explicit ctype_byname(const string&, size_t refs = 0); protected: ~ctype_byname(); }; }

28.3.4.2.4 ctype<char> specialization [facet.ctype.special]

28.3.4.2.5 Class template codecvt [locale.codecvt]

28.3.4.2.5.1 General [locale.codecvt.general]

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namespace std { class codecvt_base { public: enum result { ok, partial, error, noconv }; }; template<class internT, class externT, class stateT> class codecvt : public locale::facet, public codecvt_base { public: using intern_type = internT; using extern_type = externT; using state_type = stateT; explicit codecvt(size_t refs = 0); result out( stateT& state, const internT* from, const internT* from_end, const internT*& from_next, externT* to, externT* to_end, externT*& to_next) const; result unshift( stateT& state, externT* to, externT* to_end, externT*& to_next) const; result in( stateT& state, const externT* from, const externT* from_end, const externT*& from_next, internT* to, internT* to_end, internT*& to_next) const; int encoding() const noexcept; bool always_noconv() const noexcept; int length(stateT&, const externT* from, const externT* end, size_t max) const; int max_length() const noexcept; static locale::id id; protected: ~codecvt(); virtual result do_out( stateT& state, const internT* from, const internT* from_end, const internT*& from_next, externT* to, externT* to_end, externT*& to_next) const; virtual result do_in( stateT& state, const externT* from, const externT* from_end, const externT*& from_next, internT* to, internT* to_end, internT*& to_next) const; virtual result do_unshift( stateT& state, externT* to, externT* to_end, externT*& to_next) const; virtual int do_encoding() const noexcept; virtual bool do_always_noconv() const noexcept; virtual int do_length(stateT&, const externT* from, const externT* end, size_t max) const; virtual int do_max_length() const noexcept; }; }

The class codecvt<internT, externT, stateT> is for use when converting from one character encoding to another, such as from wide characters to multibyte characters or between wide character encodings such as UTF-32 and EUC.

The stateT argument selects the pair of character encodings being mapped between.

The specializations required in Table 86 ([locale.category]) convert the implementation-defined native character set.

codecvt<char, char, mbstate_t> implements a degenerate conversion; it does not convert at all.

codecvt<wchar_t, char, mbstate_t> converts between the native character sets for ordinary and wide characters.

Specializations on mbstate_t perform conversion between encodings known to the library implementer.

Other encodings can be converted by specializing on a program-defined stateT type.

Objects of type stateT can contain any state that is useful to communicate to or from the specialized do_in or do_out members.

28.3.4.2.5.2 Members [locale.codecvt.members]

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result out(
  stateT& state,
  const internT* from, const internT* from_end, const internT*& from_next,
  externT* to, externT* to_end, externT*& to_next) const;

Returns: do_out(state, from, from_end, from_next, to, to_end, to_next).

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result unshift(stateT& state, externT* to, externT* to_end, externT*& to_next) const;

Returns: do_unshift(state, to, to_end, to_next).

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result in(
  stateT& state,
  const externT* from, const externT* from_end, const externT*& from_next,
  internT* to, internT* to_end, internT*& to_next) const;

Returns: do_in(state, from, from_end, from_next, to, to_end, to_next).

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int encoding() const noexcept;

Returns: do_encoding().

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bool always_noconv() const noexcept;

Returns: do_always_noconv().

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int length(stateT& state, const externT* from, const externT* from_end, size_t max) const;

Returns: do_length(state, from, from_end, max).

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int max_length() const noexcept;

Returns: do_max_length().

28.3.4.2.5.3 Virtual functions [locale.codecvt.virtuals]

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result do_out(
  stateT& state,
  const internT* from, const internT* from_end, const internT*& from_next,
  externT* to, externT* to_end, externT*& to_next) const;

result do_in(
  stateT& state,
  const externT* from, const externT* from_end, const externT*& from_next,
  internT* to, internT* to_end, internT*& to_next) const;

Preconditions: (from <= from_end && to <= to_end) is well-defined and true; state is initialized, if at the beginning of a sequence, or else is equal to the result of converting the preceding characters in the sequence.

Effects: Translates characters in the source range [from, from_end), placing the results in sequential positions starting at destination to.

Converts no more than (from_end - from) source elements, and stores no more than (to_end - to) destination elements.

Stops if it encounters a character it cannot convert.

It always leaves the from_next and to_next pointers pointing one beyond the last element successfully converted.

If it returns noconv, internT and externT are the same type, and the converted sequence is identical to the input sequence [from, from_next), to_next is set equal to to, the value of state is unchanged, and there are no changes to the values in [to, to_end).

A codecvt facet that is used by basic_filebuf ([file.streams]) shall have the property that if do_out(state, from, from_end, from_next, to, to_end, to_next) would return ok, where from != from_end, then do_out(state, from, from + 1, from_next, to, to_end, to_next) shall also return ok, and that if do_in(state, from, from_end, from_next, to, to_end, to_next) would return ok, where to != to_end, then do_in(state, from, from_end, from_next, to, to + 1, to_next) shall also return ok.223

[Note 1:

As a result of operations on state, it can return ok or partial and set from_next == from and to_next != to.

— end note]

Returns: An enumeration value, as summarized in Table 88 .

Table 88: do_in/do_out result values [tab:locale.codecvt.inout]

🔗	Value	Meaning
🔗	ok	completed the conversion
🔗	partial	not all source characters converted
🔗	error	encountered a character in [from, from_end) that cannot be converted
🔗	noconv	internT and externT are the same type, and input sequence is identical to converted sequence

A return value of partial, if (from_next == from_end), indicates that either the destination sequence has not absorbed all the available destination elements, or that additional source elements are needed before another destination element can be produced.

Remarks: Its operations on state are unspecified.

[Note 2:

This argument can be used, for example, to maintain shift state, to specify conversion options (such as count only), or to identify a cache of seek offsets.

— end note]

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result do_unshift(stateT& state, externT* to, externT* to_end, externT*& to_next) const;

Preconditions: (to <= to_end) is well-defined and true; state is initialized, if at the beginning of a sequence, or else is equal to the result of converting the preceding characters in the sequence.

Effects: Places characters starting at to that should be appended to terminate a sequence when the current stateT is given by state.224

Stores no more than (to_end - to) destination elements, and leaves the to_next pointer pointing one beyond the last element successfully stored.

Returns: An enumeration value, as summarized in Table 89 .

Table 89: do_unshift result values [tab:locale.codecvt.unshift]

🔗	Value	Meaning
🔗	ok	completed the sequence
🔗	partial	space for more than to_end - to destination elements was needed to terminate a sequence given the value of state
🔗	error	an unspecified error has occurred
🔗	noconv	no termination is needed for this state_type

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int do_encoding() const noexcept;

Returns: -1 if the encoding of the externT sequence is state-dependent; else the constant number of externT characters needed to produce an internal character; or 0 if this number is not a constant.225

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bool do_always_noconv() const noexcept;

Returns: true if do_in() and do_out() return noconv for all valid argument values.

codecvt<char, char, mbstate_t> returns true.

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int do_length(stateT& state, const externT* from, const externT* from_end, size_t max) const;

Preconditions: (from <= from_end) is well-defined and true; state is initialized, if at the beginning of a sequence, or else is equal to the result of converting the preceding characters in the sequence.

Effects: The effect on the state argument is as if it called do_in(state, from, from_end, from, to, to+max, to) for to pointing to a buffer of at least max elements.

Returns: (from_next-from) where from_next is the largest value in the range [from, from_end] such that the sequence of values in the range [from, from_next) represents max or fewer valid complete characters of type internT.

The specialization codecvt<char, char, mbstate_t>, returns the lesser of max and (from_end-from).

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int do_max_length() const noexcept;

Returns: The maximum value that do_length(state, from, from_end, 1) can return for any valid range [from, from_end) and stateT value state.

The specialization codecvt<char, char, mbstate_t>::do_max_length() returns 1.

223)223)

Informally, this means that basic_filebuf assumes that the mappings from internal to external characters is 1 to N: that a codecvt facet that is used by basic_filebuf can translate characters one internal character at a time.

224)224)

Typically these will be characters to return the state to stateT().

225)225)

If encoding() yields -1, then more than max_length() externT elements can be consumed when producing a single internT character, and additional externT elements can appear at the end of a sequence after those that yield the final internT character.

28.3.4.2.6 Class template codecvt_byname [locale.codecvt.byname]

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namespace std { template<class internT, class externT, class stateT> class codecvt_byname : public codecvt<internT, externT, stateT> { public: explicit codecvt_byname(const char*, size_t refs = 0); explicit codecvt_byname(const string&, size_t refs = 0); protected: ~codecvt_byname(); }; }

28.3.4.3 The numeric category [category.numeric]

28.3.4.3.1 General [category.numeric.general]

The classes num_get<> and num_put<> handle numeric formatting and parsing.

Virtual functions are provided for several numeric types.

Implementations may (but are not required to) delegate extraction of smaller types to extractors for larger types.226

All specifications of member functions for num_put and num_get in the subclauses of [category.numeric] only apply to the specializations required in Tables 86 and 87 ([locale.category]), namely num_get<char>, num_get<wchar_t>, num_get<C, InputIterator>, num_put<char>, num_put<wchar_t>, and num_put<C, OutputIterator>.

These specializations refer to the ios_base& argument for formatting specifications ([locale.categories]), and to its imbued locale for the numpunct<> facet to identify all numeric punctuation preferences, and also for the ctype<> facet to perform character classification.

Extractor and inserter members of the standard iostreams use num_get<> and num_put<> member functions for formatting and parsing numeric values ([istream.formatted.reqmts], [ostream.formatted.reqmts]).

226)226)

Parsing "-1" correctly into, e.g., an unsigned short requires that the corresponding member get() at least extract the sign before delegating.

28.3.4.3.2 Class template num_get [locale.num.get]

28.3.4.3.2.1 General [locale.num.get.general]

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namespace std { template<class charT, class InputIterator = istreambuf_iterator<charT>> class num_get : public locale::facet { public: using char_type = charT; using iter_type = InputIterator; explicit num_get(size_t refs = 0); iter_type get(iter_type in, iter_type end, ios_base&, ios_base::iostate& err, bool& v) const; iter_type get(iter_type in, iter_type end, ios_base&, ios_base::iostate& err, long& v) const; iter_type get(iter_type in, iter_type end, ios_base&, ios_base::iostate& err, long long& v) const; iter_type get(iter_type in, iter_type end, ios_base&, ios_base::iostate& err, unsigned short& v) const; iter_type get(iter_type in, iter_type end, ios_base&, ios_base::iostate& err, unsigned int& v) const; iter_type get(iter_type in, iter_type end, ios_base&, ios_base::iostate& err, unsigned long& v) const; iter_type get(iter_type in, iter_type end, ios_base&, ios_base::iostate& err, unsigned long long& v) const; iter_type get(iter_type in, iter_type end, ios_base&, ios_base::iostate& err, float& v) const; iter_type get(iter_type in, iter_type end, ios_base&, ios_base::iostate& err, double& v) const; iter_type get(iter_type in, iter_type end, ios_base&, ios_base::iostate& err, long double& v) const; iter_type get(iter_type in, iter_type end, ios_base&, ios_base::iostate& err, void*& v) const; static locale::id id; protected: ~num_get(); virtual iter_type do_get(iter_type, iter_type, ios_base&, ios_base::iostate& err, bool& v) const; virtual iter_type do_get(iter_type, iter_type, ios_base&, ios_base::iostate& err, long& v) const; virtual iter_type do_get(iter_type, iter_type, ios_base&, ios_base::iostate& err, long long& v) const; virtual iter_type do_get(iter_type, iter_type, ios_base&, ios_base::iostate& err, unsigned short& v) const; virtual iter_type do_get(iter_type, iter_type, ios_base&, ios_base::iostate& err, unsigned int& v) const; virtual iter_type do_get(iter_type, iter_type, ios_base&, ios_base::iostate& err, unsigned long& v) const; virtual iter_type do_get(iter_type, iter_type, ios_base&, ios_base::iostate& err, unsigned long long& v) const; virtual iter_type do_get(iter_type, iter_type, ios_base&, ios_base::iostate& err, float& v) const; virtual iter_type do_get(iter_type, iter_type, ios_base&, ios_base::iostate& err, double& v) const; virtual iter_type do_get(iter_type, iter_type, ios_base&, ios_base::iostate& err, long double& v) const; virtual iter_type do_get(iter_type, iter_type, ios_base&, ios_base::iostate& err, void*& v) const; }; }

The facet num_get is used to parse numeric values from an input sequence such as an istream.

28.3.4.3.2.3 Virtual functions [facet.num.get.virtuals]

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iter_type do_get(iter_type in, iter_type end, ios_base& str,
                ios_base::iostate& err, long& val) const;
iter_type do_get(iter_type in, iter_type end, ios_base& str,
                ios_base::iostate& err, long long& val) const;
iter_type do_get(iter_type in, iter_type end, ios_base& str,
                ios_base::iostate& err, unsigned short& val) const;
iter_type do_get(iter_type in, iter_type end, ios_base& str,
                ios_base::iostate& err, unsigned int& val) const;
iter_type do_get(iter_type in, iter_type end, ios_base& str,
                ios_base::iostate& err, unsigned long& val) const;
iter_type do_get(iter_type in, iter_type end, ios_base& str,
                ios_base::iostate& err, unsigned long long& val) const;
iter_type do_get(iter_type in, iter_type end, ios_base& str,
                ios_base::iostate& err, float& val) const;
iter_type do_get(iter_type in, iter_type end, ios_base& str,
                ios_base::iostate& err, double& val) const;
iter_type do_get(iter_type in, iter_type end, ios_base& str,
                ios_base::iostate& err, long double& val) const;
iter_type do_get(iter_type in, iter_type end, ios_base& str,
                ios_base::iostate& err, void*& val) const;

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iter_type do_get(iter_type in, iter_type end, ios_base& str,
                 ios_base::iostate& err, bool& val) const;

28.3.4.3.3 Class template num_put [locale.nm.put]

28.3.4.3.3.1 General [locale.nm.put.general]

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namespace std { template<class charT, class OutputIterator = ostreambuf_iterator<charT>> class num_put : public locale::facet { public: using char_type = charT; using iter_type = OutputIterator; explicit num_put(size_t refs = 0); iter_type put(iter_type s, ios_base& f, char_type fill, bool v) const; iter_type put(iter_type s, ios_base& f, char_type fill, long v) const; iter_type put(iter_type s, ios_base& f, char_type fill, long long v) const; iter_type put(iter_type s, ios_base& f, char_type fill, unsigned long v) const; iter_type put(iter_type s, ios_base& f, char_type fill, unsigned long long v) const; iter_type put(iter_type s, ios_base& f, char_type fill, double v) const; iter_type put(iter_type s, ios_base& f, char_type fill, long double v) const; iter_type put(iter_type s, ios_base& f, char_type fill, const void* v) const; static locale::id id; protected: ~num_put(); virtual iter_type do_put(iter_type, ios_base&, char_type fill, bool v) const; virtual iter_type do_put(iter_type, ios_base&, char_type fill, long v) const; virtual iter_type do_put(iter_type, ios_base&, char_type fill, long long v) const; virtual iter_type do_put(iter_type, ios_base&, char_type fill, unsigned long) const; virtual iter_type do_put(iter_type, ios_base&, char_type fill, unsigned long long) const; virtual iter_type do_put(iter_type, ios_base&, char_type fill, double v) const; virtual iter_type do_put(iter_type, ios_base&, char_type fill, long double v) const; virtual iter_type do_put(iter_type, ios_base&, char_type fill, const void* v) const; }; }

The facet num_put is used to format numeric values to a character sequence such as an ostream.

28.3.4.3.3.3 Virtual functions [facet.num.put.virtuals]

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iter_type do_put(iter_type out, ios_base& str, char_type fill, long val) const;
iter_type do_put(iter_type out, ios_base& str, char_type fill, long long val) const;
iter_type do_put(iter_type out, ios_base& str, char_type fill, unsigned long val) const;
iter_type do_put(iter_type out, ios_base& str, char_type fill, unsigned long long val) const;
iter_type do_put(iter_type out, ios_base& str, char_type fill, double val) const;
iter_type do_put(iter_type out, ios_base& str, char_type fill, long double val) const;
iter_type do_put(iter_type out, ios_base& str, char_type fill, const void* val) const;

Stage 2:
Any character c other than a decimal point(.) is converted to a charT via use_facet<ctype<charT>>(loc).widen(c)
A local variable punct is initialized via const numpunct<charT>& punct = use_facet<numpunct<charT>>(loc);
For arithmetic types, punct.thousands_sep() characters are inserted into the sequence as determined by the value returned by punct.do_grouping() using the method described in [facet.numpunct.virtuals].
Decimal point characters(.) are replaced by punct.decimal_point().

Stage 4:
The sequence of charT's at the end of stage 3 are output via *out++ = c

🔗

iter_type do_put(iter_type out, ios_base& str, char_type fill, bool val) const;

227)227)

The conversion specification #o generates a leading 0 which is not a padding character.

28.3.4.4 The numeric punctuation facet [facet.numpunct]

28.3.4.4.1 Class template numpunct [locale.numpunct]

28.3.4.4.1.1 General [locale.numpunct.general]

🔗

namespace std { template<class charT> class numpunct : public locale::facet { public: using char_type = charT; using string_type = basic_string<charT>; explicit numpunct(size_t refs = 0); char_type decimal_point() const; char_type thousands_sep() const; string grouping() const; string_type truename() const; string_type falsename() const; static locale::id id; protected: ~numpunct(); // virtual virtual char_type do_decimal_point() const; virtual char_type do_thousands_sep() const; virtual string do_grouping() const; virtual string_type do_truename() const; // for bool virtual string_type do_falsename() const; // for bool }; }

numpunct<> specifies numeric punctuation.

The specializations required in Table 86 ([locale.category]), namely numpunct<wchar_t> and numpunct<char>, provide classic "C" numeric formats, i.e., they contain information equivalent to that contained in the "C" locale or their wide character counterparts as if obtained by a call to widen.

The syntax for number formats is as follows, where digit represents the radix set specified by the fmtflags argument value, and thousands-sep and decimal-point are the results of corresponding numpunct<charT> members.

Integer values have the format:

intval :
sign

_{o p t}

units

sign :
+
-

units :
digits
digits thousands-sep units

digits :
digit digits

_{o p t}

and floating-point values have:

floatval :
sign

_{o p t}

units fractional

_{o p t}

exponent

_{o p t}

sign

_{o p t}

decimal-point digits exponent

_{o p t}

fractional :
decimal-point digits

_{o p t}

exponent :
e sign

_{o p t}

digits

e :
e
E

where the number of digits between thousands-seps is as specified by do_grouping().

For parsing, if the digits portion contains no thousands-separators, no grouping constraint is applied.

28.3.4.4.2 Class template numpunct_byname [locale.numpunct.byname]

🔗

namespace std { template<class charT> class numpunct_byname : public numpunct<charT> { // this class is specialized for char and wchar_t. public: using char_type = charT; using string_type = basic_string<charT>; explicit numpunct_byname(const char*, size_t refs = 0); explicit numpunct_byname(const string&, size_t refs = 0); protected: ~numpunct_byname(); }; }

28.3.4.5 The collate category [category.collate]

28.3.4.5.1 Class template collate [locale.collate]

28.3.4.5.1.1 General [locale.collate.general]

🔗

namespace std { template<class charT> class collate : public locale::facet { public: using char_type = charT; using string_type = basic_string<charT>; explicit collate(size_t refs = 0); int compare(const charT* low1, const charT* high1, const charT* low2, const charT* high2) const; string_type transform(const charT* low, const charT* high) const; long hash(const charT* low, const charT* high) const; static locale::id id; protected: ~collate(); virtual int do_compare(const charT* low1, const charT* high1, const charT* low2, const charT* high2) const; virtual string_type do_transform(const charT* low, const charT* high) const; virtual long do_hash (const charT* low, const charT* high) const; }; }

The class collate<charT> provides features for use in the collation (comparison) and hashing of strings.

A locale member function template, operator(), uses the collate facet to allow a locale to act directly as the predicate argument for standard algorithms ([algorithms]) and containers operating on strings.

The specializations required in Table 86 ([locale.category]), namely collate<char> and collate<wchar_t>, apply lexicographical ordering ([alg.lex.comparison]).

Each function compares a string of characters *p in the range [low, high).

28.3.4.5.1.2 Members [locale.collate.members]

🔗

int compare(const charT* low1, const charT* high1,
            const charT* low2, const charT* high2) const;

Returns: do_compare(low1, high1, low2, high2).

🔗

string_type transform(const charT* low, const charT* high) const;

Returns: do_transform(low, high).

🔗

long hash(const charT* low, const charT* high) const;

Returns: do_hash(low, high).

28.3.4.5.1.3 Virtual functions [locale.collate.virtuals]

🔗

int do_compare(const charT* low1, const charT* high1,
               const charT* low2, const charT* high2) const;

Returns: 1 if the first string is greater than the second, -1 if less, zero otherwise.

The specializations required in Table 86 ([locale.category]), namely collate<char> and collate<wchar_t>, implement a lexicographical comparison ([alg.lex.comparison]).

🔗

string_type do_transform(const charT* low, const charT* high) const;

Returns: A basic_string<charT> value that, compared lexicographically with the result of calling transform() on another string, yields the same result as calling do_compare() on the same two strings.229

🔗

long do_hash(const charT* low, const charT* high) const;

Returns: An integer value equal to the result of calling hash() on any other string for which do_compare() returns 0 (equal) when passed the two strings.

Recommended practice: The probability that the result equals that for another string which does not compare equal should be very small, approaching (1.0/numeric_limits<unsigned long>::max()).

229)229)

This function is useful when one string is being compared to many other strings.

28.3.4.5.2 Class template collate_byname [locale.collate.byname]

🔗

namespace std { template<class charT> class collate_byname : public collate<charT> { public: using string_type = basic_string<charT>; explicit collate_byname(const char*, size_t refs = 0); explicit collate_byname(const string&, size_t refs = 0); protected: ~collate_byname(); }; }

28.3.4.6 The time category [category.time]

28.3.4.6.1 General [category.time.general]

Templates time_get<charT, InputIterator> and time_put<charT, OutputIterator> provide date and time formatting and parsing.

All specifications of member functions for time_put and time_get in the subclauses of [category.time] only apply to the specializations required in Tables 86 and 87 ([locale.category]).

Their members use their ios_base&, ios_base::iostate&, and fill arguments as described in [locale.categories], and the ctype<> facet, to determine formatting details.

28.3.4.6.2 Class template time_get [locale.time.get]

28.3.4.6.2.1 General [locale.time.get.general]

🔗

namespace std { class time_base { public: enum dateorder { no_order, dmy, mdy, ymd, ydm }; }; template<class charT, class InputIterator = istreambuf_iterator<charT>> class time_get : public locale::facet, public time_base { public: using char_type = charT; using iter_type = InputIterator; explicit time_get(size_t refs = 0); dateorder date_order() const { return do_date_order(); } iter_type get_time(iter_type s, iter_type end, ios_base& f, ios_base::iostate& err, tm* t) const; iter_type get_date(iter_type s, iter_type end, ios_base& f, ios_base::iostate& err, tm* t) const; iter_type get_weekday(iter_type s, iter_type end, ios_base& f, ios_base::iostate& err, tm* t) const; iter_type get_monthname(iter_type s, iter_type end, ios_base& f, ios_base::iostate& err, tm* t) const; iter_type get_year(iter_type s, iter_type end, ios_base& f, ios_base::iostate& err, tm* t) const; iter_type get(iter_type s, iter_type end, ios_base& f, ios_base::iostate& err, tm* t, char format, char modifier = 0) const; iter_type get(iter_type s, iter_type end, ios_base& f, ios_base::iostate& err, tm* t, const char_type* fmt, const char_type* fmtend) const; static locale::id id; protected: ~time_get(); virtual dateorder do_date_order() const; virtual iter_type do_get_time(iter_type s, iter_type end, ios_base&, ios_base::iostate& err, tm* t) const; virtual iter_type do_get_date(iter_type s, iter_type end, ios_base&, ios_base::iostate& err, tm* t) const; virtual iter_type do_get_weekday(iter_type s, iter_type end, ios_base&, ios_base::iostate& err, tm* t) const; virtual iter_type do_get_monthname(iter_type s, iter_type end, ios_base&, ios_base::iostate& err, tm* t) const; virtual iter_type do_get_year(iter_type s, iter_type end, ios_base&, ios_base::iostate& err, tm* t) const; virtual iter_type do_get(iter_type s, iter_type end, ios_base& f, ios_base::iostate& err, tm* t, char format, char modifier) const; }; }

time_get is used to parse a character sequence, extracting components of a time or date into a tm object.

Each get member parses a format as produced by a corresponding format specifier to time_put<>::put.

If the sequence being parsed matches the correct format, the corresponding members of the tm argument are set to the values used to produce the sequence; otherwise either an error is reported or unspecified values are assigned.230

If the end iterator is reached during parsing by any of the get() member functions, the member sets ios_base::eofbit in err.

230)230)

In other words, user confirmation is required for reliable parsing of user-entered dates and times, but machine-generated formats can be parsed reliably.

This allows parsers to be aggressive about interpreting user variations on standard formats.

28.3.4.6.2.2 Members [locale.time.get.members]

🔗

dateorder date_order() const;

Returns: do_date_order().

🔗

iter_type get_time(iter_type s, iter_type end, ios_base& str,
                   ios_base::iostate& err, tm* t) const;

Returns: do_get_time(s, end, str, err, t).

🔗

iter_type get_date(iter_type s, iter_type end, ios_base& str,
                   ios_base::iostate& err, tm* t) const;

Returns: do_get_date(s, end, str, err, t).

🔗

iter_type get_weekday(iter_type s, iter_type end, ios_base& str,
                      ios_base::iostate& err, tm* t) const;
iter_type get_monthname(iter_type s, iter_type end, ios_base& str,
                        ios_base::iostate& err, tm* t) const;

Returns: do_get_weekday(s, end, str, err, t) or do_get_monthname(s, end, str, err, t).

🔗

iter_type get_year(iter_type s, iter_type end, ios_base& str,
                   ios_base::iostate& err, tm* t) const;

Returns: do_get_year(s, end, str, err, t).

🔗

iter_type get(iter_type s, iter_type end, ios_base& f, ios_base::iostate& err,
              tm* t, char format, char modifier = 0) const;

Returns: do_get(s, end, f, err, t, format, modifier).

🔗

iter_type get(iter_type s, iter_type end, ios_base& f, ios_base::iostate& err,
              tm* t, const char_type* fmt, const char_type* fmtend) const;

Preconditions: [fmt, fmtend) is a valid range.

Effects: The function starts by evaluating err = ios_base::goodbit.

It then enters a loop, reading zero or more characters from s at each iteration.

Unless otherwise specified below, the loop terminates when the first of the following conditions holds:

(8.1)
The expression fmt == fmtend evaluates to true.
(8.2)
The expression err == ios_base::goodbit evaluates to false.
(8.3)
The expression s == end evaluates to true, in which case the function evaluates err = ios_base::eofbit | ios_base::failbit.
(8.4)
The next element of fmt is equal to '%', optionally followed by a modifier character, followed by a conversion specifier character, format, together forming a conversion specification valid for the POSIX function strptime.

If the number of elements in the range [fmt, fmtend) is not sufficient to unambiguously determine whether the conversion specification is complete and valid, the function evaluates err = ios_base::failbit.

Otherwise, the function evaluates s = do_get(s, end, f, err, t, format, modifier), where the value of modifier is '\0' when the optional modifier is absent from the conversion specification.

If err == ios_base::goodbit holds after the evaluation of the expression, the function increments fmt to point just past the end of the conversion specification and continues looping.
(8.5)
The expression isspace(*fmt, f.getloc()) evaluates to true, in which case the function first increments fmt until fmt == fmtend || !isspace(*fmt, f.getloc()) evaluates to true, then advances s until s == end || !isspace(*s, f.getloc()) is true, and finally resumes looping.
(8.6)
The next character read from s matches the element pointed to by fmt in a case-insensitive comparison, in which case the function evaluates ++fmt, ++s and continues looping.

Otherwise, the function evaluates err = ios_base::failbit.

[Note 1:

The function uses the ctype<charT> facet installed in f's locale to determine valid whitespace characters.

It is unspecified by what means the function performs case-insensitive comparison or whether multi-character sequences are considered while doing so.

— end note]

Returns: s.

28.3.4.6.2.3 Virtual functions [locale.time.get.virtuals]

🔗

dateorder do_date_order() const;

Returns: An enumeration value indicating the preferred order of components for those date formats that are composed of day, month, and year.231

Returns no_order if the date format specified by 'x' contains other variable components (e.g., Julian day, week number, week day).

🔗

iter_type do_get_time(iter_type s, iter_type end, ios_base& str,
                      ios_base::iostate& err, tm* t) const;

Effects: Reads characters starting at s until it has extracted those tm members, and remaining format characters, used by time_put<>::put to produce the format specified by "%H:%M:%S", or until it encounters an error or end of sequence.

Returns: An iterator pointing immediately beyond the last character recognized as possibly part of a valid time.

🔗

iter_type do_get_date(iter_type s, iter_type end, ios_base& str,
                      ios_base::iostate& err, tm* t) const;

Effects: Reads characters starting at s until it has extracted those tm members and remaining format characters used by time_put<>::put to produce one of the following formats, or until it encounters an error.

The format depends on the value returned by date_order() as shown in Table 97 .

Table 97: do_get_date effects [tab:locale.time.get.dogetdate]

🔗	date_order()	Format
🔗	no_order	"%m%d%y"
🔗	dmy	"%d%m%y"
🔗	mdy	"%m%d%y"
🔗	ymd	"%y%m%d"
🔗	ydm	"%y%d%m"

An implementation may also accept additional implementation-defined formats.

Returns: An iterator pointing immediately beyond the last character recognized as possibly part of a valid date.

🔗

iter_type do_get_weekday(iter_type s, iter_type end, ios_base& str,
                         ios_base::iostate& err, tm* t) const;
iter_type do_get_monthname(iter_type s, iter_type end, ios_base& str,
                           ios_base::iostate& err, tm* t) const;

Effects: Reads characters starting at s until it has extracted the (perhaps abbreviated) name of a weekday or month.

If it finds an abbreviation that is followed by characters that can match a full name, it continues reading until it matches the full name or fails.

It sets the appropriate tm member accordingly.

Returns: An iterator pointing immediately beyond the last character recognized as part of a valid name.

🔗

iter_type do_get_year(iter_type s, iter_type end, ios_base& str,
                      ios_base::iostate& err, tm* t) const;

Effects: Reads characters starting at s until it has extracted an unambiguous year identifier.

It is implementation-defined whether two-digit year numbers are accepted, and (if so) what century they are assumed to lie in.

Sets the t->tm_year member accordingly.

Returns: An iterator pointing immediately beyond the last character recognized as part of a valid year identifier.

🔗

iter_type do_get(iter_type s, iter_type end, ios_base& f,
                 ios_base::iostate& err, tm* t, char format, char modifier) const;

Preconditions: t points to an object.

Effects: The function starts by evaluating err = ios_base::goodbit.

It then reads characters starting at s until it encounters an error, or until it has extracted and assigned those tm members, and any remaining format characters, corresponding to a conversion specification appropriate for the POSIX function strptime, formed by concatenating '%', the modifier character, when non-NUL, and the format character.

When the concatenation fails to yield a complete valid directive the function leaves the object pointed to by t unchanged and evaluates err |= ios_base::failbit.

When s == end evaluates to true after reading a character the function evaluates err |= ios_base::eofbit.

For complex conversion specifications such as %c, %x, or %X, or conversion specifications that involve the optional modifiers E or O, when the function is unable to unambiguously determine some or all tm members from the input sequence [s, end), it evaluates err |= ios_base::eofbit.

In such cases the values of those tm members are unspecified and may be outside their valid range.

Returns: An iterator pointing immediately beyond the last character recognized as possibly part of a valid input sequence for the given format and modifier.

Remarks: It is unspecified whether multiple calls to do_get() with the address of the same tm object will update the current contents of the object or simply overwrite its members.

Portable programs should zero out the object before invoking the function.

231)231)

This function is intended as a convenience only, for common formats, and can return no_order in valid locales.

28.3.4.6.3 Class template time_get_byname [locale.time.get.byname]

🔗

namespace std { template<class charT, class InputIterator = istreambuf_iterator<charT>> class time_get_byname : public time_get<charT, InputIterator> { public: using dateorder = time_base::dateorder; using iter_type = InputIterator; explicit time_get_byname(const char*, size_t refs = 0); explicit time_get_byname(const string&, size_t refs = 0); protected: ~time_get_byname(); }; }

28.3.4.6.4 Class template time_put [locale.time.put]

28.3.4.6.4.1 General [locale.time.put.general]

🔗

namespace std { template<class charT, class OutputIterator = ostreambuf_iterator<charT>> class time_put : public locale::facet { public: using char_type = charT; using iter_type = OutputIterator; explicit time_put(size_t refs = 0); // the following is implemented in terms of other member functions. iter_type put(iter_type s, ios_base& f, char_type fill, const tm* tmb, const charT* pattern, const charT* pat_end) const; iter_type put(iter_type s, ios_base& f, char_type fill, const tm* tmb, char format, char modifier = 0) const; static locale::id id; protected: ~time_put(); virtual iter_type do_put(iter_type s, ios_base&, char_type, const tm* t, char format, char modifier) const; }; }

28.3.4.6.4.2 Members [locale.time.put.members]

🔗

iter_type put(iter_type s, ios_base& str, char_type fill, const tm* t,
              const charT* pattern, const charT* pat_end) const;
iter_type put(iter_type s, ios_base& str, char_type fill, const tm* t,
              char format, char modifier = 0) const;

Effects: The first form steps through the sequence from pattern to pat_end, identifying characters that are part of a format sequence.

Each character that is not part of a format sequence is written to s immediately, and each format sequence, as it is identified, results in a call to do_put; thus, format elements and other characters are interleaved in the output in the order in which they appear in the pattern.

Format sequences are identified by converting each character c to a char value as if by ct.narrow(c, 0), where ct is a reference to ctype<charT> obtained from str.getloc().

The first character of each sequence is equal to '%', followed by an optional modifier character mod232 and a format specifier character spec as defined for the function strftime.

If no modifier character is present, mod is zero.

For each valid format sequence identified, calls do_put(s, str, fill, t, spec, mod).

The second form calls do_put(s, str, fill, t, format, modifier).

[Note 1:

The fill argument can be used in the implementation-defined formats or by derivations.

A space character is a reasonable default for this argument.

— end note]

Returns: An iterator pointing immediately after the last character produced.

232)232)

Although the C programming language defines no modifiers, most vendors do.

28.3.4.6.4.3 Virtual functions [locale.time.put.virtuals]

🔗

iter_type do_put(iter_type s, ios_base&, char_type fill, const tm* t,
                 char format, char modifier) const;

Effects: Formats the contents of the parameter t into characters placed on the output sequence s.

Formatting is controlled by the parameters format and modifier, interpreted identically as the format specifiers in the string argument to the standard library function strftime(), except that the sequence of characters produced for those specifiers that are described as depending on the C locale are instead implementation-defined.

[Note 1:

Interpretation of the modifier argument is implementation-defined.

— end note]

Returns: An iterator pointing immediately after the last character produced.

[Note 2:

The fill argument can be used in the implementation-defined formats or by derivations.

A space character is a reasonable default for this argument.

— end note]

Recommended practice: Interpretation of the modifier should follow POSIX conventions.

Implementations should refer to other standards such as POSIX for a specification of the character sequences produced for those specifiers described as depending on the C locale.

28.3.4.6.5 Class template time_put_byname [locale.time.put.byname]

🔗

namespace std { template<class charT, class OutputIterator = ostreambuf_iterator<charT>> class time_put_byname : public time_put<charT, OutputIterator> { public: using char_type = charT; using iter_type = OutputIterator; explicit time_put_byname(const char*, size_t refs = 0); explicit time_put_byname(const string&, size_t refs = 0); protected: ~time_put_byname(); }; }

28.3.4.7 The monetary category [category.monetary]

28.3.4.7.1 General [category.monetary.general]

These templates handle monetary formats.

A template parameter indicates whether local or international monetary formats are to be used.

All specifications of member functions for money_put and money_get in the subclauses of [category.monetary] only apply to the specializations required in Tables 86 and 87 ([locale.category]).

Their members use their ios_base&, ios_base::iostate&, and fill arguments as described in [locale.categories], and the moneypunct<> and ctype<> facets, to determine formatting details.

28.3.4.7.2 Class template money_get [locale.money.get]

28.3.4.7.2.1 General [locale.money.get.general]

🔗

namespace std { template<class charT, class InputIterator = istreambuf_iterator<charT>> class money_get : public locale::facet { public: using char_type = charT; using iter_type = InputIterator; using string_type = basic_string<charT>; explicit money_get(size_t refs = 0); iter_type get(iter_type s, iter_type end, bool intl, ios_base& f, ios_base::iostate& err, long double& units) const; iter_type get(iter_type s, iter_type end, bool intl, ios_base& f, ios_base::iostate& err, string_type& digits) const; static locale::id id; protected: ~money_get(); virtual iter_type do_get(iter_type, iter_type, bool, ios_base&, ios_base::iostate& err, long double& units) const; virtual iter_type do_get(iter_type, iter_type, bool, ios_base&, ios_base::iostate& err, string_type& digits) const; }; }

28.3.4.7.2.2 Members [locale.money.get.members]

🔗

iter_type get(iter_type s, iter_type end, bool intl, ios_base& f,
              ios_base::iostate& err, long double& quant) const;
iter_type get(iter_type s, iter_type end, bool intl, ios_base& f,
              ios_base::iostate& err, string_type& quant) const;

Returns: do_get(s, end, intl, f, err, quant).

28.3.4.7.2.3 Virtual functions [locale.money.get.virtuals]

🔗

iter_type do_get(iter_type s, iter_type end, bool intl, ios_base& str,
                 ios_base::iostate& err, long double& units) const;
iter_type do_get(iter_type s, iter_type end, bool intl, ios_base& str,
                 ios_base::iostate& err, string_type& digits) const;

Effects: Reads characters from s to parse and construct a monetary value according to the format specified by a moneypunct<charT, Intl> facet reference mp and the character mapping specified by a ctype<charT> facet reference ct obtained from the locale returned by str.getloc(), and str.flags().

If a valid sequence is recognized, does not change err; otherwise, sets err to (err|str.failbit), or (err|str.failbit|str.eofbit) if no more characters are available, and does not change units or digits.

Uses the pattern returned by mp.neg_format() to parse all values.

The result is returned as an integral value stored in units or as a sequence of digits possibly preceded by a minus sign (as produced by ct.widen(c) where c is '-' or in the range from '0' through '9' (inclusive)) stored in digits.

[Example 1:

The sequence $1,056.23 in a common United States locale would yield, for units, 105623, or, for digits, "105623".

— end example]

If mp.grouping() indicates that no thousands separators are permitted, any such characters are not read, and parsing is terminated at the point where they first appear.

Otherwise, thousands separators are optional; if present, they are checked for correct placement only after all format components have been read.

Where money_base::space or money_base::none appears as the last element in the format pattern, no whitespace is consumed.

Otherwise, where money_base::space appears in any of the initial elements of the format pattern, at least one whitespace character is required.

Where money_base::none appears in any of the initial elements of the format pattern, whitespace is allowed but not required.

If (str.flags() & str.showbase) is false, the currency symbol is optional and is consumed only if other characters are needed to complete the format; otherwise, the currency symbol is required.

If the first character (if any) in the string pos returned by mp.positive_sign() or the string neg returned by mp.negative_sign() is recognized in the position indicated by sign in the format pattern, it is consumed and any remaining characters in the string are required after all the other format components.

[Example 2:

If showbase is off, then for a neg value of "()" and a currency symbol of "L", in "(100 L)" the "L" is consumed; but if neg is "-", the "L" in "-100 L" is not consumed.

— end example]

If pos or neg is empty, the sign component is optional, and if no sign is detected, the result is given the sign that corresponds to the source of the empty string.

Otherwise, the character in the indicated position must match the first character of pos or neg, and the result is given the corresponding sign.

If the first character of pos is equal to the first character of neg, or if both strings are empty, the result is given a positive sign.

Digits in the numeric monetary component are extracted and placed in digits, or into a character buffer buf1 for conversion to produce a value for units, in the order in which they appear, preceded by a minus sign if and only if the result is negative.

The value units is produced as if by233 for (int i = 0; i < n; ++i) buf2[i] = src[find(atoms, atoms+sizeof(src), buf1[i]) - atoms]; buf2[n] = 0; sscanf(buf2, "%Lf", &units); where n is the number of characters placed in buf1, buf2 is a character buffer, and the values src and atoms are defined as if by static const char src[] = "0123456789-"; charT atoms[sizeof(src)]; ct.widen(src, src + sizeof(src) - 1, atoms);

Returns: An iterator pointing immediately beyond the last character recognized as part of a valid monetary quantity.

233)233)

The semantics here are different from ct.narrow.

28.3.4.7.3 Class template money_put [locale.money.put]

28.3.4.7.3.1 General [locale.money.put.general]

🔗

namespace std { template<class charT, class OutputIterator = ostreambuf_iterator<charT>> class money_put : public locale::facet { public: using char_type = charT; using iter_type = OutputIterator; using string_type = basic_string<charT>; explicit money_put(size_t refs = 0); iter_type put(iter_type s, bool intl, ios_base& f, char_type fill, long double units) const; iter_type put(iter_type s, bool intl, ios_base& f, char_type fill, const string_type& digits) const; static locale::id id; protected: ~money_put(); virtual iter_type do_put(iter_type, bool, ios_base&, char_type fill, long double units) const; virtual iter_type do_put(iter_type, bool, ios_base&, char_type fill, const string_type& digits) const; }; }

28.3.4.7.3.2 Members [locale.money.put.members]

🔗

iter_type put(iter_type s, bool intl, ios_base& f, char_type fill, long double quant) const;
iter_type put(iter_type s, bool intl, ios_base& f, char_type fill, const string_type& quant) const;

Returns: do_put(s, intl, f, loc, quant).

28.3.4.7.3.3 Virtual functions [locale.money.put.virtuals]

🔗

iter_type do_put(iter_type s, bool intl, ios_base& str,
                 char_type fill, long double units) const;
iter_type do_put(iter_type s, bool intl, ios_base& str,
                 char_type fill, const string_type& digits) const;

Effects: Writes characters to s according to the format specified by a moneypunct<charT, Intl> facet reference mp and the character mapping specified by a ctype<charT> facet reference ct obtained from the locale returned by str.getloc(), and str.flags().

The argument units is transformed into a sequence of wide characters as if by ct.widen(buf1, buf1 + sprintf(buf1, "%.0Lf", units), buf2) for character buffers buf1 and buf2.

If the first character in digits or buf2 is equal to ct.widen('-'), then the pattern used for formatting is the result of mp.neg_format(); otherwise the pattern is the result of mp.pos_format().

Digit characters are written, interspersed with any thousands separators and decimal point specified by the format, in the order they appear (after the optional leading minus sign) in digits or buf2.

In digits, only the optional leading minus sign and the immediately subsequent digit characters (as classified according to ct) are used; any trailing characters (including digits appearing after a non-digit character) are ignored.

Calls str.width(0).

Returns: An iterator pointing immediately after the last character produced.

Remarks: The currency symbol is generated if and only if (str.flags() & str.showbase) is nonzero.

If the number of characters generated for the specified format is less than the value returned by str.width() on entry to the function, then copies of fill are inserted as necessary to pad to the specified width.

For the value af equal to (str.flags() & str.adjustfield), if (af == str.internal) is true, the fill characters are placed where none or space appears in the formatting pattern; otherwise if (af == str.left) is true, they are placed after the other characters; otherwise, they are placed before the other characters.

[Note 1:

It is possible, with some combinations of format patterns and flag values, to produce output that cannot be parsed using num_get<>::get.

— end note]

28.3.4.7.4 Class template moneypunct [locale.moneypunct]

28.3.4.7.4.1 General [locale.moneypunct.general]

🔗

namespace std { class money_base { public: enum part { none, space, symbol, sign, value }; struct pattern { char field[4]; }; }; template<class charT, bool International = false> class moneypunct : public locale::facet, public money_base { public: using char_type = charT; using string_type = basic_string<charT>; explicit moneypunct(size_t refs = 0); charT decimal_point() const; charT thousands_sep() const; string grouping() const; string_type curr_symbol() const; string_type positive_sign() const; string_type negative_sign() const; int frac_digits() const; pattern pos_format() const; pattern neg_format() const; static locale::id id; static const bool intl = International; protected: ~moneypunct(); virtual charT do_decimal_point() const; virtual charT do_thousands_sep() const; virtual string do_grouping() const; virtual string_type do_curr_symbol() const; virtual string_type do_positive_sign() const; virtual string_type do_negative_sign() const; virtual int do_frac_digits() const; virtual pattern do_pos_format() const; virtual pattern do_neg_format() const; }; }

The moneypunct<> facet defines monetary formatting parameters used by money_get<> and money_put<>.

A monetary format is a sequence of four components, specified by a pattern value p, such that the part value static_cast<part>(p.field[i]) determines the

i^{th}

component of the format234 In the field member of a pattern object, each value symbol, sign, value, and either space or none appears exactly once.

The value none, if present, is not first; the value space, if present, is neither first nor last.

Where none or space appears, whitespace is permitted in the format, except where none appears at the end, in which case no whitespace is permitted.

The value space indicates that at least one space is required at that position.

Where symbol appears, the sequence of characters returned by curr_symbol() is permitted, and can be required.

Where sign appears, the first (if any) of the sequence of characters returned by positive_sign() or negative_sign() (respectively as the monetary value is non-negative or negative) is required.

Any remaining characters of the sign sequence are required after all other format components.

Where value appears, the absolute numeric monetary value is required.

The format of the numeric monetary value is a decimal number:

value :
units fractional

_{o p t}

decimal-point digits

fractional :
decimal-point digits

_{o p t}

if frac_digits() returns a positive value, or

value :
units

otherwise.

The symbol decimal-point indicates the character returned by decimal_point().

The other symbols are defined as follows:

units :
digits
digits thousands-sep units

digits :
adigit digits

_{o p t}

In the syntax specification, the symbol adigit is any of the values ct.widen(c) for c in the range '0' through '9' (inclusive) and ct is a reference of type const ctype<charT>& obtained as described in the definitions of money_get<> and money_put<>.

The symbol thousands-sep is the character returned by thousands_sep().

The space character used is the value ct.widen(' ').

Whitespace characters are those characters c for which ci.is(space, c) returns true.

The number of digits required after the decimal point (if any) is exactly the value returned by frac_digits().

The placement of thousands-separator characters (if any) is determined by the value returned by grouping(), defined identically as the member numpunct<>::do_grouping().

234)234)

An array of char, rather than an array of part, is specified for pattern::field purely for efficiency.

28.3.4.7.4.2 Members [locale.moneypunct.members]

🔗

charT decimal_point() const; charT thousands_sep() const; string grouping() const; string_type curr_symbol() const; string_type positive_sign() const; string_type negative_sign() const; int frac_digits() const; pattern pos_format() const; pattern neg_format() const;

Each of these functions F returns the result of calling the corresponding virtual member function do_F().

28.3.4.7.4.3 Virtual functions [locale.moneypunct.virtuals]

🔗

charT do_decimal_point() const;

Returns: The radix separator to use in case do_frac_digits() is greater than zero.235

🔗

charT do_thousands_sep() const;

Returns: The digit group separator to use in case do_grouping() specifies a digit grouping pattern.236

🔗

string do_grouping() const;

Returns: A pattern defined identically as, but not necessarily equal to, the result of numpunct<charT>::do_grouping().237

🔗

string_type do_curr_symbol() const;

Returns: A string to use as the currency identifier symbol.

[Note 1:

For specializations where the second template parameter is true, this is typically four characters long: a three-letter code as specified by ISO 4217[bib] followed by a space.

— end note]

🔗

string_type do_positive_sign() const;
string_type do_negative_sign() const;

Returns: do_positive_sign() returns the string to use to indicate a positive monetary value;238 do_negative_sign() returns the string to use to indicate a negative value.

🔗

int do_frac_digits() const;

Returns: The number of digits after the decimal radix separator, if any.239

🔗

pattern do_pos_format() const;
pattern do_neg_format() const;

Returns: The specializations required in Table 87 ([locale.category]), namely

(7.1)
moneypunct<char>,
(7.2)
moneypunct<wchar_t>,
(7.3)
moneypunct<char, true>, and
(7.4)
moneypunct<wchar_t, true>,

return an object of type pattern initialized to { symbol, sign, none, value }.240

235)235)

In common U.S. locales this is '.'.

236)236)

In common U.S. locales this is ','.

237)237)

To specify grouping by 3s, the value is "\003" not "3".

238)238)

This is usually the empty string.

239)239)

In common U.S. locales, this is 2.

240)240)

Note that the international symbol returned by do_curr_symbol() usually contains a space, itself; for example, "USD ".

28.3.4.7.5 Class template moneypunct_byname [locale.moneypunct.byname]

🔗

namespace std { template<class charT, bool Intl = false> class moneypunct_byname : public moneypunct<charT, Intl> { public: using pattern = money_base::pattern; using string_type = basic_string<charT>; explicit moneypunct_byname(const char*, size_t refs = 0); explicit moneypunct_byname(const string&, size_t refs = 0); protected: ~moneypunct_byname(); }; }

28.3.4.8 The message retrieval category [category.messages]

28.3.4.8.1 General [category.messages.general]

Class messages<charT> implements retrieval of strings from message catalogs.

28.3.4.8.2 Class template messages [locale.messages]

28.3.4.8.2.1 General [locale.messages.general]

🔗

namespace std { class messages_base { public: using catalog = unspecified signed integer type; }; template<class charT> class messages : public locale::facet, public messages_base { public: using char_type = charT; using string_type = basic_string<charT>; explicit messages(size_t refs = 0); catalog open(const string& fn, const locale&) const; string_type get(catalog c, int set, int msgid, const string_type& dfault) const; void close(catalog c) const; static locale::id id; protected: ~messages(); virtual catalog do_open(const string&, const locale&) const; virtual string_type do_get(catalog, int set, int msgid, const string_type& dfault) const; virtual void do_close(catalog) const; }; }

Values of type messages_base::catalog usable as arguments to members get and close can be obtained only by calling member open.

28.3.4.8.2.2 Members [locale.messages.members]

🔗

catalog open(const string& name, const locale& loc) const;

Returns: do_open(name, loc).

🔗

string_type get(catalog cat, int set, int msgid, const string_type& dfault) const;

Returns: do_get(cat, set, msgid, dfault).

🔗

void close(catalog cat) const;

Effects: Calls do_close(cat).

28.3.4.8.2.3 Virtual functions [locale.messages.virtuals]

🔗

catalog do_open(const string& name, const locale& loc) const;

Returns: A value that may be passed to get() to retrieve a message from the message catalog identified by the string name according to an implementation-defined mapping.

The result can be used until it is passed to close().

Returns a value less than 0 if no such catalog can be opened.

Remarks: The locale argument loc is used for character set code conversion when retrieving messages, if needed.

🔗

string_type do_get(catalog cat, int set, int msgid, const string_type& dfault) const;

Preconditions: cat is a catalog obtained from open() and not yet closed.

Returns: A message identified by arguments set, msgid, and dfault, according to an implementation-defined mapping.

If no such message can be found, returns dfault.

🔗

void do_close(catalog cat) const;

Preconditions: cat is a catalog obtained from open() and not yet closed.

Effects: Releases unspecified resources associated with cat.

Remarks: The limit on such resources, if any, is implementation-defined.

28.3.4.8.3 Class template messages_byname [locale.messages.byname]

🔗

namespace std { template<class charT> class messages_byname : public messages<charT> { public: using catalog = messages_base::catalog; using string_type = basic_string<charT>; explicit messages_byname(const char*, size_t refs = 0); explicit messages_byname(const string&, size_t refs = 0); protected: ~messages_byname(); }; }

28.3.5 C library locales [c.locales]

28.3.5.1 Header <clocale> synopsis [clocale.syn]

🔗

namespace std { struct lconv; char* setlocale(int category, const char* locale); lconv* localeconv(); } #define NULL see [support.types.nullptr] #define LC_ALL see below #define LC_COLLATE see below #define LC_CTYPE see below #define LC_MONETARY see below #define LC_NUMERIC see below #define LC_TIME see below

The contents and meaning of the header are the same as the C standard library header <locale.h>.

28.3.5.2 Data races [clocale.data.races]

Calls to the function setlocale may introduce a data race ([res.on.data.races]) with other calls to setlocale or with calls to the functions listed in Table 98 .

28.4 Text encodings identification [text.encoding]

28.4.1 Header <text_encoding> synopsis [text.encoding.syn]

🔗

namespace std { struct text_encoding; // [text.encoding.hash], hash support template<class T> struct hash; template<> struct hash<text_encoding>; }

28.4.2 Class text_encoding [text.encoding.class]

28.4.2.1 Overview [text.encoding.overview]

The class text_encoding describes an interface for accessing the IANA Character Sets registry[bib].

🔗

namespace std { struct text_encoding { static constexpr size_t max_name_length = 63; // [text.encoding.id], enumeration text_encoding::id enum class id : int_least32_t { see below }; using enum id; constexpr text_encoding() = default; constexpr explicit text_encoding(string_view enc) noexcept; constexpr text_encoding(id i) noexcept; constexpr id mib() const noexcept; constexpr const char* name() const noexcept; // [text.encoding.aliases], class text_encoding::aliases_view struct aliases_view; constexpr aliases_view aliases() const noexcept; friend constexpr bool operator==(const text_encoding& a, const text_encoding& b) noexcept; friend constexpr bool operator==(const text_encoding& encoding, id i) noexcept; static consteval text_encoding literal() noexcept; static text_encoding environment(); template<id i> static bool environment_is(); private: id mib_ = id::unknown; // exposition only char name_[max_name_length + 1] = {0}; // exposition only static constexpr bool comp-name(string_view a, string_view b); // exposition only }; }

Class text_encoding is a trivially copyable type ([basic.types.general]).

28.4.2.2 General [text.encoding.general]

A registered character encoding is a character encoding scheme in the IANA Character Sets registry.

[Note 1:

The IANA Character Sets registry uses the term “character sets” to refer to character encodings.

— end note]

The primary name of a registered character encoding is the name of that encoding specified in the IANA Character Sets registry.

The set of known registered character encodings contains every registered character encoding specified in the IANA Character Sets registry except for the following:

(2.1)
NATS-DANO (33)
(2.2)
NATS-DANO-ADD (34)

Each known registered character encoding is identified by an enumerator in text_encoding::id, and has a set of zero or more aliases.

The set of aliases of a known registered character encoding is an implementation-defined superset of the aliases specified in the IANA Character Sets registry.

The set of aliases for US-ASCII includes “ASCII”.

No two aliases or primary names of distinct registered character encodings are equivalent when compared by text_encoding::comp-name.

How a text_encoding object is determined to be representative of a character encoding scheme implemented in the translation or execution environment is implementation-defined.

An object e of type text_encoding such that e.mib() == text_encoding::id::unknown is false and e.mib() == text_encoding::id::other is false maintains the following invariants:

(6.1)
*e.name() == '\0' is false, and
(6.2)
e.mib() == text_encoding(e.name()).mib() is true.

Recommended practice:

(7.1)
Implementations should not consider registered encodings to be interchangeable.

[Example 1:
Shift_JIS and Windows-31J denote different encodings.
— end example]
(7.2)
Implementations should not use the name of a registered encoding to describe another similar yet different non-registered encoding unless there is a precedent on that implementation.

[Example 2:
Big5
— end example]

28.4.2.3 Members [text.encoding.members]

🔗

constexpr explicit text_encoding(string_view enc) noexcept;

Preconditions:

(1.1)
enc represents a string in the ordinary literal encoding consisting only of elements of the basic character set ([lex.charset]).
(1.2)
enc.size() <= max_name_length is true.
(1.3)
enc.contains('\0') is false.

Postconditions:

(2.1)
If there exists a primary name or alias a of a known registered character encoding such that comp-name(a, enc) is true, mib_ has the value of the enumerator of id associated with that registered character encoding.

Otherwise, mib_ == id::other is true.
(2.2)
enc.compare(name_) == 0 is true.

🔗

constexpr text_encoding(id i) noexcept;

Preconditions: i has the value of one of the enumerators of id.

Postconditions:

(4.1)
mib_ == i is true.
(4.2)
If (mib_ == id::unknown || mib_ == id::other) is true, strlen(name_) == 0 is true.

Otherwise, ranges::contains(aliases(), string_view(name_)) is true.

🔗

constexpr id mib() const noexcept;

Returns: mib_.

🔗

constexpr const char* name() const noexcept;

Returns: name_.

Remarks: name() is an ntbs and accessing elements of name_ outside of the range

name() + [0, strlen(name()) + 1)

is undefined behavior.

🔗

constexpr aliases_view aliases() const noexcept;

Let r denote an instance of aliases_view.

If *this represents a known registered character encoding, then:

r.front() is the primary name of the registered character encoding,
r contains the aliases of the registered character encoding, and
r does not contain duplicate values when compared with strcmp.

Otherwise, r is an empty range.

Each element in r is a non-null, non-empty ntbs encoded in the literal character encoding and comprising only characters from the basic character set.

Returns: r.

[Note 1:

The order of aliases in r is unspecified.

— end note]

🔗

static consteval text_encoding literal() noexcept;

Mandates: CHAR_BIT == 8 is true.

Returns: A text_encoding object representing the ordinary character literal encoding ([lex.charset]).

🔗

static text_encoding environment();

Mandates: CHAR_BIT == 8 is true.

Returns: A text_encoding object representing the implementation-defined character encoding scheme of the environment.

On a POSIX implementation, this is the encoding scheme associated with the POSIX locale denoted by the empty string "".

[Note 2:

This function is not affected by calls to setlocale.

— end note]

Recommended practice: Implementations should return a value that is not affected by calls to the POSIX function setenv and other functions which can modify the environment ([support.runtime]).

🔗

template<id i>
  static bool environment_is();

Mandates: CHAR_BIT == 8 is true.

Returns: environment() == i.

🔗

static constexpr bool comp-name(string_view a, string_view b);

Returns: true if the two strings a and b encoded in the ordinary literal encoding are equal, ignoring, from left-to-right,

(19.1)
all elements that are not digits or letters ([character.seq.general]),
(19.2)
character case, and
(19.3)
any sequence of one or more 0 characters not immediately preceded by a numeric prefix, where a numeric prefix is a sequence consisting of a digit in the range [1, 9] optionally followed by one or more elements which are not digits or letters,

and false otherwise.

[Note 3:

This comparison is identical to the “Charset Alias Matching” algorithm described in the Unicode Technical Standard 22[bib].

— end note]

[Example 1: static_assert(comp-name("UTF-8", "utf8") == true); static_assert(comp-name("u.t.f-008", "utf8") == true); static_assert(comp-name("ut8", "utf8") == false); static_assert(comp-name("utf-80", "utf8") == false); — end example]

28.4.2.4 Comparison functions [text.encoding.cmp]

🔗

friend constexpr bool operator==(const text_encoding& a, const text_encoding& b) noexcept;

Returns: If a.mib_ == id::other && b.mib_ == id::other is true, then comp-name(a.name_,
b.name_).

Otherwise, a.mib_ == b.mib_.

🔗

friend constexpr bool operator==(const text_encoding& encoding, id i) noexcept;

Returns: encoding.mib_ == i.

Remarks: This operator induces an equivalence relation on its arguments if and only if i != id::other is true.

28.4.2.5 Class text_encoding::aliases_view [text.encoding.aliases]

🔗

struct text_encoding::aliases_view : ranges::view_interface<text_encoding::aliases_view> {
  constexpr implementation-defined begin() const;
  constexpr implementation-defined end() const;
};

text_encoding::aliases_view models copyable, ranges::view, ranges::random_access_range, and ranges::borrowed_range.

[Note 1:

text_encoding::aliases_view is not required to satisfy ranges::common_range, nor default_initializable .

— end note]

Both ranges::range_value_t<text_encoding::aliases_view> and ranges::range_reference_t<text_encoding::aliases_view> denote const char*.

ranges::iterator_t<text_encoding::aliases_view> is a constexpr iterator ([iterator.requirements.general]).

28.4.2.6 Enumeration text_encoding::id [text.encoding.id]

🔗

namespace std { enum class text_encoding::id : int_least32_t { other = 1, unknown = 2, ASCII = 3, ISOLatin1 = 4, ISOLatin2 = 5, ISOLatin3 = 6, ISOLatin4 = 7, ISOLatinCyrillic = 8, ISOLatinArabic = 9, ISOLatinGreek = 10, ISOLatinHebrew = 11, ISOLatin5 = 12, ISOLatin6 = 13, ISOTextComm = 14, HalfWidthKatakana = 15, JISEncoding = 16, ShiftJIS = 17, EUCPkdFmtJapanese = 18, EUCFixWidJapanese = 19, ISO4UnitedKingdom = 20, ISO11SwedishForNames = 21, ISO15Italian = 22, ISO17Spanish = 23, ISO21German = 24, ISO60DanishNorwegian = 25, ISO69French = 26, ISO10646UTF1 = 27, ISO646basic1983 = 28, INVARIANT = 29, ISO2IntlRefVersion = 30, NATSSEFI = 31, NATSSEFIADD = 32, ISO10Swedish = 35, KSC56011987 = 36, ISO2022KR = 37, EUCKR = 38, ISO2022JP = 39, ISO2022JP2 = 40, ISO13JISC6220jp = 41, ISO14JISC6220ro = 42, ISO16Portuguese = 43, ISO18Greek7Old = 44, ISO19LatinGreek = 45, ISO25French = 46, ISO27LatinGreek1 = 47, ISO5427Cyrillic = 48, ISO42JISC62261978 = 49, ISO47BSViewdata = 50, ISO49INIS = 51, ISO50INIS8 = 52, ISO51INISCyrillic = 53, ISO54271981 = 54, ISO5428Greek = 55, ISO57GB1988 = 56, ISO58GB231280 = 57, ISO61Norwegian2 = 58, ISO70VideotexSupp1 = 59, ISO84Portuguese2 = 60, ISO85Spanish2 = 61, ISO86Hungarian = 62, ISO87JISX0208 = 63, ISO88Greek7 = 64, ISO89ASMO449 = 65, ISO90 = 66, ISO91JISC62291984a = 67, ISO92JISC62991984b = 68, ISO93JIS62291984badd = 69, ISO94JIS62291984hand = 70, ISO95JIS62291984handadd = 71, ISO96JISC62291984kana = 72, ISO2033 = 73, ISO99NAPLPS = 74, ISO102T617bit = 75, ISO103T618bit = 76, ISO111ECMACyrillic = 77, ISO121Canadian1 = 78, ISO122Canadian2 = 79, ISO123CSAZ24341985gr = 80, ISO88596E = 81, ISO88596I = 82, ISO128T101G2 = 83, ISO88598E = 84, ISO88598I = 85, ISO139CSN369103 = 86, ISO141JUSIB1002 = 87, ISO143IECP271 = 88, ISO146Serbian = 89, ISO147Macedonian = 90, ISO150 = 91, ISO151Cuba = 92, ISO6937Add = 93, ISO153GOST1976874 = 94, ISO8859Supp = 95, ISO10367Box = 96, ISO158Lap = 97, ISO159JISX02121990 = 98, ISO646Danish = 99, USDK = 100, DKUS = 101, KSC5636 = 102, Unicode11UTF7 = 103, ISO2022CN = 104, ISO2022CNEXT = 105, UTF8 = 106, ISO885913 = 109, ISO885914 = 110, ISO885915 = 111, ISO885916 = 112, GBK = 113, GB18030 = 114, OSDEBCDICDF0415 = 115, OSDEBCDICDF03IRV = 116, OSDEBCDICDF041 = 117, ISO115481 = 118, KZ1048 = 119, UCS2 = 1000, UCS4 = 1001, UnicodeASCII = 1002, UnicodeLatin1 = 1003, UnicodeJapanese = 1004, UnicodeIBM1261 = 1005, UnicodeIBM1268 = 1006, UnicodeIBM1276 = 1007, UnicodeIBM1264 = 1008, UnicodeIBM1265 = 1009, Unicode11 = 1010, SCSU = 1011, UTF7 = 1012, UTF16BE = 1013, UTF16LE = 1014, UTF16 = 1015, CESU8 = 1016, UTF32 = 1017, UTF32BE = 1018, UTF32LE = 1019, BOCU1 = 1020, UTF7IMAP = 1021, Windows30Latin1 = 2000, Windows31Latin1 = 2001, Windows31Latin2 = 2002, Windows31Latin5 = 2003, HPRoman8 = 2004, AdobeStandardEncoding = 2005, VenturaUS = 2006, VenturaInternational = 2007, DECMCS = 2008, PC850Multilingual = 2009, PC8DanishNorwegian = 2012, PC862LatinHebrew = 2013, PC8Turkish = 2014, IBMSymbols = 2015, IBMThai = 2016, HPLegal = 2017, HPPiFont = 2018, HPMath8 = 2019, HPPSMath = 2020, HPDesktop = 2021, VenturaMath = 2022, MicrosoftPublishing = 2023, Windows31J = 2024, GB2312 = 2025, Big5 = 2026, Macintosh = 2027, IBM037 = 2028, IBM038 = 2029, IBM273 = 2030, IBM274 = 2031, IBM275 = 2032, IBM277 = 2033, IBM278 = 2034, IBM280 = 2035, IBM281 = 2036, IBM284 = 2037, IBM285 = 2038, IBM290 = 2039, IBM297 = 2040, IBM420 = 2041, IBM423 = 2042, IBM424 = 2043, PC8CodePage437 = 2011, IBM500 = 2044, IBM851 = 2045, PCp852 = 2010, IBM855 = 2046, IBM857 = 2047, IBM860 = 2048, IBM861 = 2049, IBM863 = 2050, IBM864 = 2051, IBM865 = 2052, IBM868 = 2053, IBM869 = 2054, IBM870 = 2055, IBM871 = 2056, IBM880 = 2057, IBM891 = 2058, IBM903 = 2059, IBM904 = 2060, IBM905 = 2061, IBM918 = 2062, IBM1026 = 2063, IBMEBCDICATDE = 2064, EBCDICATDEA = 2065, EBCDICCAFR = 2066, EBCDICDKNO = 2067, EBCDICDKNOA = 2068, EBCDICFISE = 2069, EBCDICFISEA = 2070, EBCDICFR = 2071, EBCDICIT = 2072, EBCDICPT = 2073, EBCDICES = 2074, EBCDICESA = 2075, EBCDICESS = 2076, EBCDICUK = 2077, EBCDICUS = 2078, Unknown8BiT = 2079, Mnemonic = 2080, Mnem = 2081, VISCII = 2082, VIQR = 2083, KOI8R = 2084, HZGB2312 = 2085, IBM866 = 2086, PC775Baltic = 2087, KOI8U = 2088, IBM00858 = 2089, IBM00924 = 2090, IBM01140 = 2091, IBM01141 = 2092, IBM01142 = 2093, IBM01143 = 2094, IBM01144 = 2095, IBM01145 = 2096, IBM01146 = 2097, IBM01147 = 2098, IBM01148 = 2099, IBM01149 = 2100, Big5HKSCS = 2101, IBM1047 = 2102, PTCP154 = 2103, Amiga1251 = 2104, KOI7switched = 2105, BRF = 2106, TSCII = 2107, CP51932 = 2108, windows874 = 2109, windows1250 = 2250, windows1251 = 2251, windows1252 = 2252, windows1253 = 2253, windows1254 = 2254, windows1255 = 2255, windows1256 = 2256, windows1257 = 2257, windows1258 = 2258, TIS620 = 2259, CP50220 = 2260 }; }

[Note 1:

The text_encoding::id enumeration contains an enumerator for each known registered character encoding.

For each encoding, the corresponding enumerator is derived from the alias beginning with “cs”, as follows

csUnicode is mapped to text_encoding::id::UCS2,
csIBBM904 is mapped to text_encoding::id::IBM904, and
the “cs” prefix is removed from other names.

— end note]

28.4.2.7 Hash support [text.encoding.hash]

🔗

template<> struct hash<text_encoding>;

The specialization is enabled ([unord.hash]).

28.5 Formatting [format]

28.5.1 Header <format> synopsis [format.syn]

🔗

namespace std { // [format.context], class template basic_format_context template<class Out, class charT> class basic_format_context; using format_context = basic_format_context<unspecified, char>; using wformat_context = basic_format_context<unspecified, wchar_t>; // [format.args], class template basic_format_args template<class Context> class basic_format_args; using format_args = basic_format_args<format_context>; using wformat_args = basic_format_args<wformat_context>; // [format.fmt.string], class template basic_format_string template<class charT, class... Args> struct basic_format_string; template<class charT> struct runtime-format-string { // exposition only private: basic_string_view<charT> str; // exposition only public: runtime-format-string(basic_string_view<charT> s) noexcept : str(s) {} runtime-format-string(const runtime-format-string&) = delete; runtime-format-string& operator=(const runtime-format-string&) = delete; }; runtime-format-string<char> runtime_format(string_view fmt) noexcept { return fmt; } runtime-format-string<wchar_t> runtime_format(wstring_view fmt) noexcept { return fmt; } template<class... Args> using format_string = basic_format_string<char, type_identity_t<Args>...>; template<class... Args> using wformat_string = basic_format_string<wchar_t, type_identity_t<Args>...>; // [format.functions], formatting functions template<class... Args> string format(format_string<Args...> fmt, Args&&... args); template<class... Args> wstring format(wformat_string<Args...> fmt, Args&&... args); template<class... Args> string format(const locale& loc, format_string<Args...> fmt, Args&&... args); template<class... Args> wstring format(const locale& loc, wformat_string<Args...> fmt, Args&&... args); string vformat(string_view fmt, format_args args); wstring vformat(wstring_view fmt, wformat_args args); string vformat(const locale& loc, string_view fmt, format_args args); wstring vformat(const locale& loc, wstring_view fmt, wformat_args args); template<class Out, class... Args> Out format_to(Out out, format_string<Args...> fmt, Args&&... args); template<class Out, class... Args> Out format_to(Out out, wformat_string<Args...> fmt, Args&&... args); template<class Out, class... Args> Out format_to(Out out, const locale& loc, format_string<Args...> fmt, Args&&... args); template<class Out, class... Args> Out format_to(Out out, const locale& loc, wformat_string<Args...> fmt, Args&&... args); template<class Out> Out vformat_to(Out out, string_view fmt, format_args args); template<class Out> Out vformat_to(Out out, wstring_view fmt, wformat_args args); template<class Out> Out vformat_to(Out out, const locale& loc, string_view fmt, format_args args); template<class Out> Out vformat_to(Out out, const locale& loc, wstring_view fmt, wformat_args args); template<class Out> struct format_to_n_result { Out out; iter_difference_t<Out> size; }; template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, format_string<Args...> fmt, Args&&... args); template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, wformat_string<Args...> fmt, Args&&... args); template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, const locale& loc, format_string<Args...> fmt, Args&&... args); template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, const locale& loc, wformat_string<Args...> fmt, Args&&... args); template<class... Args> size_t formatted_size(format_string<Args...> fmt, Args&&... args); template<class... Args> size_t formatted_size(wformat_string<Args...> fmt, Args&&... args); template<class... Args> size_t formatted_size(const locale& loc, format_string<Args...> fmt, Args&&... args); template<class... Args> size_t formatted_size(const locale& loc, wformat_string<Args...> fmt, Args&&... args); // [format.formatter], formatter template<class T, class charT = char> struct formatter; // [format.formatter.locking], formatter locking template<class T> constexpr bool enable_nonlocking_formatter_optimization = false; // [format.formattable], concept formattable template<class T, class charT> concept formattable = see below; template<class R, class charT> concept const-formattable-range = // exposition only ranges::input_range<const R> && formattable<ranges::range_reference_t<const R>, charT>; template<class R, class charT> using fmt-maybe-const = // exposition only conditional_t<const-formattable-range<R, charT>, const R, R>; // [format.parse.ctx], class template basic_format_parse_context template<class charT> class basic_format_parse_context; using format_parse_context = basic_format_parse_context<char>; using wformat_parse_context = basic_format_parse_context<wchar_t>; // [format.range], formatting of ranges // [format.range.fmtkind], variable template format_kind enum class range_format { disabled, map, set, sequence, string, debug_string }; template<class R> constexpr unspecified format_kind = unspecified; template<ranges::input_range R> requires same_as<R, remove_cvref_t<R>> constexpr range_format format_kind<R> = see below; // [format.range.formatter], class template range_formatter template<class T, class charT = char> requires same_as<remove_cvref_t<T>, T> && formattable<T, charT> class range_formatter; // [format.range.fmtdef], class template range-default-formatter template<range_format K, ranges::input_range R, class charT> struct range-default-formatter; // exposition only // [format.range.fmtmap], [format.range.fmtset], [format.range.fmtstr], specializations for maps, sets, and strings template<ranges::input_range R, class charT> requires (format_kind<R> != range_format::disabled) && formattable<ranges::range_reference_t<R>, charT> struct formatter<R, charT> : range-default-formatter<format_kind<R>, R, charT> { }; template<ranges::input_range R> requires (format_kind<R> != range_format::disabled) constexpr bool enable_nonlocking_formatter_optimization<R> = false; // [format.arguments], arguments // [format.arg], class template basic_format_arg template<class Context> class basic_format_arg; // [format.arg.store], class template format-arg-store template<class Context, class... Args> class format-arg-store; // exposition only template<class Context = format_context, class... Args> format-arg-store<Context, Args...> make_format_args(Args&... fmt_args); template<class... Args> format-arg-store<wformat_context, Args...> make_wformat_args(Args&... args); // [format.error], class format_error class format_error; }

The class template format_to_n_result has the template parameters, data members, and special members specified above.

It has no base classes or members other than those specified.

28.5.2 Format string [format.string]

28.5.2.1 General [format.string.general]

A format string for arguments args is a (possibly empty) sequence of replacement fields, escape sequences, and characters other than { and }.

Let charT be the character type of the format string.

Each character that is not part of a replacement field or an escape sequence is copied unchanged to the output.

An escape sequence is one of {{ or }}.

It is replaced with { or }, respectively, in the output.

The syntax of replacement fields is as follows:

replacement-field :
{ arg-id

_{o p t}

format-specifier

_{o p t}

}

arg-id :
0
positive-integer

positive-integer :
nonzero-digit
positive-integer digit

nonnegative-integer :
digit
nonnegative-integer digit

nonzero-digit : one of
1 2 3 4 5 6 7 8 9

digit : one of
0 1 2 3 4 5 6 7 8 9

format-specifier :
: format-spec

format-spec :
as specified by the formatter specialization for the argument type; cannot start with }

The arg-id field specifies the index of the argument in args whose value is to be formatted and inserted into the output instead of the replacement field.

If there is no argument with the index arg-id in args, the string is not a format string for args.

The optional format-specifier field explicitly specifies a format for the replacement value.

[Example 1: string s = format("{0}-{{", 8); // value of s is "8-{" — end example]

If all arg-ids in a format string are omitted (including those in the format-spec, as interpreted by the corresponding formatter specialization), argument indices 0, 1, 2, … will automatically be used in that order.

If some arg-ids are omitted and some are present, the string is not a format string.

[Note 1:

A format string cannot contain a mixture of automatic and manual indexing.

— end note]

[Example 2: string s0 = format("{} to {}", "a", "b"); // OK, automatic indexing string s1 = format("{1} to {0}", "a", "b"); // OK, manual indexing string s2 = format("{0} to {}", "a", "b"); // not a format string (mixing automatic and manual indexing), // ill-formed string s3 = format("{} to {1}", "a", "b"); // not a format string (mixing automatic and manual indexing), // ill-formed — end example]

The format-spec field contains format specifications that define how the value should be presented.

Each type can define its own interpretation of the format-spec field.

If format-spec does not conform to the format specifications for the argument type referred to by arg-id, the string is not a format string for args.

[Example 3:

(5.1)
For arithmetic, pointer, and string types the format-spec is interpreted as a std-format-spec as described in ([format.string.std]).
(5.2)
For chrono types the format-spec is interpreted as a chrono-format-spec as described in ([time.format]).
(5.3)
For user-defined formatter specializations, the behavior of the parse member function determines how the format-spec is interpreted.

— end example]

28.5.2.2 Standard format specifiers [format.string.std]

Each formatter specialization described in [format.formatter.spec] for fundamental and string types interprets format-spec as a std-format-spec.

[Note 1:

The format specification can be used to specify such details as minimum field width, alignment, padding, and decimal precision.

Some of the formatting options are only supported for arithmetic types.

— end note]

The syntax of format specifications is as follows:

std-format-spec :
fill-and-align

_{o p t}

sign

_{o p t}

_{o p t}

_{o p t}

width

_{o p t}

precision

_{o p t}

_{o p t}

type

_{o p t}

fill-and-align :
fill

_{o p t}

align

fill :
any character other than { or }

align : one of
< > ^

sign : one of
+ - space

width :
positive-integer
{ arg-id

_{o p t}

}

precision :
. nonnegative-integer
. { arg-id

_{o p t}

}

type : one of
a A b B c d e E f F g G o p P s x X ?

Field widths are specified in field width units; the number of column positions required to display a sequence of characters in a terminal.

The minimum field width is the number of field width units a replacement field minimally requires of the formatted sequence of characters produced for a format argument.

The estimated field width is the number of field width units that are required for the formatted sequence of characters produced for a format argument independent of the effects of the width option.

The padding width is the greater of 0 and the difference of the minimum field width and the estimated field width.

[Note 2:

The POSIX wcswidth function is an example of a function that, given a string, returns the number of column positions required by a terminal to display the string.

— end note]

The fill character is the character denoted by the fill option or, if the fill option is absent, the space character.

For a format specification in UTF-8, UTF-16, or UTF-32, the fill character corresponds to a single Unicode scalar value.

[Note 3:

The presence of a fill option is signaled by the character following it, which must be one of the alignment options.

If the second character of std-format-spec is not a valid alignment option, then it is assumed that the fill and align options are both absent.

— end note]

The align option applies to all argument types.

The meaning of the various alignment options is as specified in Table 99 .

[Example 1: char c = 120; string s0 = format("{:6}", 42); // value of s0 is " 42" string s1 = format("{:6}", 'x'); // value of s1 is "x " string s2 = format("{:*<6}", 'x'); // value of s2 is "x*****" string s3 = format("{:*>6}", 'x'); // value of s3 is "*****x" string s4 = format("{:*^6}", 'x'); // value of s4 is "**x***" string s5 = format("{:6d}", c); // value of s5 is " 120" string s6 = format("{:6}", true); // value of s6 is "true " string s7 = format("{:*<6.3}", "123456"); // value of s7 is "123***" string s8 = format("{:02}", 1234); // value of s8 is "1234" string s9 = format("{:*<}", "12"); // value of s9 is "12" string sA = format("{:*<6}", "12345678"); // value of sA is "12345678" string sB = format("{:🤡^6}", "x"); // value of sB is "🤡🤡x🤡🤡🤡" string sC = format("{:*^6}", "🤡🤡🤡"); // value of sC is "🤡🤡🤡" — end example]

[Note 4:

The fill, align, and 0 options have no effect when the minimum field width is not greater than the estimated field width because padding width is 0 in that case.

Since fill characters are assumed to have a field width of 1, use of a character with a different field width can produce misaligned output.

The 🤡 (U+1f921 clown face) character has a field width of 2.

The examples above that include that character illustrate the effect of the field width when that character is used as a fill character as opposed to when it is used as a formatting argument.

— end note]

Table 99: Meaning of align options [tab:format.align]

🔗	Option	Meaning
🔗	<	Forces the formatted argument to be aligned to the start of the field by inserting n fill characters after the formatted argument where n is the padding width. This is the default for non-arithmetic non-pointer types, charT, and bool, unless an integer presentation type is specified.
🔗	>	Forces the formatted argument to be aligned to the end of the field by inserting n fill characters before the formatted argument where n is the padding width. This is the default for arithmetic types other than charT and bool, pointer types, or when an integer presentation type is specified.
🔗	^	Forces the formatted argument to be centered within the field by inserting $⌊ \frac{n}{2} ⌋$ fill characters before and $⌈ \frac{n}{2} ⌉$ fill characters after the formatted argument, where n is the padding width.

The sign option is only valid for arithmetic types other than charT and bool or when an integer presentation type is specified.

The meaning of the various options is as specified in Table 100 .

Table 100: Meaning of sign options [tab:format.sign]

🔗	Option	Meaning
🔗	+	Indicates that a sign should be used for both non-negative and negative numbers. The + sign is inserted before the output of to_chars for non-negative numbers other than negative zero. [Note 5: For negative numbers and negative zero the output of to_chars will already contain the sign so no additional transformation is performed. — end note]
🔗	-	Indicates that a sign should be used for negative numbers and negative zero only (this is the default behavior).
🔗	space	Indicates that a leading space should be used for non-negative numbers other than negative zero, and a minus sign for negative numbers and negative zero.

The sign option applies to floating-point infinity and NaN.

[Example 2: double inf = numeric_limits<double>::infinity(); double nan = numeric_limits<double>::quiet_NaN(); string s0 = format("{0:},{0:+},{0:-},{0: }", 1); // value of s0 is "1,+1,1, 1" string s1 = format("{0:},{0:+},{0:-},{0: }", -1); // value of s1 is "-1,-1,-1,-1" string s2 = format("{0:},{0:+},{0:-},{0: }", inf); // value of s2 is "inf,+inf,inf, inf" string s3 = format("{0:},{0:+},{0:-},{0: }", nan); // value of s3 is "nan,+nan,nan, nan" — end example]

The # option causes the alternate form to be used for the conversion.

This option is valid for arithmetic types other than charT and bool or when an integer presentation type is specified, and not otherwise.

For integral types, the alternate form inserts the base prefix (if any) specified in Table 102 into the output after the sign character (possibly space) if there is one, or before the output of to_chars otherwise.

For floating-point types, the alternate form causes the result of the conversion of finite values to always contain a decimal-point character, even if no digits follow it.

Normally, a decimal-point character appears in the result of these conversions only if a digit follows it.

In addition, for g and G conversions, trailing zeros are not removed from the result.

The 0 option is valid for arithmetic types other than charT and bool, pointer types, or when an integer presentation type is specified.

For formatting arguments that have a value other than an infinity or a NaN, this option pads the formatted argument by inserting the 0 character n times following the sign or base prefix indicators (if any) where n is 0 if the align option is present and is the padding width otherwise.

[Example 3: char c = 120; string s1 = format("{:+06d}", c); // value of s1 is "+00120" string s2 = format("{:#06x}", 0xa); // value of s2 is "0x000a" string s3 = format("{:<06}", -42); // value of s3 is "-42 " (0 has no effect) string s4 = format("{:06}", inf); // value of s4 is " inf" (0 has no effect) — end example]

The width option specifies the minimum field width.

If the width option is absent, the minimum field width is 0.

If { arg-id

_{o p t}

} is used in a width or precision option, the value of the corresponding formatting argument is used as the value of the option.

The option is valid only if the corresponding formatting argument is of standard signed or unsigned integer type.

If its value is negative, an exception of type format_error is thrown.

If positive-integer is used in a width option, the value of the positive-integer is interpreted as a decimal integer and used as the value of the option.

For the purposes of width computation, a string is assumed to be in a locale-independent, implementation-defined encoding.

Implementations should use either UTF-8, UTF-16, or UTF-32, on platforms capable of displaying Unicode text in a terminal.

[Note 6:

This is the case for Windows®-based241 and many POSIX-based operating systems.

— end note]

For a sequence of characters in UTF-8, UTF-16, or UTF-32, an implementation should use as its field width the sum of the field widths of the first code point of each extended grapheme cluster.

Extended grapheme clusters are defined by UAX #29 of the Unicode Standard.

The following code points have a field width of 2:

(13.1)
any code point with the East_Asian_Width="W" or East_Asian_Width="F" property as described by UAX #44 of the Unicode Standard
(13.2)
U+4dc0 – U+4dff (Yijing Hexagram Symbols)
(13.3)
U+1f300 – U+1f5ff (Miscellaneous Symbols and Pictographs)
(13.4)
U+1f900 – U+1f9ff (Supplemental Symbols and Pictographs)

The field width of all other code points is 1.

For a sequence of characters in neither UTF-8, UTF-16, nor UTF-32, the field width is unspecified.

The precision option is valid for floating-point and string types.

For floating-point types, the value of this option specifies the precision to be used for the floating-point presentation type.

For string types, this option specifies the longest prefix of the formatted argument to be included in the replacement field such that the field width of the prefix is no greater than the value of this option.

If nonnegative-integer is used in a precision option, the value of the decimal integer is used as the value of the option.

When the L option is used, the form used for the conversion is called the locale-specific form.

The L option is only valid for arithmetic types, and its effect depends upon the type.

(17.1)
For integral types, the locale-specific form causes the context's locale to be used to insert the appropriate digit group separator characters.
(17.2)
For floating-point types, the locale-specific form causes the context's locale to be used to insert the appropriate digit group and radix separator characters.
(17.3)
For the textual representation of bool, the locale-specific form causes the context's locale to be used to insert the appropriate string as if obtained with numpunct::truename or numpunct::falsename.

The type determines how the data should be presented.

The available string presentation types are specified in Table 101 .

Table 101: Meaning of type options for strings [tab:format.type.string]

🔗	Type	Meaning
🔗	none, s	Copies the string to the output.
🔗	?	Copies the escaped string ([format.string.escaped]) to the output.

The meaning of some non-string presentation types is defined in terms of a call to to_chars.

In such cases, let [first, last) be a range large enough to hold the to_chars output and value be the formatting argument value.

Formatting is done as if by calling to_chars as specified and copying the output through the output iterator of the format context.

[Note 7:

Additional padding and adjustments are performed prior to copying the output through the output iterator as specified by the format specifiers.

— end note]

The available integer presentation types for integral types other than bool and charT are specified in Table 102 .

[Example 4: string s0 = format("{}", 42); // value of s0 is "42" string s1 = format("{0:b} {0:d} {0:o} {0:x}", 42); // value of s1 is "101010 42 52 2a" string s2 = format("{0:#x} {0:#X}", 42); // value of s2 is "0x2a 0X2A" string s3 = format("{:L}", 1234); // value of s3 can be "1,234" // (depending on the locale) — end example]

Table 102: Meaning of type options for integer types [tab:format.type.int]

🔗	Type	Meaning
🔗	b	to_chars(first, last, value, 2); the base prefix is 0b.
🔗	B	The same as b, except that the base prefix is 0B.
🔗	c	Copies the character static_cast<charT>(value) to the output. Throws format_error if value is not in the range of representable values for charT.
🔗	d	to_chars(first, last, value).
🔗	o	to_chars(first, last, value, 8); the base prefix is 0 if value is nonzero and is empty otherwise.
🔗	x	to_chars(first, last, value, 16); the base prefix is 0x.
🔗	X	The same as x, except that it uses uppercase letters for digits above 9 and the base prefix is 0X.
🔗	none	The same as d. [Note 8: If the formatting argument type is charT or bool, the default is instead c or s, respectively. — end note]

The available charT presentation types are specified in Table 103 .

Table 103: Meaning of type options for charT [tab:format.type.char]

🔗	Type	Meaning
🔗	none, c	Copies the character to the output.
🔗	b, B, d, o, x, X	As specified in Table 102 with value converted to the unsigned version of the underlying type.
🔗	?	Copies the escaped character ([format.string.escaped]) to the output.

The available bool presentation types are specified in Table 104 .

Table 104: Meaning of type options for bool [tab:format.type.bool]

🔗	Type	Meaning
🔗	none, s	Copies textual representation, either true or false, to the output.
🔗	b, B, d, o, x, X	As specified in Table 102 for the value static_cast<unsigned char>(value).

The available floating-point presentation types and their meanings for values other than infinity and NaN are specified in Table 105 .

For lower-case presentation types, infinity and NaN are formatted as inf and nan, respectively.

For upper-case presentation types, infinity and NaN are formatted as INF and NAN, respectively.

[Note 9:

In either case, a sign is included if indicated by the sign option.

— end note]

Table 105: Meaning of type options for floating-point types [tab:format.type.float]

🔗	Type	Meaning
🔗	a	If precision is specified, equivalent to to_chars(first, last, value, chars_format::hex, precision) where precision is the specified formatting precision; equivalent to to_chars(first, last, value, chars_format::hex) otherwise.
🔗	A	The same as a, except that it uses uppercase letters for digits above 9 and P to indicate the exponent.
🔗	e	Equivalent to to_chars(first, last, value, chars_format::scientific, precision) where precision is the specified formatting precision, or 6 if precision is not specified.
🔗	E	The same as e, except that it uses E to indicate exponent.
🔗	f, F	Equivalent to to_chars(first, last, value, chars_format::fixed, precision) where precision is the specified formatting precision, or 6 if precision is not specified.
🔗	g	Equivalent to to_chars(first, last, value, chars_format::general, precision) where precision is the specified formatting precision, or 6 if precision is not specified.
🔗	G	The same as g, except that it uses E to indicate exponent.
🔗	none	If precision is specified, equivalent to to_chars(first, last, value, chars_format::general, precision) where precision is the specified formatting precision; equivalent to to_chars(first, last, value) otherwise.

The available pointer presentation types and their mapping to to_chars are specified in Table 106 .

[Note 10:

Pointer presentation types also apply to nullptr_t.

— end note]

Table 106: Meaning of type options for pointer types [tab:format.type.ptr]

🔗	Type	Meaning
🔗	none, p	If uintptr_t is defined, to_chars(first, last, reinterpret_cast<uintptr_t>(value), 16) with the prefix 0x inserted immediately before the output of to_chars; otherwise, implementation-defined.
🔗	P	The same as p, except that it uses uppercase letters for digits above 9 and the base prefix is 0X.

241)241)

Windows® is a registered trademark of Microsoft Corporation.

This information is given for the convenience of users of this document and does not constitute an endorsement by ISO or IEC of this product.

28.5.3 Error reporting [format.err.report]

Formatting functions throw format_error if an argument fmt is passed that is not a format string for args.

They propagate exceptions thrown by operations of formatter specializations and iterators.

Failure to allocate storage is reported by throwing an exception as described in [res.on.exception.handling].

28.5.4 Class template basic_format_string [format.fmt.string]

namespace std { template<class charT, class... Args> struct basic_format_string { private: basic_string_view<charT> str; // exposition only public: template<class T> consteval basic_format_string(const T& s); basic_format_string(runtime-format-string<charT> s) noexcept : str(s.str) {} constexpr basic_string_view<charT> get() const noexcept { return str; } }; }

🔗

template<class T> consteval basic_format_string(const T& s);

Constraints: const T& models convertible_to<basic_string_view<charT>>.

Effects: Direct-non-list-initializes str with s.

Remarks: A call to this function is not a core constant expression ([expr.const]) unless there exist args of types Args such that str is a format string for args.

28.5.5 Formatting functions [format.functions]

In the description of the functions, operator + is used for some of the iterator categories for which it does not have to be defined.

In these cases the semantics of a + n are the same as in [algorithms.requirements].

🔗

template<class... Args>
  string format(format_string<Args...> fmt, Args&&... args);

Effects: Equivalent to: return vformat(fmt.str, make_format_args(args...));

🔗

template<class... Args>
  wstring format(wformat_string<Args...> fmt, Args&&... args);

Effects: Equivalent to: return vformat(fmt.str, make_wformat_args(args...));

🔗

template<class... Args>
  string format(const locale& loc, format_string<Args...> fmt, Args&&... args);

Effects: Equivalent to: return vformat(loc, fmt.str, make_format_args(args...));

🔗

template<class... Args>
  wstring format(const locale& loc, wformat_string<Args...> fmt, Args&&... args);

Effects: Equivalent to: return vformat(loc, fmt.str, make_wformat_args(args...));

🔗

string vformat(string_view fmt, format_args args);
wstring vformat(wstring_view fmt, wformat_args args);
string vformat(const locale& loc, string_view fmt, format_args args);
wstring vformat(const locale& loc, wstring_view fmt, wformat_args args);

Returns: A string object holding the character representation of formatting arguments provided by args formatted according to specifications given in fmt.

If present, loc is used for locale-specific formatting.

Throws: As specified in [format.err.report].

🔗

template<class Out, class... Args>
  Out format_to(Out out, format_string<Args...> fmt, Args&&... args);

Effects: Equivalent to: return vformat_to(std::move(out), fmt.str, make_format_args(args...));

🔗

template<class Out, class... Args>
  Out format_to(Out out, wformat_string<Args...> fmt, Args&&... args);

Effects: Equivalent to: return vformat_to(std::move(out), fmt.str, make_wformat_args(args...));

🔗

template<class Out, class... Args>
  Out format_to(Out out, const locale& loc, format_string<Args...>  fmt, Args&&... args);

Effects: Equivalent to: return vformat_to(std::move(out), loc, fmt.str, make_format_args(args...));

🔗

template<class Out, class... Args>
  Out format_to(Out out, const locale& loc, wformat_string<Args...> fmt, Args&&... args);

Effects: Equivalent to: return vformat_to(std::move(out), loc, fmt.str, make_wformat_args(args...));

🔗

template<class Out>
  Out vformat_to(Out out, string_view fmt, format_args args);
template<class Out>
  Out vformat_to(Out out, wstring_view fmt, wformat_args args);
template<class Out>
  Out vformat_to(Out out, const locale& loc, string_view fmt, format_args args);
template<class Out>
  Out vformat_to(Out out, const locale& loc, wstring_view fmt, wformat_args args);

Let charT be decltype(fmt)::value_type.

Constraints: Out satisfies output_iterator<const charT&>.

Preconditions: Out models output_iterator<const charT&>.

Effects: Places the character representation of formatting the arguments provided by args, formatted according to the specifications given in fmt, into the range [out, out + N), where N is the number of characters in that character representation.

If present, loc is used for locale-specific formatting.

Returns: out + N.

Throws: As specified in [format.err.report].

🔗

template<class Out, class... Args>
  format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
                                      format_string<Args...> fmt, Args&&... args);
template<class Out, class... Args>
  format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
                                      wformat_string<Args...> fmt, Args&&... args);
template<class Out, class... Args>
  format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
                                      const locale& loc, format_string<Args...> fmt,
                                      Args&&... args);
template<class Out, class... Args>
  format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
                                      const locale& loc, wformat_string<Args...> fmt,
                                      Args&&... args);

Let

(18.1)
charT be decltype(fmt.str)::value_type,
(18.2)
N be formatted_size(fmt, args...) for the functions without a loc parameter and formatted_size(loc, fmt, args...) for the functions with a loc parameter, and
(18.3)
M be clamp(n, 0, N).

Constraints: Out satisfies output_iterator<const charT&>.

Preconditions: Out models output_iterator<const charT&>, and formatter<

{remove_cvref_t<T}_{i}

>, charT> meets the BasicFormatter requirements ([formatter.requirements]) for each

T_{i}

in Args.

Effects: Places the first M characters of the character representation of formatting the arguments provided by args, formatted according to the specifications given in fmt, into the range [out, out + M).

If present, loc is used for locale-specific formatting.

Returns: {out + M, N}.

Throws: As specified in [format.err.report].

🔗

template<class... Args>
  size_t formatted_size(format_string<Args...> fmt, Args&&... args);
template<class... Args>
  size_t formatted_size(wformat_string<Args...> fmt, Args&&... args);
template<class... Args>
  size_t formatted_size(const locale& loc, format_string<Args...> fmt, Args&&... args);
template<class... Args>
  size_t formatted_size(const locale& loc, wformat_string<Args...> fmt, Args&&... args);

Let charT be decltype(fmt.str)::value_type.

Preconditions: formatter<

{remove_cvref_t<T}_{i}

>, charT> meets the BasicFormatter requirements ([formatter.requirements]) for each

T_{i}

in Args.

Returns: The number of characters in the character representation of formatting arguments args formatted according to specifications given in fmt.

If present, loc is used for locale-specific formatting.

Throws: As specified in [format.err.report].

28.5.6 Formatter [format.formatter]

28.5.6.1 Formatter requirements [formatter.requirements]

A type F meets the BasicFormatter requirements if it meets the

(1.1)
Cpp17DefaultConstructible (Table 30),
(1.2)
Cpp17CopyConstructible (Table 32),
(1.3)
Cpp17CopyAssignable (Table 34),
(1.4)
Cpp17Swappable ([swappable.requirements]), and
(1.5)
Cpp17Destructible (Table 35)

requirements, and the expressions shown in Table 107 are valid and have the indicated semantics.

A type F meets the Formatter requirements if it meets the BasicFormatter requirements and the expressions shown in Table 108 are valid and have the indicated semantics.

Given character type charT, output iterator type Out, and formatting argument type T, in Table 107 and Table 108:

(3.1)
f is a value of type (possibly const) F,
(3.2)
g is an lvalue of type F,
(3.3)
u is an lvalue of type T,
(3.4)
t is a value of a type convertible to (possibly const) T,
(3.5)
PC is basic_format_parse_context<charT>,
(3.6)
FC is basic_format_context<Out, charT>,
(3.7)
pc is an lvalue of type PC, and
(3.8)
fc is an lvalue of type FC.

pc.begin() points to the beginning of the format-spec ([format.string]) of the replacement field being formatted in the format string.

If format-spec is not present or empty then either pc.begin() == pc.end() or *pc.begin() == '}'.

Table 107: BasicFormatter requirements [tab:formatter.basic]

🔗	Expression	Return type	Requirement
🔗	g.parse(pc)	PC::iterator	Parses format-spec ([format.string]) for type T in the range [pc.begin(), pc.end()) until the first unmatched character. Throws format_error unless the whole range is parsed or the unmatched character is }. [Note 1: This allows formatters to emit meaningful error messages. — end note] Stores the parsed format specifiers in *this and returns an iterator past the end of the parsed range.
🔗	f.format(u, fc)	FC::iterator	Formats u according to the specifiers stored in *this, writes the output to fc.out(), and returns an iterator past the end of the output range. The output shall only depend on u, fc.locale(), fc.arg(n) for any value n of type size_t, and the range [pc.begin(), pc.end()) from the last call to f.parse(pc).

Table 108: Formatter requirements [tab:formatter]

🔗	Expression	Return type	Requirement
🔗	f.format(t, fc)	FC::iterator	Formats t according to the specifiers stored in *this, writes the output to fc.out(), and returns an iterator past the end of the output range. The output shall only depend on t, fc.locale(), fc.arg(n) for any value n of type size_t, and the range [pc.begin(), pc.end()) from the last call to f.parse(pc).
🔗	f.format(u, fc)	FC::iterator	As above, but does not modify u.

28.5.6.2 Formatter locking [format.formatter.locking]

🔗

template<class T>
  constexpr bool enable_nonlocking_formatter_optimization = false;

Remarks: Pursuant to [namespace.std], users may specialize enable_nonlocking_formatter_optimization for cv-unqualified program-defined types.

Such specializations shall be usable in constant expressions ([expr.const]) and have type const bool.

28.5.6.3 Concept formattable [format.formattable]

Let fmt-iter-for<charT> be an unspecified type that models output_iterator<const charT&> ([iterator.concept.output]).

template<class T, class Context, class Formatter = typename Context::template formatter_type<remove_const_t<T>>> concept formattable-with = // exposition only semiregular<Formatter> && requires(Formatter& f, const Formatter& cf, T&& t, Context fc, basic_format_parse_context<typename Context::char_type> pc) { { f.parse(pc) } -> same_as<typename decltype(pc)::iterator>; { cf.format(t, fc) } -> same_as<typename Context::iterator>; }; template<class T, class charT> concept formattable = formattable-with<remove_reference_t<T>, basic_format_context<fmt-iter-for<charT>, charT>>;

A type T and a character type charT model formattable if formatter<remove_cvref_t<T>, charT> meets the BasicFormatter requirements ([formatter.requirements]) and, if remove_reference_t<T> is const-qualified, the Formatter requirements.

28.5.6.4 Formatter specializations [format.formatter.spec]

The functions defined in [format.functions] use specializations of the class template formatter to format individual arguments.

Let charT be either char or wchar_t.

Each specialization of formatter is either enabled or disabled, as described below.

A debug-enabled specialization of formatter additionally provides a public, constexpr, non-static member function set_debug_format() which modifies the state of the formatter to be as if the type of the std-format-spec parsed by the last call to parse were ?.

Each header that declares the template formatter provides the following enabled specializations:

(2.1)
The debug-enabled specializations template<> struct formatter<char, char>; template<> struct formatter<char, wchar_t>; template<> struct formatter<wchar_t, wchar_t>;
(2.2)
For each charT, the debug-enabled string type specializations template<> struct formatter<charT*, charT>; template<> struct formatter<const charT*, charT>; template<size_t N> struct formatter<charT[N], charT>; template<class traits, class Allocator> struct formatter<basic_string<charT, traits, Allocator>, charT>; template<class traits> struct formatter<basic_string_view<charT, traits>, charT>;
(2.3)
For each charT, for each cv-unqualified arithmetic type ArithmeticT other than char, wchar_t, char8_t, char16_t, or char32_t, a specialization template<> struct formatter<ArithmeticT, charT>;
(2.4)
For each charT, the pointer type specializations template<> struct formatter<nullptr_t, charT>; template<> struct formatter<void*, charT>; template<> struct formatter<const void*, charT>;

The parse member functions of these formatters interpret the format specification as a std-format-spec as described in [format.string.std].

Unless specified otherwise, for each type T for which a formatter specialization is provided by the library, each of the headers provides the following specialization: template<> inline constexpr bool enable_nonlocking_formatter_optimization<T> = true;

[Note 1:

Specializations such as formatter<wchar_t, char> that would require implicit multibyte / wide string or character conversion are disabled.

— end note]

The header provides the following disabled specializations:

(4.1)
The string type specializations template<> struct formatter<char*, wchar_t>; template<> struct formatter<const char*, wchar_t>; template<size_t N> struct formatter<char[N], wchar_t>; template<class traits, class Allocator> struct formatter<basic_string<char, traits, Allocator>, wchar_t>; template<class traits> struct formatter<basic_string_view<char, traits>, wchar_t>;

For any types T and charT for which neither the library nor the user provides an explicit or partial specialization of the class template formatter, formatter<T, charT> is disabled.

If the library provides an explicit or partial specialization of formatter<T, charT>, that specialization is enabled and meets the Formatter requirements except as noted otherwise.

If F is a disabled specialization of formatter, these values are false:

(7.1)
is_default_constructible_v<F>,
(7.2)
is_copy_constructible_v<F>,
(7.3)
is_move_constructible_v<F>,
(7.4)
is_copy_assignable_v<F>, and
(7.5)
is_move_assignable_v<F>.

An enabled specialization formatter<T, charT> meets the BasicFormatter requirements ([formatter.requirements]).

[Example 1: #include <format> #include <string> enum color { red, green, blue }; const char* color_names[] = { "red", "green", "blue" }; template<> struct std::formatter<color> : std::formatter<const char*> { auto format(color c, format_context& ctx) const { return formatter<const char*>::format(color_names[c], ctx); } }; struct err {}; std::string s0 = std::format("{}", 42); // OK, library-provided formatter std::string s1 = std::format("{}", L"foo"); // error: disabled formatter std::string s2 = std::format("{}", red); // OK, user-provided formatter std::string s3 = std::format("{}", err{}); // error: disabled formatter — end example]

28.5.6.5 Formatting escaped characters and strings [format.string.escaped]

A character or string can be formatted as escaped to make it more suitable for debugging or for logging.

The escaped string E representation of a string S is constructed by encoding a sequence of characters as follows.

The associated character encoding CE for charT (Table 12) is used to both interpret S and construct E.

(2.1)
U+0022 quotation mark (") is appended to E.
(2.2)
For each code unit sequence X in S that either encodes a single character, is a shift sequence, or is a sequence of ill-formed code units, processing is in order as follows:
- (2.2.1)
  If X encodes a single character C, then:
  - (2.2.1.1)
    If C is one of the characters in Table 109, then the two characters shown as the corresponding escape sequence are appended to E.
  - (2.2.1.2)
    Otherwise, if C is not U+0020 space and
    (2.2.1.2.1)
    CE is UTF-8, UTF-16, or UTF-32 and C corresponds to a Unicode scalar value whose Unicode property General_Category has a value in the groups Separator (Z) or Other (C), as described by UAX #44 of the Unicode Standard, or
    (2.2.1.2.2)
    CE is UTF-8, UTF-16, or UTF-32 and C corresponds to a Unicode scalar value with the Unicode property Grapheme_Extend=Yes as described by UAX #44 of the Unicode Standard and C is not immediately preceded in S by a character P appended to E without translation to an escape sequence, or
    (2.2.1.2.3)
    CE is neither UTF-8, UTF-16, nor UTF-32 and C is one of an implementation-defined set of separator or non-printable characters
    then the sequence \u{hex-digit-sequence} is appended to E, where hex-digit-sequence is the shortest hexadecimal representation of C using lower-case hexadecimal digits.
  - (2.2.1.3)
    Otherwise, C is appended to E.
- (2.2.2)
  Otherwise, if X is a shift sequence, the effect on E and further decoding of S is unspecified.
  Recommended practice: A shift sequence should be represented in E such that the original code unit sequence of S can be reconstructed.
- (2.2.3)
  Otherwise (X is a sequence of ill-formed code units), each code unit U is appended to E in order as the sequence \x{hex-digit-sequence}, where hex-digit-sequence is the shortest hexadecimal representation of U using lower-case hexadecimal digits.
(2.3)
Finally, U+0022 quotation mark (") is appended to E.

Table 109: Mapping of characters to escape sequences [tab:format.escape.sequences]

🔗	Character	Escape sequence
🔗	U+0009 character tabulation	\t
🔗	U+000a line feed	\n
🔗	U+000d carriage return	\r
🔗	U+0022 quotation mark	\"
🔗	U+005c reverse solidus	\\

The escaped string representation of a character C is equivalent to the escaped string representation of a string of C, except that:

(3.1)
the result starts and ends with U+0027 apostrophe (') instead of U+0022 quotation mark ("), and
(3.2)
if C is U+0027 apostrophe, the two characters \' are appended to E, and
(3.3)
if C is U+0022 quotation mark, then C is appended unchanged.

[Example 1: string s0 = format("[{}]", "h\tllo"); // s0 has value: [h llo] string s1 = format("[{:?}]", "h\tllo"); // s1 has value: ["h\tllo"] string s2 = format("[{:?}]", "Спасибо, Виктор ♥!"); // s2 has value: ["Спасибо, Виктор ♥!"] string s3 = format("[{:?}, {:?}]", '\'', '"'); // s3 has value: ['\'', '"'] // The following examples assume use of the UTF-8 encoding string s4 = format("[{:?}]", string("\0 \n \t \x02 \x1b", 9)); // s4 has value: ["\u{0} \n \t \u{2} \u{1b}"] string s5 = format("[{:?}]", "\xc3\x28"); // invalid UTF-8, s5 has value: ["\x{c3}("] string s6 = format("[{:?}]", "🤷🏻‍♂️"); // s6 has value: ["🤷\u{200d}♂"] string s7 = format("[{:?}]", "\u0301"); // s7 has value: ["\u{301}"] string s8 = format("[{:?}]", "\\\u0301"); // s8 has value: ["\\\u{301}"] string s9 = format("[{:?}]", "e\u0301\u0323"); // s9 has value: ["ẹ́"] — end example]

28.5.6.6 Class template basic_format_parse_context [format.parse.ctx]

🔗

namespace std { template<class charT> class basic_format_parse_context { public: using char_type = charT; using const_iterator = typename basic_string_view<charT>::const_iterator; using iterator = const_iterator; private: iterator begin_; // exposition only iterator end_; // exposition only enum indexing { unknown, manual, automatic }; // exposition only indexing indexing_; // exposition only size_t next_arg_id_; // exposition only size_t num_args_; // exposition only public: constexpr explicit basic_format_parse_context(basic_string_view<charT> fmt) noexcept; basic_format_parse_context(const basic_format_parse_context&) = delete; basic_format_parse_context& operator=(const basic_format_parse_context&) = delete; constexpr const_iterator begin() const noexcept; constexpr const_iterator end() const noexcept; constexpr void advance_to(const_iterator it); constexpr size_t next_arg_id(); constexpr void check_arg_id(size_t id); template<class... Ts> constexpr void check_dynamic_spec(size_t id) noexcept; constexpr void check_dynamic_spec_integral(size_t id) noexcept; constexpr void check_dynamic_spec_string(size_t id) noexcept; }; }

An instance of basic_format_parse_context holds the format string parsing state, consisting of the format string range being parsed and the argument counter for automatic indexing.

If a program declares an explicit or partial specialization of basic_format_parse_context, the program is ill-formed, no diagnostic required.

🔗

constexpr explicit basic_format_parse_context(basic_string_view<charT> fmt) noexcept;

Effects: Initializes begin_ with fmt.begin(), end_ with fmt.end(), indexing_ with unknown, next_arg_id_ with 0, and num_args_ with 0.

[Note 1:

Any call to next_arg_id, check_arg_id, or check_dynamic_spec on an instance of basic_format_parse_context initialized using this constructor is not a core constant expression.

— end note]

🔗

constexpr const_iterator begin() const noexcept;

Returns: begin_.

🔗

constexpr const_iterator end() const noexcept;

Returns: end_.

🔗

constexpr void advance_to(const_iterator it);

Preconditions: end() is reachable from it.

Effects: Equivalent to: begin_ = it;

🔗

constexpr size_t next_arg_id();

Effects: If indexing_ != manual is true, equivalent to: if (indexing_ == unknown) indexing_ = automatic; return next_arg_id_++;

Throws: format_error if indexing_ == manual is true.

[Note 2:

This indicates mixing of automatic and manual argument indexing.

— end note]

Remarks: Let cur-arg-id be the value of next_arg_id_ prior to this call.

Call expressions where cur-arg-id >= num_args_ is true are not core constant expressions ([expr.const]).

🔗

constexpr void check_arg_id(size_t id);

Effects: If indexing_ != automatic is true, equivalent to: if (indexing_ == unknown) indexing_ = manual;

Throws: format_error if indexing_ == automatic is true.

[Note 3:

This indicates mixing of automatic and manual argument indexing.

— end note]

Remarks: A call to this function is a core constant expression ([expr.const]) only if id < num_args_ is true.

🔗

template<class... Ts>
  constexpr void check_dynamic_spec(size_t id) noexcept;

Mandates: sizeof...(Ts) ≥ 1.

The types in Ts... are unique.

Each type in Ts... is one of bool, char_type, int, unsigned int, long long int, unsigned long long int, float, double, long double, const char_type*, basic_string_view<char_type>, or const void*.

Remarks: A call to this function is a core constant expression only if

(15.1)
id < num_args_ is true and
(15.2)
the type of the corresponding format argument (after conversion to basic_format_arg<Context>) is one of the types in Ts....

🔗

constexpr void check_dynamic_spec_integral(size_t id) noexcept;

Effects: Equivalent to: check_dynamic_spec<int, unsigned int, long long int, unsigned long long int>(id);

🔗

constexpr void check_dynamic_spec_string(size_t id) noexcept;

Effects: Equivalent to: check_dynamic_spec<const char_type*, basic_string_view<char_type>>(id);

28.5.6.7 Class template basic_format_context [format.context]

🔗

namespace std { template<class Out, class charT> class basic_format_context { basic_format_args<basic_format_context> args_; // exposition only Out out_; // exposition only basic_format_context(const basic_format_context&) = delete; basic_format_context& operator=(const basic_format_context&) = delete; public: using iterator = Out; using char_type = charT; template<class T> using formatter_type = formatter<T, charT>; basic_format_arg<basic_format_context> arg(size_t id) const noexcept; std::locale locale(); iterator out(); void advance_to(iterator it); }; }

An instance of basic_format_context holds formatting state consisting of the formatting arguments and the output iterator.

If a program declares an explicit or partial specialization of basic_format_context, the program is ill-formed, no diagnostic required.

Out shall model output_iterator<const charT&>.

format_context is an alias for a specialization of basic_format_context with an output iterator that appends to string, such as back_insert_iterator<string>.

Similarly, wformat_context is an alias for a specialization of basic_format_context with an output iterator that appends to wstring.

Recommended practice: For a given type charT, implementations should provide a single instantiation of basic_format_context for appending to basic_string<charT>, vector<charT>, or any other container with contiguous storage by wrapping those in temporary objects with a uniform interface (such as a span<charT>) and polymorphic reallocation.

🔗

basic_format_arg<basic_format_context> arg(size_t id) const noexcept;

Returns: args_.get(id).

🔗

std::locale locale();

Returns: The locale passed to the formatting function if the latter takes one, and std::locale() otherwise.

🔗

iterator out();

Effects: Equivalent to: return std::move(out_);

🔗

void advance_to(iterator it);

Effects: Equivalent to: out_ = std::move(it);

[Example 1: struct S { int value; }; template<> struct std::formatter<S> { size_t width_arg_id = 0; // Parses a width argument id in the format { digit }. constexpr auto parse(format_parse_context& ctx) { auto iter = ctx.begin(); auto is_digit = [](auto c) { return c >= '0' && c <= '9'; }; auto get_char = [&]() { return iter != ctx.end() ? *iter : 0; }; if (get_char() != '{') return iter; ++iter; char c = get_char(); if (!is_digit(c) || (++iter, get_char()) != '}') throw format_error("invalid format"); width_arg_id = c - '0'; ctx.check_arg_id(width_arg_id); return ++iter; } // Formats an S with width given by the argument width_arg_id. auto format(S s, format_context& ctx) const { int width = ctx.arg(width_arg_id).visit([](auto value) -> int { if constexpr (!is_integral_v<decltype(value)>) throw format_error("width is not integral"); else if (value < 0 || value > numeric_limits<int>::max()) throw format_error("invalid width"); else return value; }); return format_to(ctx.out(), "{0:x>{1}}", s.value, width); } }; std::string s = std::format("{0:{1}}", S{42}, 10); // value of s is "xxxxxxxx42" — end example]

28.5.7 Formatting of ranges [format.range]

28.5.7.1 Variable template format_kind [format.range.fmtkind]

🔗

template<ranges::input_range R>
    requires same_as<R, remove_cvref_t<R>>
  constexpr range_format format_kind<R> = see below;

A program that instantiates the primary template of format_kind is ill-formed.

For a type R, format_kind<R> is defined as follows:

(2.1)
If same_as<remove_cvref_t<ranges::range_reference_t<R>>, R> is true, format_kind<R> is range_format::disabled.

[Note 1:
This prevents constraint recursion for ranges whose reference type is the same range type.

For example, std::filesystem::path is a range of std::filesystem::path.
— end note]
(2.2)
Otherwise, if the qualified-id R::key_type is valid and denotes a type:
- (2.2.1)
  If the qualified-id R::mapped_type is valid and denotes a type, let U be remove_cvref_t<ranges::range_reference_t<R>>.
  
  If either U is a specialization of pair or U is a specialization of tuple and tuple_size_v<U> == 2, format_kind<R> is range_format::map.
- (2.2.2)
  Otherwise, format_kind<R> is range_format::set.
(2.3)
Otherwise, format_kind<R> is range_format::sequence.

Remarks: Pursuant to [namespace.std], users may specialize format_kind for cv-unqualified program-defined types that model ranges::input_range.

Such specializations shall be usable in constant expressions ([expr.const]) and have type const range_format.

28.5.7.2 Class template range_formatter [format.range.formatter]

🔗

namespace std { template<class T, class charT = char> requires same_as<remove_cvref_t<T>, T> && formattable<T, charT> class range_formatter { formatter<T, charT> underlying_; // exposition only basic_string_view<charT> separator_ = STATICALLY-WIDEN<charT>(", "); // exposition only basic_string_view<charT> opening-bracket_ = STATICALLY-WIDEN<charT>("["); // exposition only basic_string_view<charT> closing-bracket_ = STATICALLY-WIDEN<charT>("]"); // exposition only public: constexpr void set_separator(basic_string_view<charT> sep) noexcept; constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept; constexpr formatter<T, charT>& underlying() noexcept { return underlying_; } constexpr const formatter<T, charT>& underlying() const noexcept { return underlying_; } template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx); template<ranges::input_range R, class FormatContext> requires formattable<ranges::range_reference_t<R>, charT> && same_as<remove_cvref_t<ranges::range_reference_t<R>>, T> typename FormatContext::iterator format(R&& r, FormatContext& ctx) const; }; }

The class template range_formatter is a utility for implementing formatter specializations for range types.

range_formatter interprets format-spec as a range-format-spec.

The syntax of format specifications is as follows:

range-format-spec :
range-fill-and-align

_{o p t}

width

_{o p t}

_{o p t}

range-type

_{o p t}

range-underlying-spec

_{o p t}

range-fill-and-align :
range-fill

_{o p t}

align

range-fill :
any character other than { or } or :

range-type :
m
s
?s

range-underlying-spec :
: format-spec

For range_formatter<T, charT>, the format-spec in a range-underlying-spec, if any, is interpreted by formatter<T, charT>.

The range-fill-and-align is interpreted the same way as a fill-and-align ([format.string.std]).

The productions align and width are described in [format.string].

The n option causes the range to be formatted without the opening and closing brackets.

[Note 1:

This is equivalent to invoking set_brackets({}, {}).

— end note]

The range-type specifier changes the way a range is formatted, with certain options only valid with certain argument types.

The meaning of the various type options is as specified in Table 110 .

Table 110: Meaning of range-type options [tab:formatter.range.type]

🔗	Option	Requirements	Meaning
🔗	m	T shall be either a specialization of pair or a specialization of tuple such that tuple_size_v<T> is 2.	Indicates that the opening bracket should be "{", the closing bracket should be "}", the separator should be ", ", and each range element should be formatted as if m were specified for its tuple-type. [Note 2: If the n option is provided in addition to the m option, both the opening and closing brackets are still empty. — end note]
🔗	s	T shall be charT.	Indicates that the range should be formatted as a string.
🔗	?s	T shall be charT.	Indicates that the range should be formatted as an escaped string ([format.string.escaped]).

If the range-type is s or ?s, then there shall be no n option and no range-underlying-spec.

🔗

constexpr void set_separator(basic_string_view<charT> sep) noexcept;

Effects: Equivalent to: separator_ = sep;

🔗

constexpr void set_brackets(basic_string_view<charT> opening,
                            basic_string_view<charT> closing) noexcept;

Effects: Equivalent to: opening-bracket_ = opening; closing-bracket_ = closing;

🔗

template<class ParseContext>
  constexpr typename ParseContext::iterator
    parse(ParseContext& ctx);

Effects: Parses the format specifiers as a range-format-spec and stores the parsed specifiers in *this.

Calls underlying_.parse(ctx) to parse format-spec in range-format-spec or, if the latter is not present, an empty format-spec.

The values of opening-bracket_, closing-bracket_, and separator_ are modified if and only if required by the range-type or the n option, if present.

If:

(9.1)
the range-type is neither s nor ?s,
(9.2)
underlying_.set_debug_format() is a valid expression, and
(9.3)
there is no range-underlying-spec,

then calls underlying_.set_debug_format().

Returns: An iterator past the end of the range-format-spec.

🔗

template<ranges::input_range R, class FormatContext>
    requires formattable<ranges::range_reference_t<R>, charT> &&
             same_as<remove_cvref_t<ranges::range_reference_t<R>>, T>
  typename FormatContext::iterator
    format(R&& r, FormatContext& ctx) const;

Effects: Writes the following into ctx.out(), adjusted according to the range-format-spec:

(11.1)
If the range-type was s, then as if by formatting basic_string<charT>(from_range, r).
(11.2)
Otherwise, if the range-type was ?s, then as if by formatting basic_string<charT>(from_range, r) as an escaped string ([format.string.escaped]).
(11.3)
Otherwise,
- (11.3.1)
  opening-bracket_,
- (11.3.2)
  for each element e of the range r:
  - (11.3.2.1)
    the result of writing e via underlying_ and
  - (11.3.2.2)
    separator_, unless e is the last element of r, and
- (11.3.3)
  closing-bracket_.

Returns: An iterator past the end of the output range.

28.5.7.3 Class template range-default-formatter [format.range.fmtdef]

🔗

namespace std { template<ranges::input_range R, class charT> struct range-default-formatter<range_format::sequence, R, charT> { // exposition only private: using maybe-const-r = fmt-maybe-const<R, charT>; // exposition only range_formatter<remove_cvref_t<ranges::range_reference_t<maybe-const-r>>, charT> underlying_; // exposition only public: constexpr void set_separator(basic_string_view<charT> sep) noexcept; constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept; template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx); template<class FormatContext> typename FormatContext::iterator format(maybe-const-r& elems, FormatContext& ctx) const; }; }

🔗

constexpr void set_separator(basic_string_view<charT> sep) noexcept;

Effects: Equivalent to: underlying_.set_separator(sep);

🔗

constexpr void set_brackets(basic_string_view<charT> opening,
                            basic_string_view<charT> closing) noexcept;

Effects: Equivalent to: underlying_.set_brackets(opening, closing);

🔗

template<class ParseContext>
  constexpr typename ParseContext::iterator
    parse(ParseContext& ctx);

Effects: Equivalent to: return underlying_.parse(ctx);

🔗

template<class FormatContext>
  typename FormatContext::iterator
    format(maybe-const-r& elems, FormatContext& ctx) const;

Effects: Equivalent to: return underlying_.format(elems, ctx);

28.5.7.4 Specialization of range-default-formatter for maps [format.range.fmtmap]

🔗

namespace std { template<ranges::input_range R, class charT> struct range-default-formatter<range_format::map, R, charT> { private: using maybe-const-map = fmt-maybe-const<R, charT>; // exposition only using element-type = // exposition only remove_cvref_t<ranges::range_reference_t<maybe-const-map>>; range_formatter<element-type, charT> underlying_; // exposition only public: constexpr range-default-formatter(); template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx); template<class FormatContext> typename FormatContext::iterator format(maybe-const-map& r, FormatContext& ctx) const; }; }

🔗

constexpr range-default-formatter();

Mandates: Either:

(1.1)
element-type is a specialization of pair, or
(1.2)
element-type is a specialization of tuple and tuple_size_v<element-type> == 2.

Effects: Equivalent to: underlying_.set_brackets(STATICALLY-WIDEN<charT>("{"), STATICALLY-WIDEN<charT>("}")); underlying_.underlying().set_brackets({}, {}); underlying_.underlying().set_separator(STATICALLY-WIDEN<charT>(": "));

🔗

template<class ParseContext>
  constexpr typename ParseContext::iterator
    parse(ParseContext& ctx);

Effects: Equivalent to: return underlying_.parse(ctx);

🔗

template<class FormatContext>
  typename FormatContext::iterator
    format(maybe-const-map& r, FormatContext& ctx) const;

Effects: Equivalent to: return underlying_.format(r, ctx);

28.5.7.5 Specialization of range-default-formatter for sets [format.range.fmtset]

🔗

namespace std { template<ranges::input_range R, class charT> struct range-default-formatter<range_format::set, R, charT> { private: using maybe-const-set = fmt-maybe-const<R, charT>; // exposition only range_formatter<remove_cvref_t<ranges::range_reference_t<maybe-const-set>>, charT> underlying_; // exposition only public: constexpr range-default-formatter(); template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx); template<class FormatContext> typename FormatContext::iterator format(maybe-const-set& r, FormatContext& ctx) const; }; }

🔗

constexpr range-default-formatter();

Effects: Equivalent to: underlying_.set_brackets(STATICALLY-WIDEN<charT>("{"), STATICALLY-WIDEN<charT>("}"));

🔗

template<class ParseContext>
  constexpr typename ParseContext::iterator
    parse(ParseContext& ctx);

Effects: Equivalent to: return underlying_.parse(ctx);

🔗

template<class FormatContext>
  typename FormatContext::iterator
    format(maybe-const-set& r, FormatContext& ctx) const;

Effects: Equivalent to: return underlying_.format(r, ctx);

28.5.7.6 Specialization of range-default-formatter for strings [format.range.fmtstr]

🔗

namespace std { template<range_format K, ranges::input_range R, class charT> requires (K == range_format::string || K == range_format::debug_string) struct range-default-formatter<K, R, charT> { private: formatter<basic_string<charT>, charT> underlying_; // exposition only public: template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx); template<class FormatContext> typename FormatContext::iterator format(see below& str, FormatContext& ctx) const; }; }

Mandates: same_as<remove_cvref_t<range_reference_t<R>>, charT> is true.

🔗

template<class ParseContext>
  constexpr typename ParseContext::iterator
    parse(ParseContext& ctx);

Effects: Equivalent to: auto i = underlying_.parse(ctx); if constexpr (K == range_format::debug_string) { underlying_.set_debug_format(); } return i;

🔗

template<class FormatContext>
  typename FormatContext::iterator
    format(see below& r, FormatContext& ctx) const;

The type of r is const R& if ranges::input_range<const R> is true and R& otherwise.

Effects: Let s be a basic_string<charT> such that ranges::equal(s, r) is true.

Equivalent to: return underlying_.format(s, ctx);

28.5.8 Arguments [format.arguments]

28.5.8.1 Class template basic_format_arg [format.arg]

🔗

namespace std { template<class Context> class basic_format_arg { public: class handle; private: using char_type = typename Context::char_type; // exposition only variant<monostate, bool, char_type, int, unsigned int, long long int, unsigned long long int, float, double, long double, const char_type*, basic_string_view<char_type>, const void*, handle> value; // exposition only template<class T> explicit basic_format_arg(T& v) noexcept; // exposition only public: basic_format_arg() noexcept; explicit operator bool() const noexcept; template<class Visitor> decltype(auto) visit(this basic_format_arg arg, Visitor&& vis); template<class R, class Visitor> R visit(this basic_format_arg arg, Visitor&& vis); }; }

An instance of basic_format_arg provides access to a formatting argument for user-defined formatters.

The behavior of a program that adds specializations of basic_format_arg is undefined.

🔗

basic_format_arg() noexcept;

Postconditions: !(*this).

🔗

template<class T> explicit basic_format_arg(T& v) noexcept;

Constraints: T satisfies formattable-with<Context>.

Preconditions: If decay_t<T> is char_type* or const char_type*, static_cast<const char_
type*>(v) points to a NTCTS ([defns.ntcts]).

Effects: Let TD be remove_const_t<T>.

(6.1)
If TD is bool or char_type, initializes value with v;
(6.2)
otherwise, if TD is char and char_type is wchar_t, initializes value with static_cast<wchar_t>(static_cast<unsigned char>(v));
(6.3)
otherwise, if TD is a signed integer type ([basic.fundamental]) and sizeof(TD) <= sizeof(int), initializes value with static_cast<int>(v);
(6.4)
otherwise, if TD is an unsigned integer type and sizeof(TD) <= sizeof(unsigned int), initializes value with static_cast<unsigned int>(v);
(6.5)
otherwise, if TD is a signed integer type and sizeof(TD) <= sizeof(long long int), initializes value with static_cast<long long int>(v);
(6.6)
otherwise, if TD is an unsigned integer type and sizeof(TD) <= sizeof(unsigned long long int), initializes value with static_cast<unsigned long long int>(v);
(6.7)
otherwise, if TD is a standard floating-point type, initializes value with v;
(6.8)
otherwise, if TD is a specialization of basic_string_view or basic_string and TD::value_type is char_type, initializes value with basic_string_view<char_type>(v.data(), v.size());
(6.9)
otherwise, if decay_t<TD> is char_type* or const char_type*, initializes value with static_cast<const char_type*>(v);
(6.10)
otherwise, if is_void_v<remove_pointer_t<TD>> is true or is_null_pointer_v<TD> is true, initializes value with static_cast<const void*>(v);
(6.11)
otherwise, initializes value with handle(v).

[Note 1:

Constructing basic_format_arg from a pointer to a member is ill-formed unless the user provides an enabled specialization of formatter for that pointer to member type.

— end note]

🔗

explicit operator bool() const noexcept;

Returns: !holds_alternative<monostate>(value).

🔗

template<class Visitor>
  decltype(auto) visit(this basic_format_arg arg, Visitor&& vis);

Effects: Equivalent to: return arg.value.visit(std::forward<Visitor>(vis));

🔗

template<class R, class Visitor>
  R visit(this basic_format_arg arg, Visitor&& vis);

Effects: Equivalent to: return arg.value.visit<R>(std::forward<Visitor>(vis));

The class handle allows formatting an object of a user-defined type.

🔗

namespace std { template<class Context> class basic_format_arg<Context>::handle { const void* ptr_; // exposition only void (*format_)(basic_format_parse_context<char_type>&, Context&, const void*); // exposition only template<class T> explicit handle(T& val) noexcept; // exposition only public: void format(basic_format_parse_context<char_type>&, Context& ctx) const; }; }

🔗

template<class T> explicit handle(T& val) noexcept;

Let

(11.1)
TD be remove_const_t<T>,
(11.2)
TQ be const TD if const TD satisfies formattable-with<Context> and TD otherwise.

Mandates: TQ satisfies formattable-with<Context>.

Effects: Initializes ptr_ with addressof(val) and format_ with [](basic_format_parse_context<char_type>& parse_ctx, Context& format_ctx, const void* ptr) { typename Context::template formatter_type<TD> f; parse_ctx.advance_to(f.parse(parse_ctx)); format_ctx.advance_to(f.format(*const_cast<TQ*>(static_cast<const TD*>(ptr)), format_ctx)); }

🔗

void format(basic_format_parse_context<char_type>& parse_ctx, Context& format_ctx) const;

Effects: Equivalent to: format_(parse_ctx, format_ctx, ptr_);

28.5.8.2 Class template format-arg-store [format.arg.store]

namespace std { template<class Context, class... Args> class format-arg-store { // exposition only array<basic_format_arg<Context>, sizeof...(Args)> args; // exposition only }; }

An instance of format-arg-store stores formatting arguments.

🔗

template<class Context = format_context, class... Args>
  format-arg-store<Context, Args...> make_format_args(Args&... fmt_args);

Preconditions: The type typename Context::template formatter_type<remove_const_t<

T_{i}

>>
meets the BasicFormatter requirements ([formatter.requirements]) for each

T_{i}

in Args.

Returns: An object of type format-arg-store<Context, Args...> whose args data member is initialized with {basic_format_arg<Context>(fmt_args)...}.

🔗

template<class... Args>
  format-arg-store<wformat_context, Args...> make_wformat_args(Args&... args);

Effects: Equivalent to: return make_format_args<wformat_context>(args...);

28.5.8.3 Class template basic_format_args [format.args]

namespace std { template<class Context> class basic_format_args { size_t size_; // exposition only const basic_format_arg<Context>* data_; // exposition only public: template<class... Args> basic_format_args(const format-arg-store<Context, Args...>& store) noexcept; basic_format_arg<Context> get(size_t i) const noexcept; }; template<class Context, class... Args> basic_format_args(format-arg-store<Context, Args...>) -> basic_format_args<Context>; }

An instance of basic_format_args provides access to formatting arguments.

Implementations should optimize the representation of basic_format_args for a small number of formatting arguments.

[Note 1:

For example, by storing indices of type alternatives separately from values and packing the former.

— end note]

🔗

template<class... Args>
  basic_format_args(const format-arg-store<Context, Args...>& store) noexcept;

Effects: Initializes size_ with sizeof...(Args) and data_ with store.args.data().

🔗

basic_format_arg<Context> get(size_t i) const noexcept;

Returns: i < size_ ? data_[i] : basic_format_arg<Context>().

28.5.9 Tuple formatter [format.tuple]

For each of pair and tuple, the library provides the following formatter specialization where pair-or-tuple is the name of the template:

🔗

namespace std { template<class charT, formattable<charT>... Ts> struct formatter<pair-or-tuple<Ts...>, charT> { private: tuple<formatter<remove_cvref_t<Ts>, charT>...> underlying_; // exposition only basic_string_view<charT> separator_ = STATICALLY-WIDEN<charT>(", "); // exposition only basic_string_view<charT> opening-bracket_ = STATICALLY-WIDEN<charT>("("); // exposition only basic_string_view<charT> closing-bracket_ = STATICALLY-WIDEN<charT>(")"); // exposition only public: constexpr void set_separator(basic_string_view<charT> sep) noexcept; constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept; template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx); template<class FormatContext> typename FormatContext::iterator format(see below& elems, FormatContext& ctx) const; }; template<class... Ts> constexpr bool enable_nonlocking_formatter_optimization<pair-or-tuple<Ts...>> = (enable_nonlocking_formatter_optimization<Ts> && ...); }

The parse member functions of these formatters interpret the format specification as a tuple-format-spec according to the following syntax:

tuple-format-spec :
tuple-fill-and-align

_{o p t}

width

_{o p t}

tuple-type

_{o p t}

tuple-fill-and-align :
tuple-fill

_{o p t}

align

tuple-fill :
any character other than { or } or :

tuple-type :
m
n

The tuple-fill-and-align is interpreted the same way as a fill-and-align ([format.string.std]).

The productions align and width are described in [format.string].

The tuple-type specifier changes the way a pair or tuple is formatted, with certain options only valid with certain argument types.

The meaning of the various type options is as specified in Table 111 .

Table 111: Meaning of tuple-type options [tab:formatter.tuple.type]

🔗	Option	Requirements	Meaning
🔗	m	sizeof...(Ts) == 2	Equivalent to: set_separator(STATICALLY-WIDEN<charT>(": ")); set_brackets({}, {});
🔗	n	none	Equivalent to: set_brackets({}, {});
🔗	none	none	No effects

🔗

constexpr void set_separator(basic_string_view<charT> sep) noexcept;

Effects: Equivalent to: separator_ = sep;

🔗

constexpr void set_brackets(basic_string_view<charT> opening,
                            basic_string_view<charT> closing) noexcept;

Effects: Equivalent to: opening-bracket_ = opening; closing-bracket_ = closing;

🔗

template<class ParseContext>
  constexpr typename ParseContext::iterator
    parse(ParseContext& ctx);

Effects: Parses the format specifiers as a tuple-format-spec and stores the parsed specifiers in *this.

The values of opening-bracket_, closing-bracket_, and separator_ are modified if and only if required by the tuple-type, if present.

For each element e in underlying_, calls e.parse(ctx) to parse an empty format-spec and, if e.set_debug_format() is a valid expression, calls e.set_debug_format().

Returns: An iterator past the end of the tuple-format-spec.

🔗

template<class FormatContext>
  typename FormatContext::iterator
    format(see below& elems, FormatContext& ctx) const;

The type of elems is:

(9.1)
If (formattable<const Ts, charT> && ...) is true, const pair-or-tuple<Ts...>&.
(9.2)
Otherwise pair-or-tuple<Ts...>&.

Effects: Writes the following into ctx.out(), adjusted according to the tuple-format-spec:

(10.1)
opening-bracket_,
(10.2)
for each index I in the [0, sizeof...(Ts)):
- (10.2.1)
  if I != 0, separator_,
- (10.2.2)
  the result of writing get<I>(elems) via get<I>(underlying_), and
(10.3)
closing-bracket_.

Returns: An iterator past the end of the output range.

28.5.10 Class format_error [format.error]

🔗

namespace std { class format_error : public runtime_error { public: explicit format_error(const string& what_arg); explicit format_error(const char* what_arg); }; }

The class format_error defines the type of objects thrown as exceptions to report errors from the formatting library.

🔗

format_error(const string& what_arg);

Postconditions: strcmp(what(), what_arg.c_str()) == 0.

🔗

format_error(const char* what_arg);

Postconditions: strcmp(what(), what_arg) == 0.

28.6 Regular expressions library [re]

28.6.1 General [re.general]

Subclause [re] describes components that C++ programs may use to perform operations involving regular expression matching and searching.

The following subclauses describe a basic regular expression class template and its traits that can handle char-like ([strings.general]) template arguments, two specializations of this class template that handle sequences of char and wchar_t, a class template that holds the result of a regular expression match, a series of algorithms that allow a character sequence to be operated upon by a regular expression, and two iterator types for enumerating regular expression matches, as summarized in Table 112 .

Table 112: Regular expressions library summary [tab:re.summary]

🔗		Subclause	Header
🔗	[re.req]	Requirements
🔗	[re.const]	Constants	<regex>
🔗	[re.badexp]	Exception type
🔗	[re.traits]	Traits
🔗	[re.regex]	Regular expression template
🔗	[re.submatch]	Submatches
🔗	[re.results]	Match results
🔗	[re.alg]	Algorithms
🔗	[re.iter]	Iterators
🔗	[re.grammar]	Grammar

The ECMAScript Language Specification described in Standard Ecma-262 is called ECMA-262 in this Clause.

28.6.2 Requirements [re.req]

This subclause defines requirements on classes representing regular expression traits.

[Note 1:

The class template regex_traits, defined in [re.traits], meets these requirements.

— end note]

The class template basic_regex, defined in [re.regex], needs a set of related types and functions to complete the definition of its semantics.

These types and functions are provided as a set of member typedef-names and functions in the template parameter traits used by the basic_regex class template.

This subclause defines the semantics of these members.

To specialize class template basic_regex for a character container CharT and its related regular expression traits class Traits, use basic_regex<CharT, Traits>.

In the following requirements,

(4.1)
X denotes a traits class defining types and functions for the character container type charT;
(4.2)
u is an object of type X;
(4.3)
v is an object of type const X;
(4.4)
p is a value of type const charT*;
(4.5)
I1 and I2 are input iterators ([input.iterators]);
(4.6)
F1 and F2 are forward iterators ([forward.iterators]);
(4.7)
c is a value of type const charT;
(4.8)
s is an object of type X::string_type;
(4.9)
cs is an object of type const X::string_type;
(4.10)
b is a value of type bool;
(4.11)
I is a value of type int;
(4.12)
cl is an object of type X::char_class_type; and
(4.13)
loc is an object of type X::locale_type.

A traits class X meets the regular expression traits requirements if the following types and expressions are well-formed and have the specified semantics.

🔗

typename X::char_type

Result: charT, the character container type used in the implementation of class template basic_regex.

🔗

typename X::string_type

Result: basic_string<charT>

🔗

typename X::locale_type

Result: A copy constructible type that represents the locale used by the traits class.

🔗

typename X::char_class_type

Result: A bitmask type ([bitmask.types]) representing a particular character classification.

🔗

X::length(p)

Result: size_t

Returns: The smallest i such that p[i] == 0.

Complexity: Linear in i.

🔗

v.translate(c)

Result: X::char_type

Returns: A character such that for any character d that is to be considered equivalent to c then v.translate(c) == v.translate(d).

🔗

v.translate_nocase(c)

Result: X::char_type

Returns: For all characters C that are to be considered equivalent to c when comparisons are to be performed without regard to case, then v.translate_nocase(c) == v.translate_nocase(C).

🔗

v.transform(F1, F2)

Result: X::string_type

Returns: A sort key for the character sequence designated by the iterator range [F1, F2) such that if the character sequence [G1, G2) sorts before the character sequence [H1, H2) then v.transform(G1, G2) < v.transform(H1, H2).

🔗

v.transform_primary(F1, F2)

Result: X::string_type

🔗

v.lookup_collatename(F1, F2)

Result: X::string_type

Returns: A sequence of characters that represents the collating element consisting of the character sequence designated by the iterator range [F1, F2).

Returns an empty string if the character sequence is not a valid collating element.

🔗

v.lookup_classname(F1, F2, b)

Result: X::char_class_type

Returns: Converts the character sequence designated by the iterator range [F1, F2) into a value of a bitmask type that can subsequently be passed to isctype.

Values returned from lookup_classname can be bitwise or'ed together; the resulting value represents membership in either of the corresponding character classes.

If b is true, the returned bitmask is suitable for matching characters without regard to their case.

Returns 0 if the character sequence is not the name of a character class recognized by X.

The value returned shall be independent of the case of the characters in the sequence.

🔗

v.isctype(c, cl)

Result: bool

Returns: Returns true if character c is a member of one of the character classes designated by cl, false otherwise.

🔗

v.value(c, I)

Result: int

Returns: Returns the value represented by the digit c in base I if the character c is a valid digit in base I; otherwise returns -1.

[Note 2:

The value of I will only be 8, 10, or 16.

— end note]

🔗

u.imbue(loc)

Result: X::locale_type

Effects: Imbues u with the locale loc and returns the previous locale used by u if any.

🔗

v.getloc()

Result: X::locale_type

Returns: Returns the current locale used by v, if any.

[Note 3:

Class template regex_traits meets the requirements for a regular expression traits class when it is specialized for char or wchar_t.

This class template is described in the header <regex>, and is described in [re.traits].

— end note]

28.6.3 Header <regex> synopsis [re.syn]

🔗

#include <compare> // see [compare.syn] #include <initializer_list> // see [initializer.list.syn] namespace std { // [re.const], regex constants namespace regex_constants { using syntax_option_type = T1; using match_flag_type = T2; using error_type = T3; } // [re.badexp], class regex_error class regex_error; // [re.traits], class template regex_traits template<class charT> struct regex_traits; // [re.regex], class template basic_regex template<class charT, class traits = regex_traits<charT>> class basic_regex; using regex = basic_regex<char>; using wregex = basic_regex<wchar_t>; // [re.regex.swap], basic_regex swap template<class charT, class traits> void swap(basic_regex<charT, traits>& e1, basic_regex<charT, traits>& e2); // [re.submatch], class template sub_match template<class BidirectionalIterator> class sub_match; using csub_match = sub_match<const char*>; using wcsub_match = sub_match<const wchar_t*>; using ssub_match = sub_match<string::const_iterator>; using wssub_match = sub_match<wstring::const_iterator>; // [re.submatch.op], sub_match non-member operators template<class BiIter> bool operator==(const sub_match<BiIter>& lhs, const sub_match<BiIter>& rhs); template<class BiIter> auto operator<=>(const sub_match<BiIter>& lhs, const sub_match<BiIter>& rhs); template<class BiIter, class ST, class SA> bool operator==( const sub_match<BiIter>& lhs, const basic_string<typename iterator_traits<BiIter>::value_type, ST, SA>& rhs); template<class BiIter, class ST, class SA> auto operator<=>( const sub_match<BiIter>& lhs, const basic_string<typename iterator_traits<BiIter>::value_type, ST, SA>& rhs); template<class BiIter> bool operator==(const sub_match<BiIter>& lhs, const typename iterator_traits<BiIter>::value_type* rhs); template<class BiIter> auto operator<=>(const sub_match<BiIter>& lhs, const typename iterator_traits<BiIter>::value_type* rhs); template<class BiIter> bool operator==(const sub_match<BiIter>& lhs, const typename iterator_traits<BiIter>::value_type& rhs); template<class BiIter> auto operator<=>(const sub_match<BiIter>& lhs, const typename iterator_traits<BiIter>::value_type& rhs); template<class charT, class ST, class BiIter> basic_ostream<charT, ST>& operator<<(basic_ostream<charT, ST>& os, const sub_match<BiIter>& m); // [re.results], class template match_results template<class BidirectionalIterator, class Allocator = allocator<sub_match<BidirectionalIterator>>> class match_results; using cmatch = match_results<const char*>; using wcmatch = match_results<const wchar_t*>; using smatch = match_results<string::const_iterator>; using wsmatch = match_results<wstring::const_iterator>; // match_results comparisons template<class BidirectionalIterator, class Allocator> bool operator==(const match_results<BidirectionalIterator, Allocator>& m1, const match_results<BidirectionalIterator, Allocator>& m2); // [re.results.swap], match_results swap template<class BidirectionalIterator, class Allocator> void swap(match_results<BidirectionalIterator, Allocator>& m1, match_results<BidirectionalIterator, Allocator>& m2); // [re.alg.match], function template regex_match template<class BidirectionalIterator, class Allocator, class charT, class traits> bool regex_match(BidirectionalIterator first, BidirectionalIterator last, match_results<BidirectionalIterator, Allocator>& m, const basic_regex<charT, traits>& e, regex_constants::match_flag_type flags = regex_constants::match_default); template<class BidirectionalIterator, class charT, class traits> bool regex_match(BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, regex_constants::match_flag_type flags = regex_constants::match_default); template<class charT, class Allocator, class traits> bool regex_match(const charT* str, match_results<const charT*, Allocator>& m, const basic_regex<charT, traits>& e, regex_constants::match_flag_type flags = regex_constants::match_default); template<class ST, class SA, class Allocator, class charT, class traits> bool regex_match(const basic_string<charT, ST, SA>& s, match_results<typename basic_string<charT, ST, SA>::const_iterator, Allocator>& m, const basic_regex<charT, traits>& e, regex_constants::match_flag_type flags = regex_constants::match_default); template<class ST, class SA, class Allocator, class charT, class traits> bool regex_match(const basic_string<charT, ST, SA>&&, match_results<typename basic_string<charT, ST, SA>::const_iterator, Allocator>&, const basic_regex<charT, traits>&, regex_constants::match_flag_type = regex_constants::match_default) = delete; template<class charT, class traits> bool regex_match(const charT* str, const basic_regex<charT, traits>& e, regex_constants::match_flag_type flags = regex_constants::match_default); template<class ST, class SA, class charT, class traits> bool regex_match(const basic_string<charT, ST, SA>& s, const basic_regex<charT, traits>& e, regex_constants::match_flag_type flags = regex_constants::match_default); // [re.alg.search], function template regex_search template<class BidirectionalIterator, class Allocator, class charT, class traits> bool regex_search(BidirectionalIterator first, BidirectionalIterator last, match_results<BidirectionalIterator, Allocator>& m, const basic_regex<charT, traits>& e, regex_constants::match_flag_type flags = regex_constants::match_default); template<class BidirectionalIterator, class charT, class traits> bool regex_search(BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, regex_constants::match_flag_type flags = regex_constants::match_default); template<class charT, class Allocator, class traits> bool regex_search(const charT* str, match_results<const charT*, Allocator>& m, const basic_regex<charT, traits>& e, regex_constants::match_flag_type flags = regex_constants::match_default); template<class charT, class traits> bool regex_search(const charT* str, const basic_regex<charT, traits>& e, regex_constants::match_flag_type flags = regex_constants::match_default); template<class ST, class SA, class charT, class traits> bool regex_search(const basic_string<charT, ST, SA>& s, const basic_regex<charT, traits>& e, regex_constants::match_flag_type flags = regex_constants::match_default); template<class ST, class SA, class Allocator, class charT, class traits> bool regex_search(const basic_string<charT, ST, SA>& s, match_results<typename basic_string<charT, ST, SA>::const_iterator, Allocator>& m, const basic_regex<charT, traits>& e, regex_constants::match_flag_type flags = regex_constants::match_default); template<class ST, class SA, class Allocator, class charT, class traits> bool regex_search(const basic_string<charT, ST, SA>&&, match_results<typename basic_string<charT, ST, SA>::const_iterator, Allocator>&, const basic_regex<charT, traits>&, regex_constants::match_flag_type = regex_constants::match_default) = delete; // [re.alg.replace], function template regex_replace template<class OutputIterator, class BidirectionalIterator, class traits, class charT, class ST, class SA> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const basic_string<charT, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template<class OutputIterator, class BidirectionalIterator, class traits, class charT> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template<class traits, class charT, class ST, class SA, class FST, class FSA> basic_string<charT, ST, SA> regex_replace(const basic_string<charT, ST, SA>& s, const basic_regex<charT, traits>& e, const basic_string<charT, FST, FSA>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template<class traits, class charT, class ST, class SA> basic_string<charT, ST, SA> regex_replace(const basic_string<charT, ST, SA>& s, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template<class traits, class charT, class ST, class SA> basic_string<charT> regex_replace(const charT* s, const basic_regex<charT, traits>& e, const basic_string<charT, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template<class traits, class charT> basic_string<charT> regex_replace(const charT* s, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default); // [re.regiter], class template regex_iterator template<class BidirectionalIterator, class charT = typename iterator_traits<BidirectionalIterator>::value_type, class traits = regex_traits<charT>> class regex_iterator; using cregex_iterator = regex_iterator<const char*>; using wcregex_iterator = regex_iterator<const wchar_t*>; using sregex_iterator = regex_iterator<string::const_iterator>; using wsregex_iterator = regex_iterator<wstring::const_iterator>; // [re.tokiter], class template regex_token_iterator template<class BidirectionalIterator, class charT = typename iterator_traits<BidirectionalIterator>::value_type, class traits = regex_traits<charT>> class regex_token_iterator; using cregex_token_iterator = regex_token_iterator<const char*>; using wcregex_token_iterator = regex_token_iterator<const wchar_t*>; using sregex_token_iterator = regex_token_iterator<string::const_iterator>; using wsregex_token_iterator = regex_token_iterator<wstring::const_iterator>; namespace pmr { template<class BidirectionalIterator> using match_results = std::match_results<BidirectionalIterator, polymorphic_allocator<sub_match<BidirectionalIterator>>>; using cmatch = match_results<const char*>; using wcmatch = match_results<const wchar_t*>; using smatch = match_results<string::const_iterator>; using wsmatch = match_results<wstring::const_iterator>; } }

28.6.4 Namespace std::regex_constants [re.const]

28.6.4.1 General [re.const.general]

The namespace std::regex_constants holds symbolic constants used by the regular expression library.

This namespace provides three types, syntax_option_type, match_flag_type, and error_type, along with several constants of these types.

28.6.4.2 Bitmask type syntax_option_type [re.synopt]

🔗

namespace std::regex_constants { using syntax_option_type = T1; inline constexpr syntax_option_type icase = unspecified; inline constexpr syntax_option_type nosubs = unspecified; inline constexpr syntax_option_type optimize = unspecified; inline constexpr syntax_option_type collate = unspecified; inline constexpr syntax_option_type ECMAScript = unspecified; inline constexpr syntax_option_type basic = unspecified; inline constexpr syntax_option_type extended = unspecified; inline constexpr syntax_option_type awk = unspecified; inline constexpr syntax_option_type grep = unspecified; inline constexpr syntax_option_type egrep = unspecified; inline constexpr syntax_option_type multiline = unspecified; }

The type syntax_option_type is an implementation-defined bitmask type ([bitmask.types]).

Setting its elements has the effects listed in Table 113 .

A valid value of type syntax_option_type shall have at most one of the grammar elements ECMAScript, basic, extended, awk, grep, egrep, set.

If no grammar element is set, the default grammar is ECMAScript.

Table 113: syntax_option_type effects [tab:re.synopt]

🔗	Element	Effect(s) if set
🔗	icase	Specifies that matching of regular expressions against a character container sequence shall be performed without regard to case.
🔗	nosubs	Specifies that no sub-expressions shall be considered to be marked, so that when a regular expression is matched against a character container sequence, no sub-expression matches shall be stored in the supplied match_results object.
🔗	optimize	Specifies that the regular expression engine should pay more attention to the speed with which regular expressions are matched, and less to the speed with which regular expression objects are constructed. Otherwise it has no detectable effect on the program output.
🔗	collate	Specifies that character ranges of the form "[a-b]" shall be locale sensitive.
🔗	ECMAScript	Specifies that the grammar recognized by the regular expression engine shall be that used by ECMAScript in ECMA-262, as modified in [re.grammar]. See also: ECMA-262 15.10
🔗	basic	Specifies that the grammar recognized by the regular expression engine shall be that used by basic regular expressions in POSIX. See also: POSIX, Base Definitions and Headers, Section 9.3
🔗	extended	Specifies that the grammar recognized by the regular expression engine shall be that used by extended regular expressions in POSIX. See also: POSIX, Base Definitions and Headers, Section 9.4
🔗	awk	Specifies that the grammar recognized by the regular expression engine shall be that used by the utility awk in POSIX.
🔗	grep	Specifies that the grammar recognized by the regular expression engine shall be that used by the utility grep in POSIX.
🔗	egrep	Specifies that the grammar recognized by the regular expression engine shall be that used by the utility grep when given the -E option in POSIX.
🔗	multiline	Specifies that ^ shall match the beginning of a line and $ shall match the end of a line, if the ECMAScript engine is selected.

28.6.4.3 Bitmask type match_flag_type [re.matchflag]

🔗

namespace std::regex_constants { using match_flag_type = T2; inline constexpr match_flag_type match_default = {}; inline constexpr match_flag_type match_not_bol = unspecified; inline constexpr match_flag_type match_not_eol = unspecified; inline constexpr match_flag_type match_not_bow = unspecified; inline constexpr match_flag_type match_not_eow = unspecified; inline constexpr match_flag_type match_any = unspecified; inline constexpr match_flag_type match_not_null = unspecified; inline constexpr match_flag_type match_continuous = unspecified; inline constexpr match_flag_type match_prev_avail = unspecified; inline constexpr match_flag_type format_default = {}; inline constexpr match_flag_type format_sed = unspecified; inline constexpr match_flag_type format_no_copy = unspecified; inline constexpr match_flag_type format_first_only = unspecified; }

The type match_flag_type is an implementation-defined bitmask type ([bitmask.types]).

The constants of that type, except for match_default and format_default, are bitmask elements.

The match_default and format_default constants are empty bitmasks.

Matching a regular expression against a sequence of characters [first, last) proceeds according to the rules of the grammar specified for the regular expression object, modified according to the effects listed in Table 114 for any bitmask elements set.

Table 114: regex_constants::match_flag_type effects [tab:re.matchflag]

🔗	Element	Effect(s) if set
🔗	match_not_bol	The first character in the sequence [first, last) shall be treated as though it is not at the beginning of a line, so the character `^` in the regular expression shall not match [first, first).
🔗	match_not_eol	The last character in the sequence [first, last) shall be treated as though it is not at the end of a line, so the character `"$"` in the regular expression shall not match [last, last).
🔗	match_not_bow	The expression `"\\b"` shall not match the sub-sequence [first, first).
🔗	match_not_eow	The expression `"\\b"` shall not match the sub-sequence [last, last).
🔗	match_any	If more than one match is possible then any match is an acceptable result.
🔗	match_not_null	The expression shall not match an empty sequence.
🔗	match_continuous	The expression shall only match a sub-sequence that begins at first.
🔗	match_prev_avail	`--first` is a valid iterator position. When this flag is set the flags match_not_bol and match_not_bow shall be ignored by the regular expression algorithms ([re.alg]) and iterators ([re.iter]).
🔗	format_default	When a regular expression match is to be replaced by a new string, the new string shall be constructed using the rules used by the ECMAScript replace function in ECMA-262, part 15.5.4.11 String.prototype.replace. In addition, during search and replace operations all non-overlapping occurrences of the regular expression shall be located and replaced, and sections of the input that did not match the expression shall be copied unchanged to the output string.
🔗	format_sed	When a regular expression match is to be replaced by a new string, the new string shall be constructed using the rules used by the sed utility in POSIX.
🔗	format_no_copy	During a search and replace operation, sections of the character container sequence being searched that do not match the regular expression shall not be copied to the output string.
🔗	format_first_only	When specified during a search and replace operation, only the first occurrence of the regular expression shall be replaced.

28.6.4.4 Implementation-defined error_type [re.err]

🔗

namespace std::regex_constants { using error_type = T3; inline constexpr error_type error_collate = unspecified; inline constexpr error_type error_ctype = unspecified; inline constexpr error_type error_escape = unspecified; inline constexpr error_type error_backref = unspecified; inline constexpr error_type error_brack = unspecified; inline constexpr error_type error_paren = unspecified; inline constexpr error_type error_brace = unspecified; inline constexpr error_type error_badbrace = unspecified; inline constexpr error_type error_range = unspecified; inline constexpr error_type error_space = unspecified; inline constexpr error_type error_badrepeat = unspecified; inline constexpr error_type error_complexity = unspecified; inline constexpr error_type error_stack = unspecified; }

The type error_type is an implementation-defined enumerated type ([enumerated.types]).

Values of type error_type represent the error conditions described in Table 115:

Table 115: error_type values in the C locale [tab:re.err]

🔗	Value	Error condition
🔗	error_collate	The expression contains an invalid collating element name.
🔗	error_ctype	The expression contains an invalid character class name.
🔗	error_escape	The expression contains an invalid escaped character, or a trailing escape.
🔗	error_backref	The expression contains an invalid back reference.
🔗	error_brack	The expression contains mismatched `[` and `]`.
🔗	error_paren	The expression contains mismatched `(` and `)`.
🔗	error_brace	The expression contains mismatched `{` and `}`
🔗	error_badbrace	The expression contains an invalid range in a `{}` expression.
🔗	error_range	The expression contains an invalid character range, such as `[b-a]` in most encodings.
🔗	error_space	There is insufficient memory to convert the expression into a finite state machine.
🔗	error_badrepeat	One of `*?+{` is not preceded by a valid regular expression.
🔗	error_complexity	The complexity of an attempted match against a regular expression exceeds a pre-set level.
🔗	error_stack	There is insufficient memory to determine whether the regular expression matches the specified character sequence.

28.6.5 Class regex_error [re.badexp]

🔗

namespace std { class regex_error : public runtime_error { public: explicit regex_error(regex_constants::error_type ecode); regex_constants::error_type code() const; }; }

The class regex_error defines the type of objects thrown as exceptions to report errors from the regular expression library.

🔗

regex_error(regex_constants::error_type ecode);

Postconditions: ecode == code().

🔗

regex_constants::error_type code() const;

Returns: The error code that was passed to the constructor.

28.6.6 Class template regex_traits [re.traits]

🔗

namespace std { template<class charT> struct regex_traits { using char_type = charT; using string_type = basic_string<char_type>; using locale_type = locale; using char_class_type = bitmask_type; regex_traits(); static size_t length(const char_type* p); charT translate(charT c) const; charT translate_nocase(charT c) const; template<class ForwardIterator> string_type transform(ForwardIterator first, ForwardIterator last) const; template<class ForwardIterator> string_type transform_primary( ForwardIterator first, ForwardIterator last) const; template<class ForwardIterator> string_type lookup_collatename( ForwardIterator first, ForwardIterator last) const; template<class ForwardIterator> char_class_type lookup_classname( ForwardIterator first, ForwardIterator last, bool icase = false) const; bool isctype(charT c, char_class_type f) const; int value(charT ch, int radix) const; locale_type imbue(locale_type l); locale_type getloc() const; }; }

The specializations regex_traits<char> and regex_traits<wchar_t> meet the requirements for a regular expression traits class ([re.req]).

🔗

using char_class_type = bitmask_type;

The type char_class_type is used to represent a character classification and is capable of holding an implementation specific set returned by lookup_classname.

🔗

static size_t length(const char_type* p);

Returns: char_traits<charT>::length(p).

🔗

charT translate(charT c) const;

Returns: c.

🔗

charT translate_nocase(charT c) const;

Returns: use_facet<ctype<charT>>(getloc()).tolower(c).

🔗

template<class ForwardIterator>
  string_type transform(ForwardIterator first, ForwardIterator last) const;

Effects: As if by: string_type str(first, last); return use_facet<collate<charT>>( getloc()).transform(str.data(), str.data() + str.length());

🔗

template<class ForwardIterator>
  string_type transform_primary(ForwardIterator first, ForwardIterator last) const;

Effects: If typeid(use_facet<collate<charT>>) == typeid(collate_byname<charT>) and the form of the sort key returned by collate_byname<charT>::transform(first, last) is known and can be converted into a primary sort key then returns that key, otherwise returns an empty string.

🔗

template<class ForwardIterator>
  string_type lookup_collatename(ForwardIterator first, ForwardIterator last) const;

Returns: A sequence of one or more characters that represents the collating element consisting of the character sequence designated by the iterator range [first, last).

Returns an empty string if the character sequence is not a valid collating element.

🔗

template<class ForwardIterator>
  char_class_type lookup_classname(
    ForwardIterator first, ForwardIterator last, bool icase = false) const;

Returns: An unspecified value that represents the character classification named by the character sequence designated by the iterator range [first, last).

If the parameter icase is true then the returned mask identifies the character classification without regard to the case of the characters being matched, otherwise it does honor the case of the characters being matched.242

The value returned shall be independent of the case of the characters in the character sequence.

If the name is not recognized then returns char_class_type().

Remarks: For regex_traits<char>, at least the narrow character names in Table 116 shall be recognized.

For regex_traits<wchar_t>, at least the wide character names in Table 116 shall be recognized.

🔗

bool isctype(charT c, char_class_type f) const;

Effects: Determines if the character c is a member of the character classification represented by f.

Returns: Given the following function declaration: // for exposition only template<class C> ctype_base::mask convert(typename regex_traits<C>::char_class_type f); that returns a value in which each ctype_base::mask value corresponding to a value in f named in Table 116 is set, then the result is determined as if by: ctype_base::mask m = convert<charT>(f); const ctype<charT>& ct = use_facet<ctype<charT>>(getloc()); if (ct.is(m, c)) { return true; } else if (c == ct.widen('_')) { charT w[1] = { ct.widen('w') }; char_class_type x = lookup_classname(w, w+1); return (f&x) == x; } else { return false; }

[Example 1: regex_traits<char> t; string d("d"); string u("upper"); regex_traits<char>::char_class_type f; f = t.lookup_classname(d.begin(), d.end()); f |= t.lookup_classname(u.begin(), u.end()); ctype_base::mask m = convert<char>(f); // m == ctype_base::digit|ctype_base::upper — end example]

[Example 2: regex_traits<char> t; string w("w"); regex_traits<char>::char_class_type f; f = t.lookup_classname(w.begin(), w.end()); t.isctype('A', f); // returns true t.isctype('_', f); // returns true t.isctype(' ', f); // returns false — end example]

🔗

int value(charT ch, int radix) const;

Preconditions: The value of radix is 8, 10, or 16.

Returns: The value represented by the digit ch in base radix if the character ch is a valid digit in base radix; otherwise returns -1.

🔗

locale_type imbue(locale_type loc);

Effects: Imbues *this with a copy of the locale loc.

[Note 1:

Calling imbue with a different locale than the one currently in use invalidates all cached data held by *this.

— end note]

Postconditions: getloc() == loc.

Returns: If no locale has been previously imbued then a copy of the global locale in effect at the time of construction of *this, otherwise a copy of the last argument passed to imbue.

🔗

locale_type getloc() const;

Returns: If no locale has been imbued then a copy of the global locale in effect at the time of construction of *this, otherwise a copy of the last argument passed to imbue.

Table 116: Character class names and corresponding ctype masks [tab:re.traits.classnames]

🔗	Narrow character name	Wide character name	Corresponding ctype_base::mask value
🔗	"alnum"	L"alnum"	ctype_base::alnum
🔗	"alpha"	L"alpha"	ctype_base::alpha
🔗	"blank"	L"blank"	ctype_base::blank
🔗	"cntrl"	L"cntrl"	ctype_base::cntrl
🔗	"digit"	L"digit"	ctype_base::digit
🔗	"d"	L"d"	ctype_base::digit
🔗	"graph"	L"graph"	ctype_base::graph
🔗	"lower"	L"lower"	ctype_base::lower
🔗	"print"	L"print"	ctype_base::print
🔗	"punct"	L"punct"	ctype_base::punct
🔗	"space"	L"space"	ctype_base::space
🔗	"s"	L"s"	ctype_base::space
🔗	"upper"	L"upper"	ctype_base::upper
🔗	"w"	L"w"	ctype_base::alnum
🔗	"xdigit"	L"xdigit"	ctype_base::xdigit

242)242)

For example, if the parameter icase is true then [[:lower:]] is the same as [[:alpha:]].

28.6.7 Class template basic_regex [re.regex]

28.6.7.1 General [re.regex.general]

For a char-like type charT, specializations of class template basic_regex represent regular expressions constructed from character sequences of charT characters.

In the rest of [re.regex], charT denotes a given char-like type.

Storage for a regular expression is allocated and freed as necessary by the member functions of class basic_regex.

Objects of type specialization of basic_regex are responsible for converting the sequence of charT objects to an internal representation.

It is not specified what form this representation takes, nor how it is accessed by algorithms that operate on regular expressions.

[Note 1:

Implementations will typically declare some function templates as friends of basic_regex to achieve this.

— end note]

The functions described in [re.regex] report errors by throwing exceptions of type regex_error.

🔗

namespace std { template<class charT, class traits = regex_traits<charT>> class basic_regex { public: // types using value_type = charT; using traits_type = traits; using string_type = typename traits::string_type; using flag_type = regex_constants::syntax_option_type; using locale_type = typename traits::locale_type; // [re.synopt], constants static constexpr flag_type icase = regex_constants::icase; static constexpr flag_type nosubs = regex_constants::nosubs; static constexpr flag_type optimize = regex_constants::optimize; static constexpr flag_type collate = regex_constants::collate; static constexpr flag_type ECMAScript = regex_constants::ECMAScript; static constexpr flag_type basic = regex_constants::basic; static constexpr flag_type extended = regex_constants::extended; static constexpr flag_type awk = regex_constants::awk; static constexpr flag_type grep = regex_constants::grep; static constexpr flag_type egrep = regex_constants::egrep; static constexpr flag_type multiline = regex_constants::multiline; // [re.regex.construct], construct/copy/destroy basic_regex(); explicit basic_regex(const charT* p, flag_type f = regex_constants::ECMAScript); basic_regex(const charT* p, size_t len, flag_type f = regex_constants::ECMAScript); basic_regex(const basic_regex&); basic_regex(basic_regex&&) noexcept; template<class ST, class SA> explicit basic_regex(const basic_string<charT, ST, SA>& s, flag_type f = regex_constants::ECMAScript); template<class ForwardIterator> basic_regex(ForwardIterator first, ForwardIterator last, flag_type f = regex_constants::ECMAScript); basic_regex(initializer_list<charT> il, flag_type f = regex_constants::ECMAScript); ~basic_regex(); // [re.regex.assign], assign basic_regex& operator=(const basic_regex& e); basic_regex& operator=(basic_regex&& e) noexcept; basic_regex& operator=(const charT* p); basic_regex& operator=(initializer_list<charT> il); template<class ST, class SA> basic_regex& operator=(const basic_string<charT, ST, SA>& s); basic_regex& assign(const basic_regex& e); basic_regex& assign(basic_regex&& e) noexcept; basic_regex& assign(const charT* p, flag_type f = regex_constants::ECMAScript); basic_regex& assign(const charT* p, size_t len, flag_type f = regex_constants::ECMAScript); template<class ST, class SA> basic_regex& assign(const basic_string<charT, ST, SA>& s, flag_type f = regex_constants::ECMAScript); template<class InputIterator> basic_regex& assign(InputIterator first, InputIterator last, flag_type f = regex_constants::ECMAScript); basic_regex& assign(initializer_list<charT>, flag_type f = regex_constants::ECMAScript); // [re.regex.operations], const operations unsigned mark_count() const; flag_type flags() const; // [re.regex.locale], locale locale_type imbue(locale_type loc); locale_type getloc() const; // [re.regex.swap], swap void swap(basic_regex&); }; template<class ForwardIterator> basic_regex(ForwardIterator, ForwardIterator, regex_constants::syntax_option_type = regex_constants::ECMAScript) -> basic_regex<typename iterator_traits<ForwardIterator>::value_type>; }

28.6.7.2 Constructors [re.regex.construct]

🔗

basic_regex();

Postconditions: *this does not match any character sequence.

🔗

explicit basic_regex(const charT* p, flag_type f = regex_constants::ECMAScript);

Preconditions: [p, p + char_traits<charT>::length(p)) is a valid range.

Effects: The object's internal finite state machine is constructed from the regular expression contained in the sequence of characters [p, p + char_traits<charT>::length(p)), and interpreted according to the flags f.

Postconditions: flags() returns f.

mark_count() returns the number of marked sub-expressions within the expression.

Throws: regex_error if [p, p + char_traits<charT>::length(p)) is not a valid regular expression.

🔗

basic_regex(const charT* p, size_t len, flag_type f = regex_constants::ECMAScript);

Preconditions: [p, p + len) is a valid range.

Effects: The object's internal finite state machine is constructed from the regular expression contained in the sequence of characters [p, p + len), and interpreted according the flags specified in f.

Postconditions: flags() returns f.

mark_count() returns the number of marked sub-expressions within the expression.

Throws: regex_error if [p, p + len) is not a valid regular expression.

🔗

basic_regex(const basic_regex& e);

Postconditions: flags() and mark_count() return e.flags() and e.mark_count(), respectively.

🔗

basic_regex(basic_regex&& e) noexcept;

Postconditions: flags() and mark_count() return the values that e.flags() and e.mark_count(), respectively, had before construction.

🔗

template<class ST, class SA>
  explicit basic_regex(const basic_string<charT, ST, SA>& s,
                       flag_type f = regex_constants::ECMAScript);

Effects: The object's internal finite state machine is constructed from the regular expression contained in the string s, and interpreted according to the flags specified in f.

Postconditions: flags() returns f.

mark_count() returns the number of marked sub-expressions within the expression.

Throws: regex_error if s is not a valid regular expression.

🔗

template<class ForwardIterator>
  basic_regex(ForwardIterator first, ForwardIterator last,
              flag_type f = regex_constants::ECMAScript);

Effects: The object's internal finite state machine is constructed from the regular expression contained in the sequence of characters [first, last), and interpreted according to the flags specified in f.

Postconditions: flags() returns f.

mark_count() returns the number of marked sub-expressions within the expression.

Throws: regex_error if the sequence [first, last) is not a valid regular expression.

🔗

basic_regex(initializer_list<charT> il, flag_type f = regex_constants::ECMAScript);

Effects: Same as basic_regex(il.begin(), il.end(), f).

28.6.7.3 Assignment [re.regex.assign]

🔗

basic_regex& operator=(const basic_regex& e);

Postconditions: flags() and mark_count() return e.flags() and e.mark_count(), respectively.

🔗

basic_regex& operator=(basic_regex&& e) noexcept;

Postconditions: flags() and mark_count() return the values that e.flags() and e.mark_count(), respectively, had before assignment.

e is in a valid state with unspecified value.

🔗

basic_regex& operator=(const charT* p);

Effects: Equivalent to: return assign(p);

🔗

basic_regex& operator=(initializer_list<charT> il);

Effects: Equivalent to: return assign(il.begin(), il.end());

🔗

template<class ST, class SA>
  basic_regex& operator=(const basic_string<charT, ST, SA>& s);

Effects: Equivalent to: return assign(s);

🔗

basic_regex& assign(const basic_regex& e);

Effects: Equivalent to: return *this = e;

🔗

basic_regex& assign(basic_regex&& e) noexcept;

Effects: Equivalent to: return *this = std::move(e);

🔗

basic_regex& assign(const charT* p, flag_type f = regex_constants::ECMAScript);

Effects: Equivalent to: return assign(string_type(p), f);

🔗

basic_regex& assign(const charT* p, size_t len, flag_type f = regex_constants::ECMAScript);

Effects: Equivalent to: return assign(string_type(p, len), f);

🔗

template<class ST, class SA>
  basic_regex& assign(const basic_string<charT, ST, SA>& s,
                      flag_type f = regex_constants::ECMAScript);

Effects: Assigns the regular expression contained in the string s, interpreted according the flags specified in f.

If an exception is thrown, *this is unchanged.

Postconditions: If no exception is thrown, flags() returns f and mark_count() returns the number of marked sub-expressions within the expression.

Returns: *this.

Throws: regex_error if s is not a valid regular expression.

🔗

template<class InputIterator>
  basic_regex& assign(InputIterator first, InputIterator last,
                      flag_type f = regex_constants::ECMAScript);

Effects: Equivalent to: return assign(string_type(first, last), f);

🔗

basic_regex& assign(initializer_list<charT> il,
                    flag_type f = regex_constants::ECMAScript);

Effects: Equivalent to: return assign(il.begin(), il.end(), f);

28.6.7.4 Constant operations [re.regex.operations]

🔗

unsigned mark_count() const;

Effects: Returns the number of marked sub-expressions within the regular expression.

🔗

flag_type flags() const;

Effects: Returns a copy of the regular expression syntax flags that were passed to the object's constructor or to the last call to assign.

28.6.7.5 Locale [re.regex.locale]

🔗

locale_type imbue(locale_type loc);

Effects: Returns the result of traits_inst.imbue(loc) where traits_inst is a (default-initialized) instance of the template type argument traits stored within the object.

After a call to imbue the basic_regex object does not match any character sequence.

🔗

locale_type getloc() const;

Effects: Returns the result of traits_inst.getloc() where traits_inst is a (default-initialized) instance of the template parameter traits stored within the object.

28.6.7.6 Swap [re.regex.swap]

🔗

void swap(basic_regex& e);

Effects: Swaps the contents of the two regular expressions.

Postconditions: *this contains the regular expression that was in e, e contains the regular expression that was in *this.

Complexity: Constant time.

28.6.7.7 Non-member functions [re.regex.nonmemb]

🔗

template<class charT, class traits>
  void swap(basic_regex<charT, traits>& lhs, basic_regex<charT, traits>& rhs);

Effects: Calls lhs.swap(rhs).

28.6.8 Class template sub_match [re.submatch]

28.6.8.1 General [re.submatch.general]

Class template sub_match denotes the sequence of characters matched by a particular marked sub-expression.

namespace std { template<class BidirectionalIterator> class sub_match : public pair<BidirectionalIterator, BidirectionalIterator> { public: using value_type = typename iterator_traits<BidirectionalIterator>::value_type; using difference_type = typename iterator_traits<BidirectionalIterator>::difference_type; using iterator = BidirectionalIterator; using string_type = basic_string<value_type>; bool matched; constexpr sub_match(); difference_type length() const; operator string_type() const; string_type str() const; int compare(const sub_match& s) const; int compare(const string_type& s) const; int compare(const value_type* s) const; void swap(sub_match& s) noexcept(see below); }; }

28.6.8.2 Members [re.submatch.members]

🔗

constexpr sub_match();

Effects: Value-initializes the pair base class subobject and the member matched.

🔗

difference_type length() const;

Returns: matched ? distance(first, second) : 0.

🔗

operator string_type() const;

Returns: matched ? string_type(first, second) : string_type().

🔗

string_type str() const;

Returns: matched ? string_type(first, second) : string_type().

🔗

int compare(const sub_match& s) const;

Returns: str().compare(s.str()).

🔗

int compare(const string_type& s) const;

Returns: str().compare(s).

🔗

int compare(const value_type* s) const;

Returns: str().compare(s).

🔗

void swap(sub_match& s) noexcept(see below);

Preconditions: BidirectionalIterator meets the Cpp17Swappable requirements ([swappable.requirements]).

Effects: Equivalent to: this->pair<BidirectionalIterator, BidirectionalIterator>::swap(s); std::swap(matched, s.matched);

Remarks: The exception specification is equivalent to is_nothrow_swappable_v<BidirectionalIterator>.

28.6.8.3 Non-member operators [re.submatch.op]

Let SM-CAT(I) be compare_three_way_result_t<basic_string<typename iterator_traits<I>::value_type>>

🔗

template<class BiIter>
  bool operator==(const sub_match<BiIter>& lhs, const sub_match<BiIter>& rhs);

Returns: lhs.compare(rhs) == 0.

🔗

template<class BiIter>
  auto operator<=>(const sub_match<BiIter>& lhs, const sub_match<BiIter>& rhs);

Returns: static_cast<SM-CAT(BiIter)>(lhs.compare(rhs) <=> 0).

🔗

template<class BiIter, class ST, class SA>
  bool operator==(
      const sub_match<BiIter>& lhs,
      const basic_string<typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

Returns: lhs.compare(typename sub_match<BiIter>::string_type(rhs.data(), rhs.size())) == 0

🔗

template<class BiIter, class ST, class SA>
  auto operator<=>(
      const sub_match<BiIter>& lhs,
      const basic_string<typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

Returns: static_cast<SM-CAT(BiIter)>(lhs.compare( typename sub_match<BiIter>::string_type(rhs.data(), rhs.size())) <=> 0 )

🔗

template<class BiIter>
  bool operator==(const sub_match<BiIter>& lhs,
                  const typename iterator_traits<BiIter>::value_type* rhs);

Returns: lhs.compare(rhs) == 0.

🔗

template<class BiIter>
  auto operator<=>(const sub_match<BiIter>& lhs,
                   const typename iterator_traits<BiIter>::value_type* rhs);

Returns: static_cast<SM-CAT(BiIter)>(lhs.compare(rhs) <=> 0).

🔗

template<class BiIter>
  bool operator==(const sub_match<BiIter>& lhs,
                  const typename iterator_traits<BiIter>::value_type& rhs);

Returns: lhs.compare(typename sub_match<BiIter>::string_type(1, rhs)) == 0.

🔗

template<class BiIter>
  auto operator<=>(const sub_match<BiIter>& lhs,
                   const typename iterator_traits<BiIter>::value_type& rhs);

Returns: static_cast<SM-CAT(BiIter)>(lhs.compare( typename sub_match<BiIter>::string_type(1, rhs)) <=> 0 )

🔗

template<class charT, class ST, class BiIter>
  basic_ostream<charT, ST>&
    operator<<(basic_ostream<charT, ST>& os, const sub_match<BiIter>& m);

Returns: os << m.str().

28.6.9 Class template match_results [re.results]

28.6.9.1 General [re.results.general]

Class template match_results denotes a collection of character sequences representing the result of a regular expression match.

Storage for the collection is allocated and freed as necessary by the member functions of class template match_results.

The class template match_results meets the requirements of an allocator-aware container ([container.alloc.reqmts]) and of a sequence container ([container.requirements.general], [sequence.reqmts]) except that only copy assignment, move assignment, and operations defined for const-qualified sequence containers are supported and that the semantics of the comparison operator functions are different from those required for a container.

A default-constructed match_results object has no fully established result state.

A match result is ready when, as a consequence of a completed regular expression match modifying such an object, its result state becomes fully established.

The effects of calling most member functions from a match_results object that is not ready are undefined.

The sub_match object stored at index 0 represents sub-expression 0, i.e., the whole match.

In this case the sub_match member matched is always true.

The sub_match object stored at index n denotes what matched the marked sub-expression n within the matched expression.

If the sub-expression n participated in a regular expression match then the sub_match member matched evaluates to true, and members first and second denote the range of characters [first, second) which formed that match.

Otherwise matched is false, and members first and second point to the end of the sequence that was searched.

[Note 1:

The sub_match objects representing different sub-expressions that did not participate in a regular expression match need not be distinct.

— end note]

namespace std { template<class BidirectionalIterator, class Allocator = allocator<sub_match<BidirectionalIterator>>> class match_results { public: using value_type = sub_match<BidirectionalIterator>; using const_reference = const value_type&; using reference = value_type&; using const_iterator = implementation-defined; using iterator = const_iterator; using difference_type = typename iterator_traits<BidirectionalIterator>::difference_type; using size_type = typename allocator_traits<Allocator>::size_type; using allocator_type = Allocator; using char_type = typename iterator_traits<BidirectionalIterator>::value_type; using string_type = basic_string<char_type>; // [re.results.const], construct/copy/destroy match_results() : match_results(Allocator()) {} explicit match_results(const Allocator& a); match_results(const match_results& m); match_results(const match_results& m, const Allocator& a); match_results(match_results&& m) noexcept; match_results(match_results&& m, const Allocator& a); match_results& operator=(const match_results& m); match_results& operator=(match_results&& m); ~match_results(); // [re.results.state], state bool ready() const; // [re.results.size], size size_type size() const; size_type max_size() const; bool empty() const; // [re.results.acc], element access difference_type length(size_type sub = 0) const; difference_type position(size_type sub = 0) const; string_type str(size_type sub = 0) const; const_reference operator[](size_type n) const; const_reference prefix() const; const_reference suffix() const; const_iterator begin() const; const_iterator end() const; const_iterator cbegin() const; const_iterator cend() const; // [re.results.form], format template<class OutputIter> OutputIter format(OutputIter out, const char_type* fmt_first, const char_type* fmt_last, regex_constants::match_flag_type flags = regex_constants::format_default) const; template<class OutputIter, class ST, class SA> OutputIter format(OutputIter out, const basic_string<char_type, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const; template<class ST, class SA> basic_string<char_type, ST, SA> format(const basic_string<char_type, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const; string_type format(const char_type* fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const; // [re.results.all], allocator allocator_type get_allocator() const; // [re.results.swap], swap void swap(match_results& that); }; }

28.6.9.2 Constructors [re.results.const]

Table 117 lists the postconditions of match_results copy/move constructors and copy/move assignment operators.

For move operations, the results of the expressions depending on the parameter m denote the values they had before the respective function calls.

🔗

explicit match_results(const Allocator& a);

Effects: The stored Allocator value is constructed from a.

Postconditions: ready() returns false.

size() returns 0.

🔗

match_results(const match_results& m);
match_results(const match_results& m, const Allocator& a);

Effects: For the first form, the stored Allocator value is obtained as specified in [container.reqmts].

For the second form, the stored Allocator value is constructed from a.

Postconditions: As specified in Table 117 .

🔗

match_results(match_results&& m) noexcept;
match_results(match_results&& m, const Allocator& a);

Effects: For the first form, the stored Allocator value is move constructed from m.get_allocator().

For the second form, the stored Allocator value is constructed from a.

Postconditions: As specified in Table 117 .

Throws: The second form throws nothing if a == m.get_allocator() is true.

🔗

match_results& operator=(const match_results& m);

Postconditions: As specified in Table 117 .

🔗

match_results& operator=(match_results&& m);

Postconditions: As specified in Table 117 .

Table 117: match_results copy/move operation postconditions [tab:re.results.const]

🔗	Element	Value
🔗	ready()	m.ready()
🔗	size()	m.size()
🔗	str(n)	m.str(n) for all non-negative integers n < m.size()
🔗	prefix()	m.prefix()
🔗	suffix()	m.suffix()
🔗	(*this)[n]	m[n] for all non-negative integers n < m.size()
🔗	length(n)	m.length(n) for all non-negative integers n < m.size()
🔗	position(n)	m.position(n) for all non-negative integers n < m.size()

28.6.9.3 State [re.results.state]

🔗

bool ready() const;

Returns: true if *this has a fully established result state, otherwise false.

28.6.9.4 Size [re.results.size]

🔗

size_type size() const;

Returns: One plus the number of marked sub-expressions in the regular expression that was matched if *this represents the result of a successful match.

Otherwise returns 0.

[Note 1:

The state of a match_results object can be modified only by passing that object to regex_match or regex_search.

Subclauses [re.alg.match] and [re.alg.search] specify the effects of those algorithms on their match_results arguments.

— end note]

🔗

size_type max_size() const;

Returns: The maximum number of sub_match elements that can be stored in *this.

🔗

bool empty() const;

Returns: size() == 0.

28.6.9.5 Element access [re.results.acc]

🔗

difference_type length(size_type sub = 0) const;

Preconditions: ready() == true.

Returns: (*this)[sub].length().

🔗

difference_type position(size_type sub = 0) const;

Preconditions: ready() == true.

Returns: The distance from the start of the target sequence to (*this)[sub].first.

🔗

string_type str(size_type sub = 0) const;

Preconditions: ready() == true.

Returns: string_type((*this)[sub]).

🔗

const_reference operator[](size_type n) const;

Preconditions: ready() == true.

Returns: A reference to the sub_match object representing the character sequence that matched marked sub-expression n.

If n == 0 then returns a reference to a sub_match object representing the character sequence that matched the whole regular expression.

If n >= size() then returns a sub_match object representing an unmatched sub-expression.

🔗

const_reference prefix() const;

Preconditions: ready() == true.

Returns: A reference to the sub_match object representing the character sequence from the start of the string being matched/searched to the start of the match found.

🔗

const_reference suffix() const;

Preconditions: ready() == true.

Returns: A reference to the sub_match object representing the character sequence from the end of the match found to the end of the string being matched/searched.

🔗

const_iterator begin() const;
const_iterator cbegin() const;

Returns: A starting iterator that enumerates over all the sub-expressions stored in *this.

🔗

const_iterator end() const;
const_iterator cend() const;

Returns: A terminating iterator that enumerates over all the sub-expressions stored in *this.

28.6.9.6 Formatting [re.results.form]

🔗

template<class OutputIter>
  OutputIter format(
      OutputIter out,
      const char_type* fmt_first, const char_type* fmt_last,
      regex_constants::match_flag_type flags = regex_constants::format_default) const;

Preconditions: ready() == true and OutputIter meets the requirements for a Cpp17OutputIterator ([output.iterators]).

Effects: Copies the character sequence [fmt_first, fmt_last) to OutputIter out.

Replaces each format specifier or escape sequence in the copied range with either the character(s) it represents or the sequence of characters within *this to which it refers.

The bitmasks specified in flags determine which format specifiers and escape sequences are recognized.

Returns: out.

🔗

template<class OutputIter, class ST, class SA>
  OutputIter format(
      OutputIter out,
      const basic_string<char_type, ST, SA>& fmt,
      regex_constants::match_flag_type flags = regex_constants::format_default) const;

Effects: Equivalent to: return format(out, fmt.data(), fmt.data() + fmt.size(), flags);

🔗

template<class ST, class SA>
  basic_string<char_type, ST, SA> format(
      const basic_string<char_type, ST, SA>& fmt,
      regex_constants::match_flag_type flags = regex_constants::format_default) const;

Preconditions: ready() == true.

Effects: Constructs an empty string result of type basic_string<char_type, ST, SA> and calls: format(back_inserter(result), fmt, flags);

Returns: result.

🔗

string_type format(
    const char_type* fmt,
    regex_constants::match_flag_type flags = regex_constants::format_default) const;

Preconditions: ready() == true.

Effects: Constructs an empty string result of type string_type and calls: format(back_inserter(result), fmt, fmt + char_traits<char_type>::length(fmt), flags);

Returns: result.

28.6.9.7 Allocator [re.results.all]

🔗

allocator_type get_allocator() const;

Returns: A copy of the Allocator that was passed to the object's constructor or, if that allocator has been replaced, a copy of the most recent replacement.

28.6.9.8 Swap [re.results.swap]

🔗

void swap(match_results& that);

Effects: Swaps the contents of the two sequences.

Postconditions: *this contains the sequence of matched sub-expressions that were in that, that contains the sequence of matched sub-expressions that were in *this.

Complexity: Constant time.

🔗

template<class BidirectionalIterator, class Allocator>
  void swap(match_results<BidirectionalIterator, Allocator>& m1,
            match_results<BidirectionalIterator, Allocator>& m2);

Effects: As if by m1.swap(m2).

28.6.9.9 Non-member functions [re.results.nonmember]

🔗

template<class BidirectionalIterator, class Allocator>
bool operator==(const match_results<BidirectionalIterator, Allocator>& m1,
                const match_results<BidirectionalIterator, Allocator>& m2);

Returns: true if neither match result is ready, false if one match result is ready and the other is not.

If both match results are ready, returns true only if

(1.1)
m1.empty() && m2.empty(), or
(1.2)
!m1.empty() && !m2.empty(), and the following conditions are satisfied:
- (1.2.1)
  m1.prefix() == m2.prefix(),
- (1.2.2)
  m1.size() == m2.size() && equal(m1.begin(), m1.end(), m2.begin()), and
- (1.2.3)
  m1.suffix() == m2.suffix().

[Note 1:

The algorithm equal is defined in [algorithms].

— end note]

28.6.10 Regular expression algorithms [re.alg]

28.6.10.1 Exceptions [re.except]

The algorithms described in subclause [re.alg] may throw an exception of type regex_error.

If such an exception e is thrown, e.code() shall return either regex_constants::error_complexity or regex_constants::error_stack.

28.6.10.2 regex_match [re.alg.match]

🔗

template<class BidirectionalIterator, class Allocator, class charT, class traits>
  bool regex_match(BidirectionalIterator first, BidirectionalIterator last,
                   match_results<BidirectionalIterator, Allocator>& m,
                   const basic_regex<charT, traits>& e,
                   regex_constants::match_flag_type flags = regex_constants::match_default);

Preconditions: BidirectionalIterator models bidirectional_iterator ([iterator.concept.bidir]).

Effects: Determines whether there is a match between the regular expression e, and all of the character sequence [first, last).

The parameter flags is used to control how the expression is matched against the character sequence.

When determining if there is a match, only potential matches that match the entire character sequence are considered.

Returns true if such a match exists, false otherwise.

[Example 1: std::regex re("Get|GetValue"); std::cmatch m; regex_search("GetValue", m, re); // returns true, and m[0] contains "Get" regex_match ("GetValue", m, re); // returns true, and m[0] contains "GetValue" regex_search("GetValues", m, re); // returns true, and m[0] contains "Get" regex_match ("GetValues", m, re); // returns false — end example]

Postconditions: m.ready() == true in all cases.

If the function returns false, then the effect on parameter m is unspecified except that m.size() returns 0 and m.empty() returns true.

Otherwise the effects on parameter m are given in Table 118 .

Table 118: Effects of regex_match algorithm [tab:re.alg.match]

🔗	Element	Value
🔗	m.size()	1 + e.mark_count()
🔗	m.empty()	false
🔗	m.prefix().first	first
🔗	m.prefix().second	first
🔗	m.prefix().matched	false
🔗	m.suffix().first	last
🔗	m.suffix().second	last
🔗	m.suffix().matched	false
🔗	m[0].first	first
🔗	m[0].second	last
🔗	m[0].matched	true
🔗	m[n].first	For all integers 0 < n < m.size(), the start of the sequence that matched sub-expression n. Alternatively, if sub-expression n did not participate in the match, then last.
🔗	m[n].second	For all integers 0 < n < m.size(), the end of the sequence that matched sub-expression n. Alternatively, if sub-expression n did not participate in the match, then last.
🔗	m[n].matched	For all integers 0 < n < m.size(), true if sub-expression n participated in the match, false otherwise.

🔗

template<class BidirectionalIterator, class charT, class traits>
  bool regex_match(BidirectionalIterator first, BidirectionalIterator last,
                   const basic_regex<charT, traits>& e,
                   regex_constants::match_flag_type flags = regex_constants::match_default);

Effects: Behaves “as if” by constructing an instance of match_results<BidirectionalIterator> what, and then returning the result of regex_match(first, last, what, e, flags).

🔗

template<class charT, class Allocator, class traits>
  bool regex_match(const charT* str,
                   match_results<const charT*, Allocator>& m,
                   const basic_regex<charT, traits>& e,
                   regex_constants::match_flag_type flags = regex_constants::match_default);

Returns: regex_match(str, str + char_traits<charT>::length(str), m, e, flags).

🔗

template<class ST, class SA, class Allocator, class charT, class traits>
  bool regex_match(const basic_string<charT, ST, SA>& s,
                   match_results<typename basic_string<charT, ST, SA>::const_iterator,
                                 Allocator>& m,
                   const basic_regex<charT, traits>& e,
                   regex_constants::match_flag_type flags = regex_constants::match_default);

Returns: regex_match(s.begin(), s.end(), m, e, flags).

🔗

template<class charT, class traits>
  bool regex_match(const charT* str,
                   const basic_regex<charT, traits>& e,
                   regex_constants::match_flag_type flags = regex_constants::match_default);

Returns: regex_match(str, str + char_traits<charT>::length(str), e, flags)

🔗

template<class ST, class SA, class charT, class traits>
  bool regex_match(const basic_string<charT, ST, SA>& s,
                   const basic_regex<charT, traits>& e,
                   regex_constants::match_flag_type flags = regex_constants::match_default);

Returns: regex_match(s.begin(), s.end(), e, flags).

28.6.10.3 regex_search [re.alg.search]

🔗

template<class BidirectionalIterator, class Allocator, class charT, class traits>
  bool regex_search(BidirectionalIterator first, BidirectionalIterator last,
                    match_results<BidirectionalIterator, Allocator>& m,
                    const basic_regex<charT, traits>& e,
                    regex_constants::match_flag_type flags = regex_constants::match_default);

Preconditions: BidirectionalIterator models bidirectional_iterator ([iterator.concept.bidir]).

Effects: Determines whether there is some sub-sequence within [first, last) that matches the regular expression e.

The parameter flags is used to control how the expression is matched against the character sequence.

Returns true if such a sequence exists, false otherwise.

Postconditions: m.ready() == true in all cases.

If the function returns false, then the effect on parameter m is unspecified except that m.size() returns 0 and m.empty() returns true.

Otherwise the effects on parameter m are given in Table 119 .

Table 119: Effects of regex_search algorithm [tab:re.alg.search]

🔗	Element	Value
🔗	m.size()	1 + e.mark_count()
🔗	m.empty()	false
🔗	m.prefix().first	first
🔗	m.prefix().second	m[0].first
🔗	m.prefix().matched	m.prefix().first != m.prefix().second
🔗	m.suffix().first	m[0].second
🔗	m.suffix().second	last
🔗	m.suffix().matched	m.suffix().first != m.suffix().second
🔗	m[0].first	The start of the sequence of characters that matched the regular expression
🔗	m[0].second	The end of the sequence of characters that matched the regular expression
🔗	m[0].matched	true
🔗	m[n].first	For all integers 0 < n < m.size(), the start of the sequence that matched sub-expression n. Alternatively, if sub-expression n did not participate in the match, then last.
🔗	m[n].second	For all integers 0 < n < m.size(), the end of the sequence that matched sub-expression n. Alternatively, if sub-expression n did not participate in the match, then last.
🔗	m[n].matched	For all integers 0 < n < m.size(), true if sub-expression n participated in the match, false otherwise.

🔗

template<class charT, class Allocator, class traits>
  bool regex_search(const charT* str, match_results<const charT*, Allocator>& m,
                    const basic_regex<charT, traits>& e,
                    regex_constants::match_flag_type flags = regex_constants::match_default);

Returns: regex_search(str, str + char_traits<charT>::length(str), m, e, flags).

🔗

template<class ST, class SA, class Allocator, class charT, class traits>
  bool regex_search(const basic_string<charT, ST, SA>& s,
                    match_results<typename basic_string<charT, ST, SA>::const_iterator,
                                  Allocator>& m,
                    const basic_regex<charT, traits>& e,
                    regex_constants::match_flag_type flags = regex_constants::match_default);

Returns: regex_search(s.begin(), s.end(), m, e, flags).

🔗

template<class BidirectionalIterator, class charT, class traits>
  bool regex_search(BidirectionalIterator first, BidirectionalIterator last,
                    const basic_regex<charT, traits>& e,
                    regex_constants::match_flag_type flags = regex_constants::match_default);

Effects: Behaves “as if” by constructing an object what of type match_results<BidirectionalIterator> and returning regex_search(first, last, what, e, flags).

🔗

template<class charT, class traits>
  bool regex_search(const charT* str,
                    const basic_regex<charT, traits>& e,
                    regex_constants::match_flag_type flags = regex_constants::match_default);

Returns: regex_search(str, str + char_traits<charT>::length(str), e, flags).

🔗

template<class ST, class SA, class charT, class traits>
  bool regex_search(const basic_string<charT, ST, SA>& s,
                    const basic_regex<charT, traits>& e,
                    regex_constants::match_flag_type flags = regex_constants::match_default);

Returns: regex_search(s.begin(), s.end(), e, flags).

28.6.10.4 regex_replace [re.alg.replace]

🔗

template<class OutputIterator, class BidirectionalIterator,
          class traits, class charT, class ST, class SA>
  OutputIterator
    regex_replace(OutputIterator out,
                  BidirectionalIterator first, BidirectionalIterator last,
                  const basic_regex<charT, traits>& e,
                  const basic_string<charT, ST, SA>& fmt,
                  regex_constants::match_flag_type flags = regex_constants::match_default);
template<class OutputIterator, class BidirectionalIterator, class traits, class charT>
  OutputIterator
    regex_replace(OutputIterator out,
                  BidirectionalIterator first, BidirectionalIterator last,
                  const basic_regex<charT, traits>& e,
                  const charT* fmt,
                  regex_constants::match_flag_type flags = regex_constants::match_default);

Effects: Constructs a regex_iterator object i as if by regex_iterator<BidirectionalIterator, charT, traits> i(first, last, e, flags) and uses i to enumerate through all of the matches m of type match_results<BidirectionalIterator> that occur within the sequence [first, last).

If no such matches are found and !(flags & regex_constants::format_no_copy), then calls out = copy(first, last, out)

If any matches are found then, for each such match:

(1.1)
If !(flags & regex_constants::format_no_copy), calls out = copy(m.prefix().first, m.prefix().second, out)
(1.2)
Then calls out = m.format(out, fmt, flags) for the first form of the function and out = m.format(out, fmt, fmt + char_traits<charT>::length(fmt), flags) for the second.

Finally, if such a match is found and !(flags & regex_constants::format_no_copy), calls out = copy(last_m.suffix().first, last_m.suffix().second, out) where last_m is a copy of the last match found.

If flags & regex_constants::format_first_only is nonzero, then only the first match found is replaced.

Returns: out.

🔗

template<class traits, class charT, class ST, class SA, class FST, class FSA>
  basic_string<charT, ST, SA>
    regex_replace(const basic_string<charT, ST, SA>& s,
                  const basic_regex<charT, traits>& e,
                  const basic_string<charT, FST, FSA>& fmt,
                  regex_constants::match_flag_type flags = regex_constants::match_default);
template<class traits, class charT, class ST, class SA>
  basic_string<charT, ST, SA>
    regex_replace(const basic_string<charT, ST, SA>& s,
                  const basic_regex<charT, traits>& e,
                  const charT* fmt,
                  regex_constants::match_flag_type flags = regex_constants::match_default);

Effects: Constructs an empty string result of type basic_string<charT, ST, SA> and calls: regex_replace(back_inserter(result), s.begin(), s.end(), e, fmt, flags);

Returns: result.

🔗

template<class traits, class charT, class ST, class SA>
  basic_string<charT>
    regex_replace(const charT* s,
                  const basic_regex<charT, traits>& e,
                  const basic_string<charT, ST, SA>& fmt,
                  regex_constants::match_flag_type flags = regex_constants::match_default);
template<class traits, class charT>
  basic_string<charT>
    regex_replace(const charT* s,
                  const basic_regex<charT, traits>& e,
                  const charT* fmt,
                  regex_constants::match_flag_type flags = regex_constants::match_default);

Effects: Constructs an empty string result of type basic_string<charT> and calls: regex_replace(back_inserter(result), s, s + char_traits<charT>::length(s), e, fmt, flags);

Returns: result.

28.6.11 Regular expression iterators [re.iter]

28.6.11.1 Class template regex_iterator [re.regiter]

28.6.11.1.1 General [re.regiter.general]

The class template regex_iterator is an iterator adaptor.

It represents a new view of an existing iterator sequence, by enumerating all the occurrences of a regular expression within that sequence.

A regex_iterator uses regex_search to find successive regular expression matches within the sequence from which it was constructed.

After the iterator is constructed, and every time operator++ is used, the iterator finds and stores a value of match_results<BidirectionalIterator>.

If the end of the sequence is reached (regex_search returns false), the iterator becomes equal to the end-of-sequence iterator value.

The default constructor constructs an end-of-sequence iterator object, which is the only legitimate iterator to be used for the end condition.

The result of operator* on an end-of-sequence iterator is not defined.

For any other iterator value a const match_results<BidirectionalIterator>& is returned.

The result of operator-> on an end-of-sequence iterator is not defined.

For any other iterator value a const match_results<BidirectionalIterator>* is returned.

It is impossible to store things into regex_iterators.

Two end-of-sequence iterators are always equal.

An end-of-sequence iterator is not equal to a non-end-of-sequence iterator.

Two non-end-of-sequence iterators are equal when they are constructed from the same arguments.

An object of type regex_iterator that is not an end-of-sequence iterator holds a zero-length match if match[0].matched == true and match[0].first == match[0].second.

[Note 1:

For example, this can occur when the part of the regular expression that matched consists only of an assertion (such as '^', '$', '\b', '\B').

— end note]

28.6.11.1.2 Constructors [re.regiter.cnstr]

🔗

regex_iterator();

Effects: Constructs an end-of-sequence iterator.

🔗

regex_iterator(BidirectionalIterator a, BidirectionalIterator b,
               const regex_type& re,
               regex_constants::match_flag_type m = regex_constants::match_default);

Effects: Initializes begin and end to a and b, respectively, sets pregex to addressof(re), sets flags to m, then calls regex_search(begin, end, match, *pregex, flags).

If this call returns false the constructor sets *this to the end-of-sequence iterator.

28.6.11.1.3 Comparisons [re.regiter.comp]

🔗

bool operator==(const regex_iterator& right) const;

Returns: true if *this and right are both end-of-sequence iterators or if the following conditions all hold:

(1.1)
begin == right.begin,
(1.2)
end == right.end,
(1.3)
pregex == right.pregex,
(1.4)
flags == right.flags, and
(1.5)
match[0] == right.match[0];

otherwise false.

28.6.11.1.4 Indirection [re.regiter.deref]

🔗

const value_type& operator*() const;

Returns: match.

🔗

const value_type* operator->() const;

Returns: addressof(match).

28.6.11.1.5 Increment [re.regiter.incr]

🔗

regex_iterator& operator++();

Effects: Constructs a local variable start of type BidirectionalIterator and initializes it with the value of match[0].second.

If the iterator holds a zero-length match and start == end the operator sets *this to the end-of-sequence iterator and returns *this.

Otherwise, if the iterator holds a zero-length match, the operator calls: regex_search(start, end, match, *pregex, flags | regex_constants::match_not_null | regex_constants::match_continuous)

If the call returns true the operator returns *this.

Otherwise the operator increments start and continues as if the most recent match was not a zero-length match.

If the most recent match was not a zero-length match, the operator sets flags to flags | regex_constants::match_prev_avail and calls regex_search(start, end, match, *pregex, flags).

If the call returns false the iterator sets *this to the end-of-sequence iterator.

The iterator then returns *this.

In all cases in which the call to regex_search returns true, match.prefix().first shall be equal to the previous value of match[0].second, and for each index i in the half-open range [0, match.size()) for which match[i].matched is true, match.position(i) shall return distance(begin, match[i].first).

[Note 1:

This means that match.position(i) gives the offset from the beginning of the target sequence, which is often not the same as the offset from the sequence passed in the call to regex_search.

— end note]

It is unspecified how the implementation makes these adjustments.

[Note 2:

This means that an implementation can call an implementation-specific search function, in which case a program-defined specialization of regex_search will not be called.

— end note]

🔗

regex_iterator operator++(int);

Effects: As if by: regex_iterator tmp = *this; ++(*this); return tmp;

28.6.11.2 Class template regex_token_iterator [re.tokiter]

28.6.11.2.1 General [re.tokiter.general]

The class template regex_token_iterator is an iterator adaptor; that is to say it represents a new view of an existing iterator sequence, by enumerating all the occurrences of a regular expression within that sequence, and presenting one or more sub-expressions for each match found.

Each position enumerated by the iterator is a sub_match class template instance that represents what matched a particular sub-expression within the regular expression.

When class regex_token_iterator is used to enumerate a single sub-expression with index

- 1

the iterator performs field splitting: that is to say it enumerates one sub-expression for each section of the character container sequence that does not match the regular expression specified.

After it is constructed, the iterator finds and stores a value regex_iterator<BidirectionalIterator> position and sets the internal count N to zero.

It also maintains a sequence subs which contains a list of the sub-expressions which will be enumerated.

Every time operator++ is used the count N is incremented; if N exceeds or equals subs.size(), then the iterator increments member position and sets count N to zero.

If the end of sequence is reached (position is equal to the end of sequence iterator), the iterator becomes equal to the end-of-sequence iterator value, unless the sub-expression being enumerated has index

- 1

, in which case the iterator enumerates one last sub-expression that contains all the characters from the end of the last regular expression match to the end of the input sequence being enumerated, provided that this would not be an empty sub-expression.

The default constructor constructs an end-of-sequence iterator object, which is the only legitimate iterator to be used for the end condition.

The result of operator* on an end-of-sequence iterator is not defined.

For any other iterator value a const sub_match<BidirectionalIterator>& is returned.

The result of operator-> on an end-of-sequence iterator is not defined.

For any other iterator value a const sub_match<BidirectionalIterator>* is returned.

It is impossible to store things into regex_token_iterators.

Two end-of-sequence iterators are always equal.

An end-of-sequence iterator is not equal to a non-end-of-sequence iterator.

Two non-end-of-sequence iterators are equal when they are constructed from the same arguments.

namespace std { template<class BidirectionalIterator, class charT = typename iterator_traits<BidirectionalIterator>::value_type, class traits = regex_traits<charT>> class regex_token_iterator { public: using regex_type = basic_regex<charT, traits>; using iterator_category = forward_iterator_tag; using iterator_concept = input_iterator_tag; using value_type = sub_match<BidirectionalIterator>; using difference_type = ptrdiff_t; using pointer = const value_type*; using reference = const value_type&; regex_token_iterator(); regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, int submatch = 0, regex_constants::match_flag_type m = regex_constants::match_default); regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, const vector<int>& submatches, regex_constants::match_flag_type m = regex_constants::match_default); regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, initializer_list<int> submatches, regex_constants::match_flag_type m = regex_constants::match_default); template<size_t N> regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, const int (&submatches)[N], regex_constants::match_flag_type m = regex_constants::match_default); regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type&& re, int submatch = 0, regex_constants::match_flag_type m = regex_constants::match_default) = delete; regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type&& re, const vector<int>& submatches, regex_constants::match_flag_type m = regex_constants::match_default) = delete; regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type&& re, initializer_list<int> submatches, regex_constants::match_flag_type m = regex_constants::match_default) = delete; template<size_t N> regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type&& re, const int (&submatches)[N], regex_constants::match_flag_type m = regex_constants::match_default) = delete; regex_token_iterator(const regex_token_iterator&); regex_token_iterator& operator=(const regex_token_iterator&); bool operator==(const regex_token_iterator&) const; bool operator==(default_sentinel_t) const { return *this == regex_token_iterator(); } const value_type& operator*() const; const value_type* operator->() const; regex_token_iterator& operator++(); regex_token_iterator operator++(int); private: using position_iterator = regex_iterator<BidirectionalIterator, charT, traits>; // exposition only position_iterator position; // exposition only const value_type* result; // exposition only value_type suffix; // exposition only size_t N; // exposition only vector<int> subs; // exposition only }; }

A suffix iterator is a regex_token_iterator object that points to a final sequence of characters at the end of the target sequence.

In a suffix iterator the member result holds a pointer to the data member suffix, the value of the member suffix.match is true, suffix.first points to the beginning of the final sequence, and suffix.second points to the end of the final sequence.

[Note 1:

For a suffix iterator, data member suffix.first is the same as the end of the last match found, and suffix.second is the same as the end of the target sequence.

— end note]

The current match is (*position).prefix() if subs[N] == -1, or (*position)[subs[N]] for any other value of subs[N].

28.6.11.2.2 Constructors [re.tokiter.cnstr]

🔗

regex_token_iterator();

Effects: Constructs the end-of-sequence iterator.

🔗

regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
                     const regex_type& re,
                     int submatch = 0,
                     regex_constants::match_flag_type m = regex_constants::match_default);

regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
                     const regex_type& re,
                     const vector<int>& submatches,
                     regex_constants::match_flag_type m = regex_constants::match_default);

regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
                     const regex_type& re,
                     initializer_list<int> submatches,
                     regex_constants::match_flag_type m = regex_constants::match_default);

template<size_t N>
  regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
                       const regex_type& re,
                       const int (&submatches)[N],
                       regex_constants::match_flag_type m = regex_constants::match_default);

Preconditions: Each of the initialization values of submatches is >= -1.

Effects: The first constructor initializes the member subs to hold the single value submatch.

The second, third, and fourth constructors initialize the member subs to hold a copy of the sequence of integer values pointed to by the iterator range [begin(submatches), end(submatches)).

Each constructor then sets N to 0, and position to position_iterator(a, b, re, m).

If position is not an end-of-sequence iterator the constructor sets result to the address of the current match.

Otherwise if any of the values stored in subs is equal to

- 1

the constructor sets *this to a suffix iterator that points to the range [a, b), otherwise the constructor sets *this to an end-of-sequence iterator.

28.6.11.2.3 Comparisons [re.tokiter.comp]

🔗

bool operator==(const regex_token_iterator& right) const;

Returns: true if *this and right are both end-of-sequence iterators, or if *this and right are both suffix iterators and suffix == right.suffix; otherwise returns false if *this or right is an end-of-sequence iterator or a suffix iterator.

Otherwise returns true if position == right.position, N == right.N, and subs == right.subs.

Otherwise returns false.

28.6.11.2.4 Indirection [re.tokiter.deref]

🔗

const value_type& operator*() const;

Returns: *result.

🔗

const value_type* operator->() const;

Returns: result.

28.6.11.2.5 Increment [re.tokiter.incr]

🔗

regex_token_iterator& operator++();

Effects: Constructs a local variable prev of type position_iterator, initialized with the value of position.

If *this is a suffix iterator, sets *this to an end-of-sequence iterator.

Otherwise, if N + 1 < subs.size(), increments N and sets result to the address of the current match.

Otherwise, sets N to 0 and increments position.

If position is not an end-of-sequence iterator the operator sets result to the address of the current match.

Otherwise, if any of the values stored in subs is equal to

- 1

and prev->suffix().length() is not 0 the operator sets *this to a suffix iterator that points to the range [prev->suffix().first, prev->suffix().second).

Otherwise, sets *this to an end-of-sequence iterator.

Returns: *this

🔗

regex_token_iterator& operator++(int);

Effects: Constructs a copy tmp of *this, then calls ++(*this).

Returns: tmp.

28.6.12 Modified ECMAScript regular expression grammar [re.grammar]

The regular expression grammar recognized by basic_regex objects constructed with the ECMAScript flag is that specified by ECMA-262, except as specified below.

Objects of type specialization of basic_regex store within themselves a default-constructed instance of their traits template parameter, henceforth referred to as traits_inst.

This traits_inst object is used to support localization of the regular expression; basic_regex member functions shall not call any locale dependent C or C++ API, including the formatted string input functions.

Instead they shall call the appropriate traits member function to achieve the required effect.

The following productions within the ECMAScript grammar are modified as follows:

ClassAtom ::
-
ClassAtomNoDash
ClassAtomExClass
ClassAtomCollatingElement
ClassAtomEquivalence

IdentityEscape ::
SourceCharacter but not c

The following new productions are then added:

ClassAtomExClass ::
[: ClassName :]

ClassAtomCollatingElement ::
[. ClassName .]

ClassAtomEquivalence ::
[= ClassName =]

ClassName ::
ClassNameCharacter
ClassNameCharacter ClassName

ClassNameCharacter ::
SourceCharacter but not one of . or = or :

The productions ClassAtomExClass, ClassAtomCollatingElement and ClassAtomEquivalence provide functionality equivalent to that of the same features in regular expressions in POSIX.

The regular expression grammar may be modified by any regex_constants::syntax_option_type flags specified when constructing an object of type specialization of basic_regex according to the rules in Table 113 .

A ClassName production, when used in ClassAtomExClass, is not valid if traits_inst.lookup_classname returns zero for that name.

The names recognized as valid ClassNames are determined by the type of the traits class, but at least the following names shall be recognized: alnum, alpha, blank, cntrl, digit, graph, lower, print, punct, space, upper, xdigit, d, s, w.

In addition the following expressions shall be equivalent:

\d and [[:digit:]] \D and [^[:digit:]] \s and [[:space:]] \S and [^[:space:]] \w and [_[:alnum:]] \W and [^_[:alnum:]]

A ClassName production when used in a ClassAtomCollatingElement production is not valid if the value returned by traits_inst.lookup_collatename for that name is an empty string.

The results from multiple calls to traits_inst.lookup_classname can be bitwise or'ed together and subsequently passed to traits_inst.isctype.

A ClassName production when used in a ClassAtomEquivalence production is not valid if the value returned by traits_inst.lookup_collatename for that name is an empty string or if the value returned by traits_inst.transform_primary for the result of the call to traits_inst.lookup_collatename is an empty string.

When the sequence of characters being transformed to a finite state machine contains an invalid class name the translator shall throw an exception object of type regex_error.

If the CV of a UnicodeEscapeSequence is greater than the largest value that can be held in an object of type charT the translator shall throw an exception object of type regex_error.

[Note 1:

This means that values of the form "\uxxxx" that do not fit in a character are invalid.

— end note]

Where the regular expression grammar requires the conversion of a sequence of characters to an integral value, this is accomplished by calling traits_inst.value.

The behavior of the internal finite state machine representation when used to match a sequence of characters is as described in ECMA-262.

The behavior is modified according to any match_flag_type flags ([re.matchflag]) specified when using the regular expression object in one of the regular expression algorithms ([re.alg]).

The behavior is also localized by interaction with the traits class template parameter as follows:

(14.1)
During matching of a regular expression finite state machine against a sequence of characters, two characters c and d are compared using the following rules:
- (14.1.1)
  if (flags() & regex_constants::icase) the two characters are equal if traits_inst.translate_nocase(c) == traits_inst.translate_nocase(d);
- (14.1.2)
  otherwise, if flags() & regex_constants::collate the two characters are equal if traits_inst.translate(c) == traits_inst.translate(d);
- (14.1.3)
  otherwise, the two characters are equal if c == d.
(14.2)
During matching of a regular expression finite state machine against a sequence of characters, comparison of a collating element range c1-c2 against a character c is conducted as follows: if flags() & regex_constants::collate is false then the character c is matched if c1 <= c && c <= c2, otherwise c is matched in accordance with the following algorithm:
string_type str1 = string_type(1, flags() & icase ? traits_inst.translate_nocase(c1) : traits_inst.translate(c1)); string_type str2 = string_type(1, flags() & icase ? traits_inst.translate_nocase(c2) : traits_inst.translate(c2)); string_type str = string_type(1, flags() & icase ? traits_inst.translate_nocase(c) : traits_inst.translate(c)); return traits_inst.transform(str1.begin(), str1.end()) <= traits_inst.transform(str.begin(), str.end()) && traits_inst.transform(str.begin(), str.end()) <= traits_inst.transform(str2.begin(), str2.end());
(14.3)
During matching of a regular expression finite state machine against a sequence of characters, testing whether a collating element is a member of a primary equivalence class is conducted by first converting the collating element and the equivalence class to sort keys using traits::transform_primary, and then comparing the sort keys for equality.
(14.4)
During matching of a regular expression finite state machine against a sequence of characters, a character c is a member of a character class designated by an iterator range [first, last) if traits_inst.isctype(c, traits_inst.lookup_classname(first, last, flags() & icase)) is true.

28.7 Null-terminated sequence utilities [text.c.strings]

28.7.1 Header <cctype> synopsis [cctype.syn]

🔗

namespace std { int isalnum(int c); int isalpha(int c); int isblank(int c); int iscntrl(int c); int isdigit(int c); int isgraph(int c); int islower(int c); int isprint(int c); int ispunct(int c); int isspace(int c); int isupper(int c); int isxdigit(int c); int tolower(int c); int toupper(int c); }

The contents and meaning of the header are the same as the C standard library header <ctype.h>.

28.7.2 Header <cwctype> synopsis [cwctype.syn]

🔗

namespace std { using wint_t = see below; using wctrans_t = see below; using wctype_t = see below; int iswalnum(wint_t wc); int iswalpha(wint_t wc); int iswblank(wint_t wc); int iswcntrl(wint_t wc); int iswdigit(wint_t wc); int iswgraph(wint_t wc); int iswlower(wint_t wc); int iswprint(wint_t wc); int iswpunct(wint_t wc); int iswspace(wint_t wc); int iswupper(wint_t wc); int iswxdigit(wint_t wc); int iswctype(wint_t wc, wctype_t desc); wctype_t wctype(const char* property); wint_t towlower(wint_t wc); wint_t towupper(wint_t wc); wint_t towctrans(wint_t wc, wctrans_t desc); wctrans_t wctrans(const char* property); } #define WEOF see below

The contents and meaning of the header are the same as the C standard library header <wctype.h>.

28.7.3 Header <cwchar> synopsis [cwchar.syn]

🔗

namespace std { using size_t = see [support.types.layout]; // freestanding using mbstate_t = see below; // freestanding using wint_t = see below; // freestanding struct tm; int fwprintf(FILE* stream, const wchar_t* format, ...); int fwscanf(FILE* stream, const wchar_t* format, ...); int swprintf(wchar_t* s, size_t n, const wchar_t* format, ...); int swscanf(const wchar_t* s, const wchar_t* format, ...); int vfwprintf(FILE* stream, const wchar_t* format, va_list arg); int vfwscanf(FILE* stream, const wchar_t* format, va_list arg); int vswprintf(wchar_t* s, size_t n, const wchar_t* format, va_list arg); int vswscanf(const wchar_t* s, const wchar_t* format, va_list arg); int vwprintf(const wchar_t* format, va_list arg); int vwscanf(const wchar_t* format, va_list arg); int wprintf(const wchar_t* format, ...); int wscanf(const wchar_t* format, ...); wint_t fgetwc(FILE* stream); wchar_t* fgetws(wchar_t* s, int n, FILE* stream); wint_t fputwc(wchar_t c, FILE* stream); int fputws(const wchar_t* s, FILE* stream); int fwide(FILE* stream, int mode); wint_t getwc(FILE* stream); wint_t getwchar(); wint_t putwc(wchar_t c, FILE* stream); wint_t putwchar(wchar_t c); wint_t ungetwc(wint_t c, FILE* stream); double wcstod(const wchar_t* nptr, wchar_t** endptr); float wcstof(const wchar_t* nptr, wchar_t** endptr); long double wcstold(const wchar_t* nptr, wchar_t** endptr); long int wcstol(const wchar_t* nptr, wchar_t** endptr, int base); long long int wcstoll(const wchar_t* nptr, wchar_t** endptr, int base); unsigned long int wcstoul(const wchar_t* nptr, wchar_t** endptr, int base); unsigned long long int wcstoull(const wchar_t* nptr, wchar_t** endptr, int base); wchar_t* wcscpy(wchar_t* s1, const wchar_t* s2); // freestanding wchar_t* wcsncpy(wchar_t* s1, const wchar_t* s2, size_t n); // freestanding wchar_t* wmemcpy(wchar_t* s1, const wchar_t* s2, size_t n); // freestanding wchar_t* wmemmove(wchar_t* s1, const wchar_t* s2, size_t n); // freestanding wchar_t* wcscat(wchar_t* s1, const wchar_t* s2); // freestanding wchar_t* wcsncat(wchar_t* s1, const wchar_t* s2, size_t n); // freestanding int wcscmp(const wchar_t* s1, const wchar_t* s2); // freestanding int wcscoll(const wchar_t* s1, const wchar_t* s2); int wcsncmp(const wchar_t* s1, const wchar_t* s2, size_t n); // freestanding size_t wcsxfrm(wchar_t* s1, const wchar_t* s2, size_t n); int wmemcmp(const wchar_t* s1, const wchar_t* s2, size_t n); // freestanding const wchar_t* wcschr(const wchar_t* s, wchar_t c); // freestanding; see [library.c] wchar_t* wcschr(wchar_t* s, wchar_t c); // freestanding; see [library.c] size_t wcscspn(const wchar_t* s1, const wchar_t* s2); // freestanding const wchar_t* wcspbrk(const wchar_t* s1, const wchar_t* s2); // freestanding; see [library.c] wchar_t* wcspbrk(wchar_t* s1, const wchar_t* s2); // freestanding; see [library.c] const wchar_t* wcsrchr(const wchar_t* s, wchar_t c); // freestanding; see [library.c] wchar_t* wcsrchr(wchar_t* s, wchar_t c); // freestanding; see [library.c] size_t wcsspn(const wchar_t* s1, const wchar_t* s2); // freestanding const wchar_t* wcsstr(const wchar_t* s1, const wchar_t* s2); // freestanding; see [library.c] wchar_t* wcsstr(wchar_t* s1, const wchar_t* s2); // freestanding; see [library.c] wchar_t* wcstok(wchar_t* s1, const wchar_t* s2, wchar_t** ptr); // freestanding const wchar_t* wmemchr(const wchar_t* s, wchar_t c, size_t n); // freestanding; see [library.c] wchar_t* wmemchr(wchar_t* s, wchar_t c, size_t n); // freestanding; see [library.c] size_t wcslen(const wchar_t* s); // freestanding wchar_t* wmemset(wchar_t* s, wchar_t c, size_t n); // freestanding size_t wcsftime(wchar_t* s, size_t maxsize, const wchar_t* format, const tm* timeptr); wint_t btowc(int c); int wctob(wint_t c); // [c.mb.wcs], multibyte / wide string and character conversion functions int mbsinit(const mbstate_t* ps); size_t mbrlen(const char* s, size_t n, mbstate_t* ps); size_t mbrtowc(wchar_t* pwc, const char* s, size_t n, mbstate_t* ps); size_t wcrtomb(char* s, wchar_t wc, mbstate_t* ps); size_t mbsrtowcs(wchar_t* dst, const char** src, size_t len, mbstate_t* ps); size_t wcsrtombs(char* dst, const wchar_t** src, size_t len, mbstate_t* ps); } #define NULL see [support.types.nullptr] // freestanding #define WCHAR_MAX see below // freestanding #define WCHAR_MIN see below // freestanding #define WEOF see below // freestanding

The contents and meaning of the header <cwchar> are the same as the C standard library header <wchar.h>, except that it does not declare a type wchar_t.

[Note 1:

The functions wcschr, wcspbrk, wcsrchr, wcsstr, and wmemchr have different signatures in this document, but they have the same behavior as in the C standard library ([library.c]).

— end note]

28.7.4 Header <cuchar> synopsis [cuchar.syn]

🔗

namespace std { using mbstate_t = see below; using size_t = see [support.types.layout]; size_t mbrtoc8(char8_t* pc8, const char* s, size_t n, mbstate_t* ps); size_t c8rtomb(char* s, char8_t c8, mbstate_t* ps); size_t mbrtoc16(char16_t* pc16, const char* s, size_t n, mbstate_t* ps); size_t c16rtomb(char* s, char16_t c16, mbstate_t* ps); size_t mbrtoc32(char32_t* pc32, const char* s, size_t n, mbstate_t* ps); size_t c32rtomb(char* s, char32_t c32, mbstate_t* ps); }

The contents and meaning of the header are the same as the C standard library header <uchar.h>, except that it declares the additional mbrtoc8 and c8rtomb functions and does not declare types char16_t nor char32_t.

28.7.5 Multibyte / wide string and character conversion functions [c.mb.wcs]

[Note 1:

The headers <cstdlib>, <cuchar>, and <cwchar> declare the functions described in this subclause.

— end note]

🔗

int mbsinit(const mbstate_t* ps);
int mblen(const char* s, size_t n);
size_t mbstowcs(wchar_t* pwcs, const char* s, size_t n);
size_t wcstombs(char* s, const wchar_t* pwcs, size_t n);

Effects: These functions have the semantics specified in the C standard library.

See also: ISO/IEC 9899:2018, 7.22.7.1, 7.22.8, 7.29.6.2.1

🔗

int mbtowc(wchar_t* pwc, const char* s, size_t n);
int wctomb(char* s, wchar_t wchar);

Effects: These functions have the semantics specified in the C standard library.

Remarks: Calls to these functions may introduce a data race ([res.on.data.races]) with other calls to the same function.

🔗	State	stdio equivalent
🔗	basefield == oct	%o
🔗	basefield == hex	%X
🔗	basefield == 0	%i
🔗	signed integral type	%d
🔗	unsigned integral type	%u

🔗	State	stdio equivalent
🔗	floatfield == ios_base::fixed && !uppercase	%f
🔗	floatfield == ios_base::fixed	%F
🔗	floatfield == ios_base::scientific && !uppercase	%e
🔗	floatfield == ios_base::scientific	%E
🔗	floatfield == (ios_base::fixed \| ios_base::scientific) && !uppercase	%a
🔗	floatfield == (ios_base::fixed \| ios_base::scientific)	%A
🔗	!uppercase	%g
🔗	otherwise	%G

🔗	Type(s)	State	stdio equivalent
🔗	an integral type	showpos	+
🔗		showbase	#
🔗	a floating-point type	showpos	+
🔗		showpoint	#

🔗	State	Location
🔗	adjustfield == ios_base::left	pad after
🔗	adjustfield == ios_base::right	pad before
🔗	adjustfield == internal and a sign occurs in the representation	pad after the sign
🔗	adjustfield == internal and representation after stage 1 began with 0x or 0X	pad after x or X
🔗	otherwise	pad before

🔗	Type	Length modifier
🔗	short	h
🔗	unsigned short	h
🔗	long	l
🔗	unsigned long	l
🔗	long long	ll
🔗	unsigned long long	ll
🔗	double	l
🔗	long double	L

🔗	fprintf	isprint	iswdigit	localeconv	tolower
🔗	fscanf	ispunct	iswgraph	mblen	toupper
🔗	isalnum	isspace	iswlower	mbstowcs	towlower
🔗	isalpha	isupper	iswprint	mbtowc	towupper
🔗	isblank	iswalnum	iswpunct	setlocale	wcscoll
🔗	iscntrl	iswalpha	iswspace	strcoll	wcstod
🔗	isdigit	iswblank	iswupper	strerror	wcstombs
🔗	isgraph	iswcntrl	iswxdigit	strtod	wcsxfrm
🔗	islower	iswctype	isxdigit	strxfrm	wctomb

28 Text processing library [text]

28.1 General [text.general]

28.2 Primitive numeric conversions [charconv]

28.2.1 Header <charconv> synopsis [charconv.syn]

28.2.2 Primitive numeric output conversion [charconv.to.chars]

28.2.3 Primitive numeric input conversion [charconv.from.chars]

28.3 Localization library [localization]

28.3.1 General [localization.general]

28.3.2 Header <locale> synopsis [locale.syn]

28.3.3 Locales [locales]

28.3.3.1 Class locale [locale]

28.3.3.1.1 General [locale.general]

28.3.3.1.2 Types [locale.types]

28.3.3.1.2.1 Type locale​::​category [locale.category]

28.3.3.1.2.2 Class locale​::​facet [locale.facet]

28.3.3.1.2.3 Class locale​::​id [locale.id]

28.3.3.1.3 Constructors and destructor [locale.cons]

28.3.3.1.4 Members [locale.members]

28.3.3.1.5 Operators [locale.operators]

28.3.3.1.6 Static members [locale.statics]

28.3.3.2 locale globals [locale.global.templates]

28.3.3.3 Convenience interfaces [locale.convenience]

28.3.3.3.1 Character classification [classification]

28.3.3.3.2 Character conversions [conversions.character]

28.3.4 Standard locale categories [locale.categories]

28.3.4.1 General [locale.categories.general]

28.3.4.2 The ctype category [category.ctype]

28.3.4.2.1 General [category.ctype.general]

28.3.4.2.2 Class template ctype [locale.ctype]

28.3.4.2.2.1 General [locale.ctype.general]

28.3.4.2.2.2 ctype members [locale.ctype.members]

28.3.4.2.2.3 ctype virtual functions [locale.ctype.virtuals]

28.3.4.2.3 Class template ctype_byname [locale.ctype.byname]

28.3.4.2.4 ctype<char> specialization [facet.ctype.special]

28.3.4.2.4.1 General [facet.ctype.special.general]

28.3.4.2.4.2 Destructor [facet.ctype.char.dtor]

28.3.4.2.4.3 Members [facet.ctype.char.members]

28.3.4.2.4.4 Static members [facet.ctype.char.statics]

28.3.4.2.4.5 Virtual functions [facet.ctype.char.virtuals]

28.3.4.2.5 Class template codecvt [locale.codecvt]

28.3.4.2.5.1 General [locale.codecvt.general]

28.3.4.2.5.2 Members [locale.codecvt.members]

28.3.4.2.5.3 Virtual functions [locale.codecvt.virtuals]

28.3.4.2.6 Class template codecvt_byname [locale.codecvt.byname]

28.3.4.3 The numeric category [category.numeric]

28.3.4.3.1 General [category.numeric.general]

28.3.4.3.2 Class template num_get [locale.num.get]

28.3.4.3.2.1 General [locale.num.get.general]

28.3.4.3.2.2 Members [facet.num.get.members]

28.3.4.3.2.3 Virtual functions [facet.num.get.virtuals]

28.3.4.3.3 Class template num_put [locale.nm.put]

28.3.4.3.3.1 General [locale.nm.put.general]

28.3.4.3.3.2 Members [facet.num.put.members]

28.3.4.3.3.3 Virtual functions [facet.num.put.virtuals]

28.3.4.4 The numeric punctuation facet [facet.numpunct]

28.3.4.4.1 Class template numpunct [locale.numpunct]

28.3.4.4.1.1 General [locale.numpunct.general]

28.3.4.4.1.2 Members [facet.numpunct.members]

28.3.4.4.1.3 Virtual functions [facet.numpunct.virtuals]

28.3.4.4.2 Class template numpunct_byname [locale.numpunct.byname]

28.3.4.5 The collate category [category.collate]

28.3.4.5.1 Class template collate [locale.collate]

28.3.4.5.1.1 General [locale.collate.general]

28.3.4.5.1.2 Members [locale.collate.members]

28.3.4.5.1.3 Virtual functions [locale.collate.virtuals]

28.3.4.5.2 Class template collate_byname [locale.collate.byname]

28.3.4.6 The time category [category.time]

28.3.4.6.1 General [category.time.general]

28.3.4.6.2 Class template time_get [locale.time.get]

28.3.4.6.2.1 General [locale.time.get.general]

28.3.4.6.2.2 Members [locale.time.get.members]

28.3.4.6.2.3 Virtual functions [locale.time.get.virtuals]

28.3.4.6.3 Class template time_get_byname [locale.time.get.byname]

28.3.4.6.4 Class template time_put [locale.time.put]

28.3.4.6.4.1 General [locale.time.put.general]

28.3.4.6.4.2 Members [locale.time.put.members]

28.3.4.6.4.3 Virtual functions [locale.time.put.virtuals]

28.3.4.6.5 Class template time_put_byname [locale.time.put.byname]

28.3.4.7 The monetary category [category.monetary]

28.3.4.7.1 General [category.monetary.general]

28.3.3.1.2.1 Type locale::category [locale.category]

28.3.3.1.2.2 Class locale::facet [locale.facet]

28.3.3.1.2.3 Class locale::id [locale.id]

28.4.2.5 Class text_encoding::aliases_view [text.encoding.aliases]

28.4.2.6 Enumeration text_encoding::id [text.encoding.id]

28.6.4 Namespace std::regex_constants [re.const]