20 General utilities library [utilities]

20.20 Formatting [format]

20.20.5 Formatter [format.formatter]

20.20.5.1 Formatter requirements [formatter.requirements]

A type F meets the Formatter requirements if:
Given character type charT, output iterator type Out, and formatting argument type T, in Table 69:
  • f is a value of type F,
  • u is an lvalue of type T,
  • t is a value of a type convertible to (possibly const) T,
  • PC is basic_­format_­parse_­context<charT>,
  • FC is basic_­format_­context<Out, charT>,
  • pc is an lvalue of type PC, and
  • 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 empty then either pc.begin() == pc.end() or *pc.begin() == '}'.
Table 69: Formatter requirements [tab:formatter]
Expression
Return type
Requirement
f.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(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(), 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.

20.20.5.2 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.
[Note 1:
Enabled specializations meet the Formatter requirements, and disabled specializations do not.
— end note]
Each header that declares the template formatter provides the following enabled specializations:
  • The specializations template<> struct formatter<char, char>; template<> struct formatter<char, wchar_t>; template<> struct formatter<wchar_t, wchar_t>;
  • For each charT, the string type specializations template<> struct formatter<charT*, charT>; template<> struct formatter<const charT*, charT>; template<size_t N> struct formatter<const 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>;
  • 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>;
  • 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].
[Note 2:
Specializations such as formatter<wchar_­t, char> and formatter<const char*, wchar_­t> that would require implicit multibyte / wide string or character conversion are disabled.
— end note]
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 except as noted otherwise.
If F is a disabled specialization of formatter, these values are false:
  • is_­default_­constructible_­v<F>,
  • is_­copy_­constructible_­v<F>,
  • is_­move_­constructible_­v<F>,
  • is_­copy_­assignable_­v<F>, and
  • is_­move_­assignable_­v<F>.
An enabled specialization formatter<T, charT> meets the Formatter requirements ([formatter.requirements]).
[Example 1: #include <format> 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) { 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]

20.20.5.3 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, size_t num_args = 0) 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); }; }
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.
constexpr explicit basic_format_parse_context(basic_string_view<charT> fmt, size_t num_args = 0) noexcept;
Effects: Initializes begin_­ with fmt.begin(), end_­ with fmt.end(), indexing_­ with unknown, next_­arg_­id_­ with 0, and num_­args_­ with num_­args.
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, equivalent to: if (indexing_ == unknown) indexing_ = automatic; return next_arg_id_++;
Throws: format_­error if indexing_­ == manual which indicates mixing of automatic and manual argument indexing.
constexpr void check_arg_id(size_t id);
Effects: If indexing_­ != automatic, equivalent to: if (indexing_ == unknown) indexing_ = manual;
Throws: format_­error if indexing_­ == automatic which indicates mixing of automatic and manual argument indexing.
Remarks: Call expressions where id >= num_­args_­ are not core constant expressions ([expr.const]).

20.20.5.4 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 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; 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.
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;
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();
Returns: out_­.
void advance_to(iterator it);
Effects: Equivalent to: out_­ = 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 get_char = [&]() { return iter != ctx.end() ? *iter : 0; }; if (get_char() != '{') return iter; ++iter; char c = get_char(); if (!isdigit(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) { int width = visit_format_arg([](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; }, ctx.arg(width_arg_id)); 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]