27 Strings library [strings]

27.2 Character traits [char.traits]

27.2.1 General [char.traits.general]

Subclause [char.traits] defines requirements on classes representing character traits, and defines a class template char_traits<charT>, along with five specializations, char_traits<char>, char_traits<char8_t>, char_traits<char16_t>, char_traits<char32_t>, and char_traits<wchar_t>, that meet those requirements.

Most classes specified in [string.classes], [string.view], and [input.output] need a set of related types and functions to complete the definition of their semantics.

These types and functions are provided as a set of member typedef-names and functions in the template parameter traits used by each such template.

Subclause [char.traits] defines the semantics of these members.

To specialize those templates to generate a string, string view, or iostream class to handle a particular character container type ([defns.character.container]) C, that and its related character traits class X are passed as a pair of parameters to the string, string view, or iostream template as parameters charT and traits.

If X::char_type is not the same type as C, the program is ill-formed.

27.2.2 Character traits requirements [char.traits.require]

In Table 82, X denotes a traits class defining types and functions for the character container type C; c and d denote values of type C; p and q denote values of type const C*; s denotes a value of type C*; n, i and j denote values of type size_t; e and f denote values of type X::int_type; pos denotes a value of type X::pos_type; and r denotes an lvalue of type C.

No expression which is part of the character traits requirements specified in [char.traits.require] shall exit via an exception.

Table 82: Character traits requirements [tab:char.traits.req]

🔗	Expression	Return type	Assertion/note	Complexity
🔗			pre-/post-condition
🔗	X::char_type	C		compile-time
🔗	X::int_type		(described in [char.traits.typedefs])	compile-time
🔗	X::off_type		(described in [iostreams.limits.pos] and [iostream.forward])	compile-time
🔗	X::pos_type		(described in [iostreams.limits.pos] and [iostream.forward])	compile-time
🔗	X::state_type		(described in [char.traits.typedefs])	compile-time
🔗	X::eq(c,d)	bool	Returns: whether c is to be treated as equal to d.	constant
🔗	X::lt(c,d)	bool	Returns: whether c is to be treated as less than d.	constant
🔗	X::compare(p,q,n)	int	Returns: 0 if for each i in [0,n), X::eq(p[i],q[i]) is true; else, a negative value if, for some j in [0,n), X::lt(p[j],q[j]) is true and for each i in [0,j) X::eq(p[i],q[i]) is true; else a positive value.	linear
🔗	X::length(p)	size_t	Returns: the smallest i such that X::eq(p[i],charT()) is true.	linear
🔗	X::find(p,n,c)	const X::char_type*	Returns: the smallest q in [p,p+n) such that X::eq(*q,c) is true, nullptr otherwise.	linear
🔗	X::move(s,p,n)	X::char_type*	for each i in [0, n), performs X::assign(s[i],p[i]). Copies correctly even where the ranges [p, p+n) and [s, s+n) overlap. Returns: s.	linear
🔗	X::copy(s,p,n)	X::char_type*	Preconditions: The ranges [p, p+n) and [s, s+n) do not overlap. Returns: s. for each i in [0, n), performs X::assign(s[i],p[i]).	linear
🔗	X::assign(r,d)	(not used)	assigns r=d.	constant
🔗	X::assign(s,n,c)	X::char_type*	for each i in [0, n), performs X::assign(s[i],c). Returns: s.	linear
🔗	X::not_eof(e)	int_type	Returns: e if X::eq_int_type(e,X::eof()) is false, otherwise a value f such that X::eq_int_type(f,X::eof()) is false.	constant
🔗	X::to_char_type(e)	X::char_type	Returns: if for some c, X::eq_int_type(e,X::to_int_type(c)) is true, c; else some unspecified value.	constant
🔗	X::to_int_type(c)	X::int_type	Returns: some value e, constrained by the definitions of to_char_type and eq_int_type.	constant
🔗	X::eq_int_type(e,f)	bool	Returns: for all c and d, X::eq(c,d) is equal to X::eq_int_type(X::to_int_type(c), X::to_int_type(d)); otherwise, yields true if e and f are both copies of X::eof(); otherwise, yields false if one of e and f is a copy of X::eof() and the other is not; otherwise the value is unspecified.	constant
🔗	X::eof()	X::int_type	Returns: a value e such that X::eq_int_type(e,X::to_int_type(c)) is false for all values c.	constant

The class template

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template<class charT> struct char_traits; is provided in the header <string> as a basis for explicit specializations.

27.2.3 Traits typedefs [char.traits.typedefs]

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using int_type = see below;

Preconditions: int_type shall be able to represent all of the valid characters converted from the corresponding char_type values, as well as an end-of-file value, eof().215

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using state_type = see below;

Preconditions: state_type meets the Cpp17Destructible (Table 35), Cpp17CopyAssignable (Table 34), Cpp17CopyConstructible (Table 32), and Cpp17DefaultConstructible (Table 30) requirements.

215)215)

If eof() can be held in char_type then some iostreams operations can give surprising results.

27.2.4 char_traits specializations [char.traits.specializations]

27.2.4.1 General [char.traits.specializations.general]

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namespace std { template<> struct char_traits<char>; template<> struct char_traits<char8_t>; template<> struct char_traits<char16_t>; template<> struct char_traits<char32_t>; template<> struct char_traits<wchar_t>; }

The header <string> defines five specializations of the class template char_traits: char_traits<char>, char_traits<char8_t>, char_traits<char16_t>, char_traits<char32_t>, and char_traits<wchar_t>.

27.2.4.2 struct char_traits<char> [char.traits.specializations.char]

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namespace std { template<> struct char_traits<char> { using char_type = char; using int_type = int; using off_type = streamoff; using pos_type = streampos; using state_type = mbstate_t; using comparison_category = strong_ordering; static constexpr void assign(char_type& c1, const char_type& c2) noexcept; static constexpr bool eq(char_type c1, char_type c2) noexcept; static constexpr bool lt(char_type c1, char_type c2) noexcept; static constexpr int compare(const char_type* s1, const char_type* s2, size_t n); static constexpr size_t length(const char_type* s); static constexpr const char_type* find(const char_type* s, size_t n, const char_type& a); static constexpr char_type* move(char_type* s1, const char_type* s2, size_t n); static constexpr char_type* copy(char_type* s1, const char_type* s2, size_t n); static constexpr char_type* assign(char_type* s, size_t n, char_type a); static constexpr int_type not_eof(int_type c) noexcept; static constexpr char_type to_char_type(int_type c) noexcept; static constexpr int_type to_int_type(char_type c) noexcept; static constexpr bool eq_int_type(int_type c1, int_type c2) noexcept; static constexpr int_type eof() noexcept; }; }

The type mbstate_t is defined in <cwchar> and can represent any of the conversion states that can occur in an implementation-defined set of supported multibyte character encoding rules.

The two-argument member assign is defined identically to the built-in operator =.

The two-argument members eq and lt are defined identically to the built-in operators == and < for type unsigned char.

The member eof() returns EOF.

27.2.4.3 struct char_traits<char8_t> [char.traits.specializations.char8.t]

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namespace std { template<> struct char_traits<char8_t> { using char_type = char8_t; using int_type = unsigned int; using off_type = streamoff; using pos_type = u8streampos; using state_type = mbstate_t; using comparison_category = strong_ordering; static constexpr void assign(char_type& c1, const char_type& c2) noexcept; static constexpr bool eq(char_type c1, char_type c2) noexcept; static constexpr bool lt(char_type c1, char_type c2) noexcept; static constexpr int compare(const char_type* s1, const char_type* s2, size_t n); static constexpr size_t length(const char_type* s); static constexpr const char_type* find(const char_type* s, size_t n, const char_type& a); static constexpr char_type* move(char_type* s1, const char_type* s2, size_t n); static constexpr char_type* copy(char_type* s1, const char_type* s2, size_t n); static constexpr char_type* assign(char_type* s, size_t n, char_type a); static constexpr int_type not_eof(int_type c) noexcept; static constexpr char_type to_char_type(int_type c) noexcept; static constexpr int_type to_int_type(char_type c) noexcept; static constexpr bool eq_int_type(int_type c1, int_type c2) noexcept; static constexpr int_type eof() noexcept; }; }

The two-argument members assign, eq, and lt are defined identically to the built-in operators =, ==, and < respectively.

The member eof() returns an implementation-defined constant that cannot appear as a valid UTF-8 code unit.

27.2.4.4 struct char_traits<char16_t> [char.traits.specializations.char16.t]

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namespace std { template<> struct char_traits<char16_t> { using char_type = char16_t; using int_type = uint_least16_t; using off_type = streamoff; using pos_type = u16streampos; using state_type = mbstate_t; using comparison_category = strong_ordering; static constexpr void assign(char_type& c1, const char_type& c2) noexcept; static constexpr bool eq(char_type c1, char_type c2) noexcept; static constexpr bool lt(char_type c1, char_type c2) noexcept; static constexpr int compare(const char_type* s1, const char_type* s2, size_t n); static constexpr size_t length(const char_type* s); static constexpr const char_type* find(const char_type* s, size_t n, const char_type& a); static constexpr char_type* move(char_type* s1, const char_type* s2, size_t n); static constexpr char_type* copy(char_type* s1, const char_type* s2, size_t n); static constexpr char_type* assign(char_type* s, size_t n, char_type a); static constexpr int_type not_eof(int_type c) noexcept; static constexpr char_type to_char_type(int_type c) noexcept; static constexpr int_type to_int_type(char_type c) noexcept; static constexpr bool eq_int_type(int_type c1, int_type c2) noexcept; static constexpr int_type eof() noexcept; }; }

The two-argument members assign, eq, and lt are defined identically to the built-in operators =, ==, and <, respectively.

The member eof() returns an implementation-defined constant that cannot appear as a valid UTF-16 code unit.

27.2.4.5 struct char_traits<char32_t> [char.traits.specializations.char32.t]

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namespace std { template<> struct char_traits<char32_t> { using char_type = char32_t; using int_type = uint_least32_t; using off_type = streamoff; using pos_type = u32streampos; using state_type = mbstate_t; using comparison_category = strong_ordering; static constexpr void assign(char_type& c1, const char_type& c2) noexcept; static constexpr bool eq(char_type c1, char_type c2) noexcept; static constexpr bool lt(char_type c1, char_type c2) noexcept; static constexpr int compare(const char_type* s1, const char_type* s2, size_t n); static constexpr size_t length(const char_type* s); static constexpr const char_type* find(const char_type* s, size_t n, const char_type& a); static constexpr char_type* move(char_type* s1, const char_type* s2, size_t n); static constexpr char_type* copy(char_type* s1, const char_type* s2, size_t n); static constexpr char_type* assign(char_type* s, size_t n, char_type a); static constexpr int_type not_eof(int_type c) noexcept; static constexpr char_type to_char_type(int_type c) noexcept; static constexpr int_type to_int_type(char_type c) noexcept; static constexpr bool eq_int_type(int_type c1, int_type c2) noexcept; static constexpr int_type eof() noexcept; }; }

The two-argument members assign, eq, and lt are defined identically to the built-in operators =, ==, and <, respectively.

The member eof() returns an implementation-defined constant that cannot appear as a Unicode code point.

27.2.4.6 struct char_traits<wchar_t> [char.traits.specializations.wchar.t]

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namespace std { template<> struct char_traits<wchar_t> { using char_type = wchar_t; using int_type = wint_t; using off_type = streamoff; using pos_type = wstreampos; using state_type = mbstate_t; using comparison_category = strong_ordering; static constexpr void assign(char_type& c1, const char_type& c2) noexcept; static constexpr bool eq(char_type c1, char_type c2) noexcept; static constexpr bool lt(char_type c1, char_type c2) noexcept; static constexpr int compare(const char_type* s1, const char_type* s2, size_t n); static constexpr size_t length(const char_type* s); static constexpr const char_type* find(const char_type* s, size_t n, const char_type& a); static constexpr char_type* move(char_type* s1, const char_type* s2, size_t n); static constexpr char_type* copy(char_type* s1, const char_type* s2, size_t n); static constexpr char_type* assign(char_type* s, size_t n, char_type a); static constexpr int_type not_eof(int_type c) noexcept; static constexpr char_type to_char_type(int_type c) noexcept; static constexpr int_type to_int_type(char_type c) noexcept; static constexpr bool eq_int_type(int_type c1, int_type c2) noexcept; static constexpr int_type eof() noexcept; }; }

The two-argument members assign, eq, and lt are defined identically to the built-in operators =, ==, and <, respectively.

The member eof() returns WEOF.