This subclause defines requirements on classes representing character traits, and defines a class template char_traits<charT>, along with four specializations, char_traits<char>, char_traits<char16_t>,
char_traits<char32_t>, and char_traits<wchar_t>, that satisfy those requirements.
This subclause specifies a class template, char_traits<charT>, and four explicit specializations of it, char_traits<char>, char_traits<char16_t>, char_traits<char32_t>, and char_traits<wchar_t>, all of which appear in the header <string> and satisfy the requirements below.
24.2.2 Traits typedefs [char.traits.typedefs]using char_type = CHAR_T;
The type char_type is used to refer to the character container type in the implementation of the library classes defined in [string.classes] and Clause [input.output].
using int_type = INT_T;
Requires: For a certain character container type char_type, a related container type INT_T shall be a type or class which can represent all of the valid characters converted from the corresponding char_type values, as well as an end-of-file value, eof(). The type int_type represents a character container type which can hold end-of-file to be used as a return type of the iostream class member functions.224
using off_type = implementation-defined; using pos_type = implementation-defined;
using state_type = STATE_T;
namespace std {
template<> struct char_traits<char>;
template<> struct char_traits<char16_t>;
template<> struct char_traits<char32_t>;
template<> struct char_traits<wchar_t>;
}
The header <string> shall define four specializations of the class template char_traits: char_traits<char>, char_traits<char16_t>, char_traits<char32_t>, and char_traits<wchar_t>.
24.2.3.1 struct char_traits<char> [char.traits.specializations.char]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;
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 char_type* move(char_type* s1, const char_type* s2, size_t n);
static char_type* copy(char_type* s1, const char_type* s2, size_t n);
static 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 defined types for int_type, pos_type, off_type, and state_type shall be int, streampos, streamoff, and mbstate_t respectively.
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 shall be defined identically to the built-in operator =. The two-argument members eq and lt shall be defined identically to the built-in operators == and < for type unsigned char.
The member eof() shall return EOF.
24.2.3.2 struct char_traits<char16_t> [char.traits.specializations.char16_t]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;
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 char_type* move(char_type* s1, const char_type* s2, size_t n);
static char_type* copy(char_type* s1, const char_type* s2, size_t n);
static 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 shall be defined identically to the built-in operators =, ==, and < respectively.
The member eof() shall return an implementation-defined constant that cannot appear as a valid UTF-16 code unit.
24.2.3.3 struct char_traits<char32_t> [char.traits.specializations.char32_t]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;
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 char_type* move(char_type* s1, const char_type* s2, size_t n);
static char_type* copy(char_type* s1, const char_type* s2, size_t n);
static 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 shall be defined identically to the built-in operators =, ==, and < respectively.
The member eof() shall return an implementation-defined constant that cannot appear as a Unicode code point.
24.2.3.4 struct char_traits<wchar_t> [char.traits.specializations.wchar.t]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;
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 char_type* move(char_type* s1, const char_type* s2, size_t n);
static char_type* copy(char_type* s1, const char_type* s2, size_t n);
static 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 defined types for int_type, pos_type, and state_type shall be wint_t, wstreampos, and mbstate_t respectively.
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 members assign, eq, and lt shall be defined identically to the built-in operators =, ==, and < respectively.
The member eof() shall return WEOF.
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