24 Strings library [strings]

24.2 Character traits [char.traits]

24.2.3 char_­traits specializations [char.traits.specializations]

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>.
The requirements for the members of these specializations are given in [char.traits.require].

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 streampos shall be an implementation-defined type that satisfies the requirements for pos_­type in [iostreams.limits.pos] and [iostream.forward].
The type streamoff shall be an implementation-defined type that satisfies the requirements for off_­type in [iostreams.limits.pos] and [iostream.forward].
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 type u16streampos shall be an implementation-defined type that satisfies the requirements for pos_­type in [iostreams.limits.pos] and [iostream.forward].
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 type u32streampos shall be an implementation-defined type that satisfies the requirements for pos_­type in [iostreams.limits.pos] and [iostream.forward].
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 wstreampos shall be an implementation-defined type that satisfies the requirements for pos_­type in [iostreams.limits.pos] and [iostream.forward].
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.