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Showing content from https://timsong-cpp.github.io/cppwp/n4140/char.traits below:

[char.traits]

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 struct 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.

typedef CHAR_T char_type;

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].

typedef INT_T int_type;

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.233

typedef implementation-defined off_type; typedef implementation-defined pos_type;

typedef STATE_T state_type;

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 template struct char_traits: char_traits<char>, char_traits<char16_t>, char_traits<char32_t>, and char_traits<wchar_t>.

namespace std {
  template<> struct char_traits<char> {
    typedef char        char_type;
    typedef int         int_type;
    typedef streamoff   off_type;
    typedef streampos   pos_type;
    typedef mbstate_t   state_type;

    static 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 int compare(const char_type* s1, const char_type* s2, size_t n);
    static size_t length(const char_type* s);
    static 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.

namespace std {
  template<> struct char_traits<char16_t> {
    typedef char16_t        char_type;
    typedef uint_least16_t  int_type;
    typedef streamoff       off_type;
    typedef u16streampos    pos_type;
    typedef mbstate_t       state_type;

    static 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 int compare(const char_type* s1, const char_type* s2, size_t n);
    static size_t length(const char_type* s);
    static 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.

namespace std {
  template<> struct char_traits<char32_t> {
    typedef char32_t        char_type;
    typedef uint_least32_t  int_type;
    typedef streamoff       off_type;
    typedef u32streampos    pos_type;
    typedef mbstate_t       state_type;

    static 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 int compare(const char_type* s1, const char_type* s2, size_t n);
    static size_t length(const char_type* s);
    static 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.

namespace std {
  template<> struct char_traits<wchar_t> {
    typedef wchar_t      char_type;
    typedef wint_t       int_type;
    typedef streamoff    off_type;
    typedef wstreampos   pos_type;
    typedef mbstate_t    state_type;

    static 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 int compare(const char_type* s1, const char_type* s2, size_t n);
    static size_t length(const char_type* s);
    static 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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