NAME¶

std::char_traits - std::char_traits

Synopsis¶

Defined in header <string>
template<

class CharT

> class char_traits;

The char_traits class is a traits class template that abstracts basic character and
string operations for a given character type. The defined operation set is such that
generic algorithms almost always can be implemented in terms of it. It is thus
possible to use such algorithms with almost any possible character or string type,
just by supplying a customized char_traits class.

The char_traits class template serves as a basis for explicit instantiations. The
user can provide a specialization for any custom character types. Several
specializations are defined for the standard character types.

If an operation on traits emits an exception, the behavior is undefined.

Standard specializations¶

Member typedefs of standard specializations are as follows.

Specialization char_type int_type pos_type
std::char_traits<char> char int std::streampos
std::char_traits<wchar_t> wchar_t std::wint_t std::wstreampos
std::char_traits<char16_t> (C++11) char16_t std::uint_least16_t std::u16streampos
std::char_traits<char32_t> (C++11) char32_t std::uint_least32_t std::u32streampos
std::char_traits<char8_t> (C++20) char8_t unsigned int std::u8streampos

Member type Definition (same among all standard specializations)
off_type std::streamoff
state_type std::mbstate_t
comparison_category (C++20) std::strong_ordering

The semantics of the member functions of standard specializations are defined are as
follows.

Specialization assign eq lt eof
std::char_traits<char> = == for < for EOF
unsigned char unsigned char
std::char_traits<wchar_t> = == < WEOF
invalid
std::char_traits<char16_t> (C++11) = == < UTF-16 code
unit
invalid
std::char_traits<char32_t> (C++11) = == < UTF-32 code
unit
std::char_traits<char8_t> (C++20) = == < invalid UTF-8
code unit

Standard specializations of char_traits class template satisfy the requirements of
CharTraits.

Member types¶

Type Definition
char_type CharT
int_type an integer type that can hold all values of char_type plus EOF
off_type implementation-defined
pos_type implementation-defined
state_type implementation-defined

Member functions¶

assign assigns a character
[static] (public static member function)
eq compares two characters
lt (public static member function)
[static]
move moves one character sequence onto another
[static] (public static member function)
copy copies a character sequence
[static] (public static member function)
compare lexicographically compares two character sequences
[static] (public static member function)
length returns the length of a character sequence
[static] (public static member function)
find finds a character in a character sequence
[static] (public static member function)
to_char_type converts int_type to equivalent char_type
[static] (public static member function)
to_int_type converts char_type to equivalent int_type
[static] (public static member function)
eq_int_type compares two int_type values
[static] (public static member function)
eof returns an eof value
[static] (public static member function)
not_eof checks whether a character is eof value
[static] (public static member function)

Example¶

User-defined character traits may be used to provide case-insensitive comparison

// Run this code

#include <string>
#include <string_view>
#include <iostream>
#include <cctype>

struct ci_char_traits : public std::char_traits<char> {
static char to_upper(char ch) {
return std::toupper((unsigned char) ch);
}
static bool eq(char c1, char c2) {
return to_upper(c1) == to_upper(c2);
}
static bool lt(char c1, char c2) {
return to_upper(c1) < to_upper(c2);
}
static int compare(const char* s1, const char* s2, std::size_t n) {
while ( n-- != 0 ) {
if ( to_upper(*s1) < to_upper(*s2) ) return -1;
if ( to_upper(*s1) > to_upper(*s2) ) return 1;
++s1; ++s2;
}
return 0;
}
static const char* find(const char* s, std::size_t n, char a) {
auto const ua (to_upper(a));
while ( n-- != 0 )
{
if (to_upper(*s) == ua)
return s;
s++;
}
return nullptr;
}
};

template<class DstTraits, class CharT, class SrcTraits>
constexpr std::basic_string_view<CharT, DstTraits>
traits_cast(const std::basic_string_view<CharT, SrcTraits> src) noexcept
{
return {src.data(), src.size()};
}

int main()
{
using namespace std::literals;

constexpr auto s1 = "Hello"sv;
constexpr auto s2 = "heLLo"sv;

if (traits_cast<ci_char_traits>(s1) == traits_cast<ci_char_traits>(s2))
std::cout << s1 << " and " << s2 << " are equal\n";
}

Output:¶

Hello and heLLo are equal

See also¶

basic_string stores and manipulates sequences of characters
(class template)