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 explicit
specializations are provided for the standard character types (see below), other
specializations are not required to satisfy the requirements of CharTraits.

Specializations¶

The standard library provides the following standard specializations:

Defined in header <string>
std::char_traits<char> the standard character traits of char
std::char_traits<wchar_t> the standard character traits of wchar_t
std::char_traits<char8_t> (C++20) the standard character traits of char8_t
std::char_traits<char16_t> (C++11) the standard character traits of char16_t
std::char_traits<char32_t> (C++11) the standard character traits of char32_t

All these specializations satisfy the requirements of CharTraits.

Member types¶

The standard specializations define the following member types required by
CharTraits:

CharT Member type
char_type int_type off_type pos_type state_type char char int std::streampos wchar_t wchar_t std::wint_t std::wstreampos char8_t char8_t unsigned int std::streamoff std::u8streampos std::mbstate_t
char16_t char16_t std::uint_least16_t std::u16streampos char32_t char32_t std::uint_least32_t std::u32streampos

On top of that, the standard specializations also define the member (since C++20)
type comparison_category as std::strong_ordering.

Member functions¶

The standard specializations define the following static member functions required
by CharTraits:

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)

Notes¶

CharTraits does not require defining the types and functions listed above as direct
members, it only requires types like X::type and expressions like X::func(args) are
valid and have the required semantics. Users-defined character traits can be derived
from other character traits classes and only override some of their members, see the
example below.

Example¶

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

// Run this code

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

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)
{
const auto 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)
basic_string_view read-only string view
(C++17) (class template)
wraps a given abstract device (std::basic_streambuf)
basic_istream and provides high-level input interface
(class template)
wraps a given abstract device (std::basic_streambuf)
basic_ostream and provides high-level output interface
(class template)
basic_streambuf abstracts a raw device
(class template)