std::char_traits

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< cpp‎ | string
 
C++
 
Strings library
 
std::char_traits
 
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::streamoff  std::streampos  std::mbstate_t 
wchar_t wchar_t std::wint_t std::wstreampos
char8_t char8_t unsigned int std::u8streampos
 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 type comparison_category as std::strong_ordering.

(since C++20)

Member functions

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

[static]
 assigns a character
(public static member function)
[static]
 compares two characters
(public static member function)
[static]
 moves one character sequence onto another
(public static member function)
[static]
 copies a character sequence
(public static member function)
[static]
 lexicographically compares two character sequences
(public static member function)
[static]
 returns the length of a character sequence
(public static member function)
[static]
 finds a character in a character sequence
(public static member function)
[static]
 converts int_type to equivalent char_type
(public static member function)
[static]
 converts char_type to equivalent int_type
(public static member function)
[static]
 compares two int_type values
(public static member function)
[static]
 returns an eof value
(public static member function)
[static]
 checks whether a character is eof value
(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

 stores and manipulates sequences of characters
(class template)
(C++17)
 read-only string view
(class template)
wraps a given abstract device (std::basic_streambuf)
and provides high-level input interface
(class template)
wraps a given abstract device (std::basic_streambuf)
and provides high-level output interface
(class template)
abstracts a raw device
(class template)
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