From cppreference.com
template<classCharT>classchar_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
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
.
Specializations
The standard library provides the following standard specializations:
Defined in header
std::char_traits<char>the standard character traits of charstd::char_traits<wchar_t>the standard character traits of wchar_tstd::char_traits<char8_t>(C++20)the standard character traits of char8_tstd::char_traits<char16_t>(C++11)the standard character traits of char16_tstd::char_traits<char32_t>(C++11)the standard character traits of char32_tAll these specializations satisfy the requirements of
.
Member types
The standard specializations define the following member types required by
:
CharTMember type char_typeint_typeoff_typepos_typestate_typecharcharint
wchar_twchar_t
char8_tchar8_tunsignedint
char16_tchar16_t
char32_tchar32_t
On top of that, the standard specializations also define the member type comparison_category as
.
(since C++20)Member functions
The standard specializations define the following static member functions required by
:
[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
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
:
Run this code
#include<cctype>#include<iostream>#include<string>#include<string_view>structci_char_traits:publicstd::char_traits<char>{staticcharto_upper(charch){returnstd::toupper((unsignedchar)ch);}staticbooleq(charc1,charc2){returnto_upper(c1)==to_upper(c2);}staticboollt(charc1,charc2){returnto_upper(c1)<to_upper(c2);}staticintcompare(constchar*s1,constchar*s2,std::size_tn){while(n--!=0){if(to_upper(*s1)<to_upper(*s2))return-1;if(to_upper(*s1)>to_upper(*s2))return1;++s1;++s2;}return0;}staticconstchar*find(constchar*s,std::size_tn,chara){constautoua{to_upper(a)};while(n--!=0){if(to_upper(*s)==ua)returns;s++;}returnnullptr;}};template<classDstTraits,classCharT,classSrcTraits>constexprstd::basic_string_view<CharT,DstTraits>traits_cast(conststd::basic_string_view<CharT,SrcTraits>src)noexcept{return{src.data(),src.size()};}intmain(){usingnamespacestd::literals;constexprautos1="Hello"sv;constexprautos2="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