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std::regex_traits<CharT>:: length

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std::regex_traits
static std:: size_t length ( const char_type * p ) ;

Calculates the length of a null-terminated character sequence, that is, the smallest i such that p [ i ] == 0 .

Standard library specializations of std::regex_traits execute std:: char_traits < CharT > :: length ( p ) ; .

Parameters

p - pointer to the first element of the null-terminated character sequence

Return value

The length of the null-terminated character string.

Example

Run this code 
#include <iostream>
#include <regex>
int main()
{
    std::cout << std::regex_traits<char>::length("Кошка") << '\n'
              << std::regex_traits<wchar_t>::length(L"Кошка") << '\n';
}

Output: 

10
5