std::char_traits<char>:: length, std::char_traits<wchar_t>:: length, std::char_traits<char8_t>:: length, std::char_traits<char16_t>:: length, std::char_traits<char32_t>:: length

From cppreference.com
C++
Strings library
std::char_traits
static std:: size_t length ( const char_type * s ) ;
(constexpr since C++17)

Returns the length of the character sequence pointed to by s , that is, the position of the terminating null character ( char_type ( ) ).

See CharTraits for the general requirements on character traits for X::length .

Parameters

s - pointer to a character sequence to return length of

Return value

The length of character sequence pointed to by s .

Complexity

Linear.

Example

Run this code 
#include <iomanip>
#include <iostream>
#include <string>
 
void print(const char* str)
{
    std::cout << std::quoted(str) << " has length = "
              << std::char_traits<char>::length(str) << '\n';
}
 
int main()
{
    print("foo");
 
    std::string s{"booo"};
    print(s.c_str());
}

Output: 

"foo" has length = 3
"booo" has length = 4
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