std::mblen

#include <clocale>
#include <cstdlib>
#include <iomanip>
#include <iostream>
#include <stdexcept>
#include <string_view>

//        mblen()
// :    
// std::mbstowcs(nullptr, s.c_str(), s.size())
std::size_t strlen_mb(const std::string_view s)
{
    std::mblen(nullptr, 0); //   
    std::size_t result = 0;
    const char* ptr = s.data();
    for (const char* const end = ptr + s.size(); ptr < end; ++result)
    {
        const int next = std::mblen(ptr, end - ptr);
        if (next == -1)
            throw std::runtime_error("strlen_mb():  ");
        ptr += next;
    }
    return result;
}

void dump_bytes(const std::string_view str)
{
    std::cout << std::hex << std::uppercase << std::setfill('0');
    for (unsigned char c : str)
        std::cout << std::setw(2) << static_cast<int>(c) << ' ';
    std::cout << std::dec << '\n';
}

int main()
{
    //  mblen()     UTF-8
    std::setlocale(LC_ALL, "en_US.utf8");
    //    UTF-8
    const std::string_view str = "z\u00df\u6c34\U0001f34c"; //  u8"z"
    std::cout << std::quoted(str) << "  " << strlen_mb(str)
              << " ,    " << str.size() << " : ";
    dump_bytes(str);
}