manual_cvt.cpp

#include <iostream>
#include <locale>
#include <string>

struct cvt32_8 : public std::codecvt<char32_t, char, std::mbstate_t>
{};

int main()
{
    cvt32_8 cvt;
    char32_t str[] = U"konzele";

    std::mbstate_t mb;
    for (auto c : str)
    {
        char buf[5];
        const char32_t* end32;
        char* end8;
        cvt.out(mb, &c, &c + 1, end32, buf, buf + 4, end8);
        *end8 = 0;
        std::cout << buf;
    }
    std::cout << '\n';

    std::string foo = "21312";
    const char* fbeg = foo.data();
    const auto fe = fbeg + foo.size();
    for (auto c : foo)
    {
        char32_t out;
        char32_t* u;
        cvt.in(mb, fbeg, fe, fbeg, &out, &out + 1, u);
        std::cout << out << ", ";
    }

    std::cout << '\n';
    return 0;
}