Example for https://essential-cpp.programming-books.io/user-defined-literals-d3696e4f87894401af6c2f073d356228 (made with https://codeeval.dev)

main.cc

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

int main()
{
    using namespace std::literals::string_literals;

    std::string s = "hello world"s;
    std::u16string s16 = u"hello world"s;
    std::u32string s32 = U"hello world"s;
    std::wstring ws = L"hello world"s;
    
    std::cout << s << std::endl;

    std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t> utf16conv;
    std::cout << utf16conv.to_bytes(s16) << std::endl;

    std::wstring_convert<std::codecvt_utf8_utf16<char32_t>, char32_t> utf32conv;
    std::cout << utf32conv.to_bytes(s32) << std::endl;

    std::wcout << ws << std::endl;
}