Converting between character encodings

#include <iostream>
#include <codecvt>
#include <locale>
#include <string>
using namespace std;

int main() {
    // converts between wstring and utf8 string
    wstring_convert<codecvt_utf8_utf16<wchar_t>> wchar_to_utf8;
    // converts between u16string and utf8 string
    wstring_convert<codecvt_utf8_utf16<char16_t>, char16_t> utf16_to_utf8;
    
    wstring wstr = L"foobar";
    string utf8str = wchar_to_utf8.to_bytes(wstr);
    wstring wstr2 = wchar_to_utf8.from_bytes(utf8str);
    
    wcout << wstr << endl;
    cout << utf8str << endl;
    wcout << wstr2 << endl;
    
    u16string u16str = u"foobar";
    string utf8str2 = utf16_to_utf8.to_bytes(u16str);
    u16string u16str2 = utf16_to_utf8.from_bytes(utf8str2);
    
    return 0;
}

using utf16_char = unsigned short;
wstring_convert<codecvt_utf8_utf16<utf16_char>, utf16_char> conv_utf8_utf16;

void strings::utf16_to_utf8(const std::u16string& utf16, std::string& utf8)
{
  std::basic_string<utf16_char> tmp;
  tmp.resize(utf16.length());
  std::copy(utf16.begin(), utf16.end(), tmp.begin());
  utf8 = conv_utf8_utf16.to_bytes(tmp);
}
void strings::utf8_to_utf16(const std::string& utf8, std::u16string& utf16)
{ 
  std::basic_string<utf16_char> tmp = conv_utf8_utf16.from_bytes(utf8);
  utf16.clear();
  utf16.resize(tmp.length());
  std::copy(tmp.begin(), tmp.end(), utf16.begin());
}