#include <fstream>
#include <iostream>
#include <string>
#include <locale>
#include <codecvt>
...
 
// Write file in UTF-8
std::wofstream wof;
wof.imbue(std::locale(std::locale::empty(), new std::codecvt_utf8<wchar_t,0x10ffff,std::generate_header>));
wof.open(L"file.txt");
wof << L"This is a test.";
wof << L"This is another test.";
wof << L"\nThis is the final test.\n";
wof.close();
 
// Read file in UTF-8
std::wifstream wif(L"file.txt");
wif.imbue(std::locale(std::locale::empty(), new std::codecvt_utf8<wchar_t,0x10ffff, std::consume_header>));
 
std::wstringstream wss;
wss << wif.rdbuf();