example.cpp

#include "utf.hpp"
#include <iostream>
#include <string>

int main()
{
    // A UTF-8 literal
    std::string utf8 =  u8"z\u00df\u6c34\U0001d10b"; // or u8"zß水𝄋"
                        // or "\x7a\xc3\x9f\xe6\xb0\xb4\xf0\x9d\x84\x8b";

    // Create a UTF-16 version in a 2-byte/16-bit char string
    std::u16string utf16 = UTF::widen(utf8);
    
    // Convert the UTF-16 version back to UTF-8 for fun
    std::string new_utf8 = UTF::narrow(utf16);

    // Print out the origional UTF-8 string as hex
    std::cout << "Origional UTF8 string has " << std::dec << utf8.size() << " code units:" << std::endl;
    for (unsigned char c : utf8)
        std::cout << std::hex << std::showbase << (unsigned int)c << ' ';
    std::cout << std::endl << std::endl;
    
    // Print out the newly converted UTF-8 version
    std::cout << "UTF8 conversion produced " << std::dec << new_utf8.size() << " code units:" << std::endl;
    for (unsigned char c : new_utf8)
        std::cout << std::hex << std::showbase << (unsigned int)c << ' ';
    std::cout << std::endl << std::endl;
 
    // Print out the UTF-16 version
    std::cout << "UTF16 conversion produced " << std::dec << utf16.size() << " code units:" << std::endl;
    for (char16_t c : utf16)
        std::cout << std::hex << std::showbase << (unsigned long)c << ' ';
    std::cout << std::endl << std::endl;
    
    return 0;
}

utf.hpp

/*
 * Basic C++11 library to convert between UTF-8 and UTF-16 using the STL.
 *
 *
 * By Matt Sturgeon 2016, may be used either as public domain or under the MIT Licence.
 *
 * TODO:
 * - Consider adding some wrappers around cout, cerr and ifstream to use UTF-16 if on Windows
 * - Consider adding support for converting to wstring and u32string (widestring will either be 
 *   16bit or 32 bit. On Windows it is 16bit.)
 *
 */

#ifndef CONVERT_UTF_H
#define CONVERT_UTF_H

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


namespace UTF {
	// C++ wide string converter object
	// Convert UTF-8 <--> UTF-16
	static std::wstring_convert <std::codecvt_utf8_utf16<char16_t>, char16_t> convert {};


	// Convert a UTF-8 string to a UTF-16 u16string (2 byte wide char)
	static std::u16string widen (const std::string &str) {
		return convert.from_bytes(str.data());
	}
	static std::u16string widen (const char *str) {
		return widen(std::string(str));
	} 

	// Convert a 2 byte wide char UTF-16 string into a 1 byte char UTF-8 string
	static std::string narrow (const std::u16string &str) {
		return convert.to_bytes(str.data());
	}
	static std::string narrow (const char16_t *str) {
		return narrow(std::u16string(str));
	}
}; // UTF

#endif // CONVERT_UTF_H

recommend using ICU library because codecvt will get deprecated