How to convert UTF-8 std::string to UTF-16 std::wstring?

This is how you do it with C++11:

std::string str = "your string in utf8";
std::wstring_convert<std::codecvt_utf8_utf16<char16_t>> converter;
std::wstring wstr = converter.from_bytes(str);

And these are the headers you need:

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

A more complete example available here: http://en.cppreference.com/w/cpp/locale/wstring_convert/from_bytes

Here's some code. Only lightly tested and there's probably a few improvements. Call this function to convert a UTF-8 string to a UTF-16 wstring. If it thinks the input string is not UTF-8 then it will throw an exception, otherwise it returns the equivalent UTF-16 wstring.

std::wstring utf8_to_utf16(const std::string& utf8)
{
    std::vector<unsigned long> unicode;
    size_t i = 0;
    while (i < utf8.size())
    {
        unsigned long uni;
        size_t todo;
        bool error = false;
        unsigned char ch = utf8[i++];
        if (ch <= 0x7F)
        {
            uni = ch;
            todo = 0;
        }
        else if (ch <= 0xBF)
        {
            throw std::logic_error("not a UTF-8 string");
        }
        else if (ch <= 0xDF)
        {
            uni = ch&0x1F;
            todo = 1;
        }
        else if (ch <= 0xEF)
        {
            uni = ch&0x0F;
            todo = 2;
        }
        else if (ch <= 0xF7)
        {
            uni = ch&0x07;
            todo = 3;
        }
        else
        {
            throw std::logic_error("not a UTF-8 string");
        }
        for (size_t j = 0; j < todo; ++j)
        {
            if (i == utf8.size())
                throw std::logic_error("not a UTF-8 string");
            unsigned char ch = utf8[i++];
            if (ch < 0x80 || ch > 0xBF)
                throw std::logic_error("not a UTF-8 string");
            uni <<= 6;
            uni += ch & 0x3F;
        }
        if (uni >= 0xD800 && uni <= 0xDFFF)
            throw std::logic_error("not a UTF-8 string");
        if (uni > 0x10FFFF)
            throw std::logic_error("not a UTF-8 string");
        unicode.push_back(uni);
    }
    std::wstring utf16;
    for (size_t i = 0; i < unicode.size(); ++i)
    {
        unsigned long uni = unicode[i];
        if (uni <= 0xFFFF)
        {
            utf16 += (wchar_t)uni;
        }
        else
        {
            uni -= 0x10000;
            utf16 += (wchar_t)((uni >> 10) + 0xD800);
            utf16 += (wchar_t)((uni & 0x3FF) + 0xDC00);
        }
    }
    return utf16;
}