Convert UTF-8 to UTF-16 and vice versa

string_convert.cpp

wstring utf8_to_utf16(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;
	}

	string utf16_to_utf8(const wstring& src){
		string dest;
		dest.clear();
		for (size_t i = 0; i < src.size(); i++){
			wchar_t w = src[i];
			if (w <= 0x7f)
				dest.push_back((char)w);
			else if (w <= 0x7ff){
				dest.push_back(0xc0 | ((w >> 6) & 0x1f));
				dest.push_back(0x80 | (w & 0x3f));
			}
			else if (w <= 0xffff){
				dest.push_back(0xe0 | ((w >> 12) & 0x0f));
				dest.push_back(0x80 | ((w >> 6) & 0x3f));
				dest.push_back(0x80 | (w & 0x3f));
			}
			else if (w <= 0x10ffff){
				dest.push_back(0xf0 | ((w >> 18) & 0x07));
				dest.push_back(0x80 | ((w >> 12) & 0x3f));
				dest.push_back(0x80 | ((w >> 6) & 0x3f));
				dest.push_back(0x80 | (w & 0x3f));
			}
			else
				dest.push_back('?');
		}
		return dest;
	}