std::string/wstring を使って、UNICODE と UTF-8 を相互変換(抜粋コード)

UNICODE2UTF8

std::string ToUtf8Str( const wchar_t* value )
{
	_ASSERTE( value != nullptr );
	if( value == nullptr || *value == L'\0' ){
		return "";
	}
	int needLength = WideCharToMultiByte( CP_UTF8, 0, value, -1, nullptr, 0, nullptr, nullptr );
	if( needLength < 1 ){
		return "";
	}
	std::string	result;
	result.resize( needLength-1 );	//	\0を含むバイト数が返ってくる
	WideCharToMultiByte( CP_UTF8, 0, value, -1, &result[0], needLength, nullptr, nullptr );
	return result;
}
std::wstring ToUnicodeStr( const char* value )
{
	_ASSERTE( value != nullptr );
	if( value == nullptr || *value == '\0' ){
		return L"";
	}
	int needLength = MultiByteToWideChar( CP_UTF8, 0, value, -1, nullptr, 0 );
	std::wstring	result;
	result.resize( needLength-1 );
	MultiByteToWideChar( CP_UTF8, 0, value, -1, &result[0], needLength );
	return result;
}