UTF8转GB2312(UTF8解码)

小弟C++上手没多久，代码不严谨之处敬请见谅。英语也不是很好，有的是直接使用的拼音。

string MyUTF_8toGB2312(string str)
{
    int changdu = MultiByteToWideChar(,,str.c_str(),-,NULL,);
    wchar_t* IUnicode = new wchar_t[changdu*];
    MultiByteToWideChar(,,str.c_str(),-,IUnicode,changdu);
    changdu = WideCharToMultiByte(,,IUnicode,-,NULL,,,);
    char* Ichar = new char[changdu*];
    WideCharToMultiByte(,,IUnicode,-,Ichar,changdu*,,);
    string temp = Ichar;
    delete []IUnicode;
    IUnicode=NULL;
    delete []Ichar;
    Ichar=NULL;
    return temp;
}