Qt c++ aggregate 'std::stringstream ss' has incomplete type and cannot be defined

I have this function in my program that converts integers to strings:

    QString Stats_Manager::convertInt(int num)
    {
        stringstream ss;
        ss << num;
        return ss.str();
    }

But when ever i run this i get the error:

aggregate 'std::stringstream ss' has incomplete type and cannot be defined

Im not really sure what that means. But if you know how to fix it or need any more code please just comment. Thanks.