Question

The following codes try to show how to overload assignment operator:

#include <iostream>
using namespace std;

template <typename T>
class A
{
public:
    A()   {};
    A( T &obj) {value = obj;};
    ~A() {};
    T value;

    template <typename E>
    A<T>& operator = ( A<E> &obj)
    {

        if(this == &obj)
            return *this;

        value = obj.value;
        return *this;
    }

};



int main()
{
    int temp;
    temp = 3;
    A<int> myobjects(temp);
    cout<<myobjects.value<<endl;

     float f_value;
     f_value = 10.7;
     A<float> fobjects(f_value);
     myobjects = fobjects;
     cout<<myobjects.value<<endl;

    return 0;
}

However, when I compile it with VC10, I have found the following error:

 error C2440: '==' : cannot convert from 'A<T> *' to 'A<T> *const '

If I change the overloading function in the following way:

  template <typename E>
    A<T>& operator = ( A<E> &obj)
    {

     // if(this == &obj)
        //  return *this;

        value = obj.value;
        return *this;
    }

It will work. Why does this error occur in the commented-out code when called in this way?

Answer 1

You’re missing part of the error message. It should say:

error C2440: '==' : cannot convert from 'A<T> *' [with T = float] to 'A<T> *const' [with T = int]

You may have to look at nearby lines in the build log to see that extra information. And the compiler is doing no favors by using T twice but representing two different types.