operator++ with templates

[newbie30]

template <class T>
T operator++(T num)
{
     cout<<"In overload function"<<endl;
	 return T result = num++;
}

int main()
{
   int num=5;
   int result = operator++(num); //whats the problem with this???

}

Why does this fail to compile?

[yzaykov]

Because you're trying to declare a variable inside a return statement.
And by the way... I'd think for a minute before committing/submitting this... whatever it does.

[newbie30]

The return statement is not the problem here.

I am visual studio 2008 if thats any help.

[yzaykov]

Oh, really?! Then you tell me why this fails to compile:

int main()
{
	return int i = 0;
}

[newbie30]

No need to be smart, what i was saying was even when i change the return part around it still fails. The error i get is before it reaches the return part.

To make it clearer for you

template <class T>
T operator++(T num)
{
     cout<<"In overload function"<<endl;
	  T result = num++;
                  return result
}

int main()
{
   int num=5;
   int result = operator++(num); //whats the problem with this???

}

This still fails.

[laserlight]

whats the problem with this?

For one thing, you cannot overload operator++ for int.

Basically, it looks like you are trying to implement postfix operator++ in terms of prefix operator++, which is weird: the opposite is more normal. Nonetheless, if you want an example:

#include <iostream>

class X
{
public:
    X(int num) : num(num) {}

    int get() const
    {
        return num;
    }

    X operator++(int)
    {
        X temp(*this);
        ++num;
        return temp;
    }
private:
    int num;
};

template <class T>
T& operator++(T& x)
{
    x++;
    return x;
}

int main()
{
    X x(5);
    X result = operator++(x);
    std::cout << x.get() << ' ' << result.get() << std::endl;
}

Personally, I feel that a more useful thing would be to define postfix operator++ in terms of prefix operator++, e.g.,

template<class T>
T operator++(T& x, int)
{
    T temp(x);
    ++x;
    return temp;
}

[jefranki]

No need to be smart, what i was saying was even when i change the return part around it still fails. The error i get is before it reaches the return part.

To make it clearer for you

template <class T>
T operator++(T num)
{
     cout<<"In overload function"<<endl;
	  T result = num++;
                  return result
}

This still fails.

You are running into a circular definition problem. You have num defined as type T and operator++ for type T is using operator++ for type T inside itself.

You also missed the semi-colon on your return line.

[laserlight]

You are running into a circular definition problem. You have num defined as type T and operator++ for type T is using operator++ for type T inside itself.

No, notice that postfix operator++ is used in the implementation of the function template prefix operator++. Incidentally, note that my example also corrects the function signature and return type.

[jefranki]

No, notice that postfix operator++ is used in the implementation of the function template prefix operator++. Incidentally, note that my example also corrects the function signature and return type.

I opened the editor before your post came up. At any rate, I missed the prefix vs. postfix element.

[Zaccheus@Work]

Why does this fail to compile?

What does the compiler say?