operator () overload with template C++

Question

I have a simple class for which I want to overload operator as below

class MyClass
{
   public:
      int first;

      template <typename T>
      T operator () () const { return first; }  
};

And the somewhere else I have

MyClass obj;

int i = obj(); // This gives me an error saying could not deduce
               // template argument for T

Can someone help me with this error, much appreciated. Thank you.

edit:

This has something to do with the operator(), for example if i replace the function with

    template <typename T>
    T get() const { return first;}

it works. Appreciate all the responses.

Answer 1

Did you try int i = obj<int>();?

Answer 2

If you wish the function call to be implicit then you'll have to apply the template to the class like this:

template <typename T>
class MyClass
{
  public:
  T first;

  T operator () () const { return first; }  
};

If it should be casted to another type then it should be:

template <typename T>
class MyClass
{
  public:
  T first;

  template <typename U>
  U operator () () const { return (U)first; }  
};

Answer 3

I am not sure why this is a template in the first place, you always return an int, right? Thanks

Answer 4

Compiler would face a quite an ambiguity when trying to deduce template arguments from code like this

template T operator () () const { return first; }

because for

int i = obj();

T can be not only int, but any type "castable" to int.

Answer 5

What you want to do is to provide a generic convector from Data to the userType. Consider something like this :

#include<iostream>
#include<string>
using namespace std;
class Data{
    public:
     std::string str;
     double var;

        template <typename UserType>
     operator UserType() const { return  UserType(var);}
};

int main()
{
Data d;
d.var = 5.5;
cout << int(d);
cout<<"\n";
return 0;
}

Is this what you needed?