Implicit Conversion Operator Overloading syntax

Question

I'm an intermediate C++ user and I encountered the following situation. The class definition shown below compiles fine with a g++ compiler. But I cannot put my finger on what exactly the whole syntax means.
My guess is that the function operator int() returns an int type.

Moreover, I cannot figure out how to use the overloaded operator () in main()

class A
{
   public:
     A(int n) { _num = n; }  //constructor 

     operator int();

   private:
     int _num;
};

A::operator int()  // Is this equivalent to "int A::operator()" ??
{
  return _num;
}

int main()
{
  int x = 10;
  A objA(x);  //creating & initializing

  // how to use operator() ?
  // int ret = objA();   // compiler error when uncommented

  return 0;
}

Any help will be appreciated.

Answer 1

operator int() is a conversion function that declares a user-defined conversion from A to int so that you can write code like

A a;
int x = a; // invokes operator int()

This is different from int operator()() , which declares a function-call operator that takes no arguments and returns an int . The function-call operator allows you to write code like

A a;
int x = a(); // invokes operator()()

Which one you want to use depends entirely on the behavior that you want to get. Note that conversion operators (eg, operator int() ) can get invoked at unexpected times and can cause pernicious errors.

Answer 2

you can use this one

#include <iostream>
using namespace std;
class A{
public:
    A(int n)  { _num=n;}
    operator int();

private:
    int _num;

};
A::operator int(){

    return _num;

}
int main(){

    A  a(10);
    cout<<a.operator int()<<endl;
    return 0;

}