C++ operator overloading

Question

Why does the below C++ program output "ACCA"? Why is operator int() called twice?

#include "stdafx.h"
#include <iostream>

using namespace std;

class Base {
public:
    Base(int m_var=1):i(m_var){
        cout<<"A";
    }
    Base(Base& Base){
        cout<<"B";
        i=Base.i;
    }
    operator int() {
        cout<<"C";
        return i;
    }
private:
    int i;
};

int main()
{
    Base obj;
    obj = obj+obj;
    return 0;
}

Answer 1

First, this line:

Base obj;

Default-constructs object obj by picking the constructor that accepts an integer, with default value 1. This is responsible for the first A being printed to the standard output.

Then, this expression:

obj + obj

Requires picking a viable overload of operator +. In this case, since obj has a user-defined conversion to int, the built-in operator + is picked, and both arguments are converted to int. This is responsible for the two Cs being printed to the standard output.

Then, the assignment to obj in:

obj = obj + obj

Needs to invoke the implicitly generated operator = for Base. The implicitly generated operator = has signature:

Base& operator = (Base const&);

This means that the expression on the right side of the equal sign, which is of type int, must be converted into a temporary Base object from which obj is assigned (the reference parameter of the implicitly-generated operator = is bound to this temporary).

But the creation of this temporary from an int in turn requires invoking the converting construction of Base that accepts an int again, which is responsible for the second A being printed to the standard output.