Experimenting with Classes

[dalbir4444]

I wrote this simple program to see how objects are instantiated and destroyed:

#include <iostream>
using namespace std;

class A
{
    public:
        A(){cout<<"Initialized"<<endl;}
        ~A(){cout<<"Destroyed"<<endl;}
};

void print(A a)
{}

int main(void)
{
    A a;
    print(a);
}

I was surprised to see that output was:

Initialized
Destroyed
Destroyed

because I had expected it to be

Initialized
Initialized
Destroyed
Destroyed

Furthermore, if I changed the return type of the print function to A, the output showed another "Destroyed" statement.

However, If I changed the print function to

A print(A a)
{
    A b;
    return b;
}

the output was now:

Initialized
Initialized
Destroyed
Destroyed
Destroyed

It would be helpful if somebody could explain what's going in each of these three cases.

[kdub]

In your call to print(a) you're using the trivial, compiler-defined copy constructor to pass the argument to the 'print' function.

The signature and body of the implicitly-defined copy constructor in your case is
A( const A& otherA ) {};

If you were to explicitly define it as
A( const A& other ){cout<<"Initialized"<<endl;}

then you should get your expected result.

[dalbir4444]

Thanks. It makes sense now.