Output of C++ Program | Set 13

Predict the output of following C++ program.

#include<iostream>
using namespace std;

class A
{
    // data members of A
public:
    A ()           { cout << "\n A's constructor"; /* Initialize data members */ }
    A (const A &a) { cout << "\n A's Copy constructor";  /* copy data members */}
    A& operator= (const A &a) // Assignemt Operator
    {
        // Handle self-assignment:
        if(this == &a) return *this;

        // Copy data members
        cout << "\n A's Assignment Operator";  return *this;
    }
};

class B
{
    A a;
    // Other members of B
public:
    B(A &a) { this->a = a; cout << "\n B's constructor"; }
};

int main()
{
    A a1;
    B b(a1);
    return 0;
}

Output:

 A's constructor
 A's constructor
 A's Assignment Operator
 B's constructor

The first line of output is printed by the statement “A a1;” in main().
The second line is printed when B’s member ‘a’ is initialized. This is important.
The third line is printed by the statement “this->a = a;” in B’s constructor.
The fourth line is printed by cout statement in B’s constructor.

If we take a closer look at the above code, the constructor of class B is not efficient as member ‘a’ is first constructed with default constructor, and then the values from the parameter are copied using assignment operator. It may be a concern when class A is big, which generally is the case with many practical classes. See the following optimized code.

#include<iostream>
using namespace std;

class A
{
    // data members of A
public:
    A()           { cout << "\n A's constructor"; /* Initialize data members */ }
    A(const A &a) { cout << "\n A's Copy constructor"; /* Copy data members */ }
    A& operator= (const A &a) // Assignemt Operator
    {
        // Handle self-assignment:
        if(this == &a) return *this;

        // Copy data members
        cout << "\n A's Assignment Operator";  return *this;
    }
};

class B
{
    A a;
    // Other members of B
public:
    B(A &a):a(a) {  cout << "\n B's constructor"; }
};

int main()
{
    A a;
    B b(a);
    return 0;
}

Output:

 A's constructor
 A's Copy constructor
 B's constructor

The constructor of class B now uses initializer list to initialize its member ‘a’. When Initializer list is used, the member ‘a’ of class B is initialized directly from the parameter. So a call to A’s constructor is reduced.
In general, it is a good idea to use Initializer List to initialize all members of a class, because it saves one extra assignment of members. See point 6 of this post for more details.

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