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 look 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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