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 << "
 A's constructor"; /* Initialize data members */ 
    }
    A (const A &a) 
    { 
        cout << "
 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 << "
 A's Assignment Operator";  
        return *this;
    }
};

class B
{
    A a;
    // Other members of B
public:
    B(A &a) 
    { 
        this->a = a; 
        cout << "
 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 << "
 A's constructor"; /* Initialize data members */ 
    }
    A(const A &a) 
    { 
        cout << "
 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 << "
 A's Assignment Operator";  
        return *this;
    }
};

class B
{
    A a;
    // Other members of B
public:
    B(A &a):a(a) 
    {  
        cout << "
 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.

　　Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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　　2013-11-27　　16:13:26