Copy Constructor, C++ tutorial

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Copy Constructor

The copy constructor is used to copy the content of one object to another object. A copy constructor always takes a reference object as an argument. A copy constructor can be defined in two ways:

i) Inside the class definition

ii) Outside the class definition

i)  Inside the class definition

The general form of defining a copy constructor holds the following syntax.

class classname

{

        private:

                //Data members

        public:

                classname(classname &object_name)

                {

                        //Copy Constructor Statements

                }

};

ii) Outside the class definition

The general form of defining a copy constructor outside the class holds the following syntax.

class classname

{

        private:

                //Data members

        public:

                classname(classname &object_name); // Copy Constructor                       prototype

};


classname::classname(classname &object_name) < --------------------- Outside the class

{

        // Copy  Constructor Statements

}

Remember

When the copy constructor is defined outside the class, its prototype must be declared in the public category of the class. 

Invoking a copy constructor

We must pass the initial values as arguments to copy constructor after initializing the source object

Method1

class_name destination_object=source_object;

Example

          demo d1(4500);

        demo d2=d1;

Method2

class_name destination_object(source_object);

Example

          demo d1(4500);

        demo d2(d1);

The programmer can use either of the methods.

Program 4.3

Write a program to provide same salary to three employees using copy constructor.

#include<iostream.h>

#include<constream.h>

class employee

{

        private:

                int salary;

        public:

                employee(int);

                employee(employee&);

                void display();

};

employee::employee(int val)

{

        salary=val;

}

employee::employee(employee &e)

{

        salary=e.salary;

}

void employee::display()

{

        cout<<salary<<endl;

}

main()

{

        clrscr();

        employee e1(4500);

        employee e2(e1);

        employee e3=e2;

        cout<<"Salary of  employee1 :";e1.display();

        cout<<"Salary of  employee2 :";e2.display();

        cout<<"Salary of  employee3 :";e3.display();

        return(0);

}

Output

Salary of  employee1 :4500

Salary of  employee2 :4500

Salary of  employee3 :4500

Explanation

In the above program, the source object e1 is initialized first, then its content is assigned to destination object e2 and e3 as declared in the main( ) function.

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