How to implement the deep copy constructor using pointer in C++

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I tried to implement the pointer, but was not able to reproduce it.

Please help me to go ahead...

C++

#include<iostream>
using namespace std;
class A
{
public:
int a1;
int *b1;
A(int a, int b)
{
cout<<"Constructor\n";
a1 = a;
b1 = new int(b);
}
A(const A& a)
{
cout<<"Copy constructor\n";
a1 = a.a1;
b1 = new int;
b1 = (a.b1);
}
};
#include<stdio.h>
 
int main()
{
A *a = new A(88, 56);
A *a1;
a1 = a;
printf("Address of a : %p\n", a);
printf("Address of a1 : %p\n", a1);

What I have tried:

I tried with static allocation, but using pointer, am missing something. Please help!

Posted 12-Oct-23 4:08am

mathiv327

Updated 12-Oct-23 9:38am

Deeksha Shenoy

v2

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Rick York · Answer 1 · 2023-10-12T05:38:00

This has been heavily revised. I added a default constructor, an assignment constructor, and an assignment method :

C++

class A
{
public:
    int   a1 { 0 };
    int * b1 { nullptr };

    A() {}                  // default constructor

    A( int x, int y )       // assignment constructor
    {
        Set( x, y );
    }

    A( const A & other )    // copy constructor
    {
        Set( other.a1, * other.b1 );
    }

    // copy operator

    A & operator = ( const A & other )
    {
        // prevent self-assignment

        if( this != & other )
        {
            // do copy operation

            Set( other.a1, * other.b1 );
        }
        return *this;
    }

    void Set( int x, int y )   // assignment method
    {
        a1 = x;
        if( b1 )
            delete b1;
        b1 = new int( y );
    }
};


int main()
{
    A a;
    A b( 42, 43 );

    a = b;
    trace( "a.a1 is %d, a.b1 is %d\n", a.a1, * a.b1 );

    A c( 56, 57 );
    trace( "c.a1 is %d, c.b1 is %d\n", c.a1, * c.b1 );

    A d( a );
    trace( "d.a1 is %d, d.b1 is %d\n", d.a1, * a.b1 );

    return 0;
}

New assignments need to re-allocate the b1 pointer if there is one so it MUST be initialized appropriately as shown. That is one reason why initialization is so important.

Note how all constructors are routed through the Set method. This can be a useful tactic to use, especially if your members are not public.

BTW - trace is my own output function that I use with GUI programs. You can replace it with printf or use cout redirection like your were.

k5054 · Answer 2 · 2023-10-12T05:54:00

The statement A *a1 only defines a pointer to an object, but does not allocate any memory to the pointer. There's at least two options that I can think of to invoke a copy constructor when using pointers to objects:
1) create the new pointer object using by dereferencing the poiner

C++

A *a2 = new A(*a)

2) create a pointer version of the copy constructor

C++

class A {
  // ...
  A(const A *a) {
    // construct new A object
  }
};

int main() 
{
    A *a1 = new A(1, 2);
    A  *a2 = new A(a1);
}

If the object is complex, then a private member function that does the work might be a good idea.

How to implement the deep copy constructor using pointer in C++

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