Copy Constructor in C++

Copy Constructor

Here, in this section, we will discuss the copy constructor in C++. Copy Constructor in C++is a type wherein we use an existing object to create multiple construction initializations for newer objects that will be created. It is a special member function of the class.

copy constructor in C++

What is a Copy Constructor?

class_name (class-name &){
     ....
}
  • Copy Constructor is a type of constructor which is used to create a copy of an already existing object of a class type.
  • It is usually of the form X (X&), where X is the class name.
  • The compiler provides a default Copy Constructor to all the classes.

Code

Run
#include <iostream>
using namespace std;
class PrepInsta 
{ 
private: 
    int x, y; 
public: 
    PrepInsta()
    { // empty default constuctor
    }
    
    PrepInsta(int x1, int y1) 
    { 
        x = x1; 
        y = y1;

        cout << "Parameterized constructor called here" << endl;

    } 

    // User defined Copy constructor 
    PrepInsta(const PrepInsta &p2) 
    {
        x = p2.x; 
        y = p2.y; 
        cout << "Copy constructor called here" << endl;
    } 

    int getX()            
    {  
        return x; 

    } 
    int getY()            
    {  
        return y; 

    } 
}; 

int main() 
{ 
    // Trying to call parameterized constructor here
    PrepInsta p1(10, 15); 
    
    // Trying to call copy constructor here 
    PrepInsta p2 = p1; 
    
    // Trying to call Copy constructor here (Another way of doing so)
    PrepInsta p3(p1);
    PrepInsta p4;
 
    // Here there is no copy constructor called only assignment operator happens
    p4 = p1;
    
    cout << "\nFor p4 no copy constructor called only assignment operation happens\n" << endl;

    // displaying values for both constructors 
    cout << "p1.x = " << p1.getX() << ", p1.y = " << p1.getY();
    cout << "\np2.x = " << p2.getX() << ", p2.y = " << p2.getY(); 

    cout << "\np3.x = " << p3.getX() << ", p3.y = " << p3.getY(); 
    cout << "\np4.x = " << p4.getX() << ", p4.y = " << p4.getY();
    
    return 0; 
}

Output

Parameterized constructor called here
Copy constructor called here
Copy constructor called here

For p4 no copy constructor called only assignment operation happens

p1.x = 10, p1.y = 15
p2.x = 10, p2.y = 15
p3.x = 10, p3.y = 15
p4.x = 10, p4.y = 15

Types of Copy Constructors

It is really great way of creating new object initialisations and is still widely used by programmers.

There are two types of copy constructors which are –

  • Default Copy Constructors (Does Shallow Copying)
  • User Defined Copy Constructors (Does Deep Copying)

We will learn more about these two in detail below –

Shallow Copy

When we do not create our own copy constructor. C++ compiler will create a default copy constructor for each class which does a member-wise copy between objects.

This default copy constructor does shallow copy.

Code

Let us see an example to see how this works –

Run
#include <iostream>
#include <string.h>
using namespace std;

class PrepInsta
{
    char *prepString;
    
    public:
    
    // Note : No user defined Copy constuctor created here
    
    PrepInsta(const char *prepString1)
    {
        //We are invoiking Dynamic memory allocation here
        prepString = new char[16]; 
        strcpy(prepString, prepString1);
    }
  
    /* concatenate method */
    void concatenate(const char *prepString1)
    {
        // using function to Concatenating two strings
        strcat(prepString, prepString1); 
    }
  
    void print()
    {
        cout << prepString << endl;
    }
};

/* main function */
int main()
{
    PrepInsta obj1("Prep");
    
    //Copy constructor
    PrepInsta obj2 = obj1; 
  
    cout << "Before concatenation - " << endl;
    
    obj1.print();
    obj2.print();
    
    // obj1 is invoking concatenate()
    // but change will also be reflected in obj2 (shallow copy)
    // see more explanation in below comments
    obj1.concatenate("Insta");
  
    cout << "\nAfter concatenation - " << endl;
    
    // as we didn't create any user defined copy constructor
    // compiler created default copy constuctor in backend on its own
    // So both obj1 & obj2 variables share same memory (shallow copy)
    // any change in obj1 also reflects in obj2
    obj1.print();
    obj2.print();

  
    return 0;
}

Output

Before concatenation - 
Prep
Prep

After concatenation -
PrepInsta
PrepInsta
Any change in obj1 also reflects in obj2 (shallow copy)

Deep Copy

When we create our own user-defined copy constructor. The individual copies of memory are created for each object and thus there is no sharing of memory.

This user-defined copy constructor does Deep copy.

Code

Let us see an example to see how this works –

Run
#include <iostream>
#include <string.h> 
using namespace std;

class PrepInsta
{
    char *prepString;
    
    public:
    PrepInsta (const char *str)
    {
        //We are invoking Dynamic memory allocation here
        prepString = new char[16];
        strcpy(prepString, str);
   }
   
   // here we are doing the change of adding additional code
   // for user defined copy constructor (Deep Copy)
    PrepInsta (const PrepInsta &x2)
    {
        //We are invoking Dynamic memory allocation here
        prepString = new char[16]; 
        strcpy(prepString, x2.prepString);
    }
   
    void concatenate(const char *prepString1)
    {
        // using function to Concatenating two strings
        strcat(prepString, prepString1);
    }
   
   ~PrepInsta()
    { 
        delete [] prepString;
    }
   
   void print()
   {
        cout << prepString << endl;
   }
};

/* main function */
int main()
{
    PrepInsta p1("Prep");
    PrepInsta p2 = p1; //copy constructor
   
   cout << "Before concatenation - " << endl;
    
    p1.print();
    p2.print();
   
    p1.concatenate("Insta");
   
    cout << "\nAfter concatenation - " << endl;
   
    p1.print();
    p2.print(); 
   
    return 0;
}

Output

Before concatenation - 
Prep
Prep

After concatenation -
PrepInsta
Prep
Changing obj1 doesn’t reflect/cause any changes in obj2(Deep Copy)

Prime Course Trailer

Related Banners

Get PrepInsta Prime & get Access to all 200+ courses offered by PrepInsta in One Subscription

Get over 200+ course One Subscription

Courses like AI/ML, Cloud Computing, Ethical Hacking, C, C++, Java, Python, DSA (All Languages), Competitive Coding (All Languages), TCS, Infosys, Wipro, Amazon, DBMS, SQL and others

Checkout list of all the video courses in PrepInsta Prime Subscription

Checkout list of all the video courses in PrepInsta Prime Subscription