Difference Between Copy Constructor and Assignment Operator in C++

A copy constructor is a type of constructor that uses another object from the same class that has been created previously, to initialize an object, whereas the assignment operator is used to assign a value to a variable.

In this article, we will understand the difference between the copy constructor and the assignment operator in C++.

Copy Constructor

A copy constructor creates a new object by copying an existing object of the same class which has been created previously. It is of two types, i.e., Default and User-defined copy constructor.

• In the default copy constructor, the copy constructor is not declared, but the C++ compiler creates a default copy constructor that copies all member variables as they are. It performs a shallow copy.
• The User-Defined Copy Constructor is defined by a user and performs a deep copy.

Example 1

The following example demonstrates the default copy constructor performing a shallow copy where the address is copied, not the actual content, and upon changing the original data, the copied data also gets changed:

#include <iostream>
using namespace std;

class Box
{
public:
   int length;

   Box(int l)
   {
      length = l;
   }

   void display()
   {
      cout << length << endl;
   }
};

int main()
{
   Box b1(10);
   cout << "Length of box 1: ";
   b1.display();

   // Copy constructor copies b1 into b2

   Box b2(b1);
   cout << "\nLength of box 2: ";
   b2.display();
   return 0;
}

The output of the above code is:

Length of box 1: 10
Length of box 2: 10

Example 2

The following example demonstrates the user-defined constructor that performs a deep copy:

#include <iostream>
using namespace std;

class Box
{
public:
   int length;

   // Constructor
   Box(int l)
   {
      length = l;
   }

   // User-defined copy constructor
   Box(const Box &b)
   {
      length = b.length; // Copy value manually
   }

   void display()
   {
      cout << length;
   }
};

int main()
{
   Box b1(10);
   cout << "Length of box 1: ";
   b1.display();
   // User-defined copy constructor called
   Box b2(b1);
   cout << "\nLength of box 2: ";
   b2.display();
   return 0;
}

The output of the above code is:

Length of box 1: 10
Length of box 2: 10

Assignment Operator

The assignment operators in C++ are used to assign values to variables and to set or update the value stored in the variable. The left operand of the assignment operator is the variable name, and the right operand of the operator is the value of that variable. The datatype must be the same for both the operands; if not, a compilation error will be raised. Some of the assignment operators are as follows: '=', '+=', '-=', and so on.

Example

Here is an example of assignment operator:

#include <iostream>
int main()
{
   int a = 5, b;
   b = a;
   std::cout << "The value of a is " << a << "\n";
   std::cout << "The value of b is " << b;
   return 0;
}

The output of the above code is:

The value of a is 5
The value of b is 5

Differences Between Copy Constructor and Assignment Operator

The differences between the copy constructor and assignment operator are mentioned below:

                     Box b1(50);
                     Box b2(b1);
                  

                     Box b1(50);
                     Box b2 = b1;