Why Assignment Operator Overloading Must Return Reference?

Operator overloading in C++ allows us to define custom behaviors for operators when applied to user-defined types. One of the most commonly overloaded operators is the assignment operator (=), which is used to assign the value of one object to another. However, when overloading the assignment operator, it’s important to ensure that it returns a reference to the object being assigned. But why is this necessary?

In this article, we will learn why the assignment operator must return a reference in C++ when overloading, and what could go wrong if it doesn’t.

Why Must the Assignment Operator Return a Reference?

When overloading the assignment operator in C++, it’s important that it returns a reference to the object being assigned. There are several key reasons for this:

1. Chaining of Assignment Operations

In C++, assignment operations can be chained together.

For example:

A a, b, c;
a = b = c;

To support this chaining, the assignment operator must return a reference to the object being assigned. This allows the operation b = c to return b, enabling a = b to work as expected.

2. Consistency with Built-in Types

For built-in types, the assignment operator in C++ returns a reference to the left-hand operand. To maintain consistency and intuitive behavior for user-defined types, overloaded assignment operators should also return a reference.

For example:

int a, b, c;
a = b = c = 5;

3. Avoiding Unnecessary Object Copies

If the assignment operator returned an object by value instead of by reference, it would result in the creation of a temporary object, which is immediately discarded. This unnecessary copying is inefficient and could lead to performance issues, especially for large objects or objects managing dynamic resources.

C++ Program to Demonstrate Properly Overloading the Assignment Operator

The below example demonstartes how to properly overload the assignment operator in C++.

// C++ program to demonstrate Properly Overloading the Assignment Operator
#include <iostream>
using namespace std;

class MyClass{
  private:
    int *data;

  public:
  
    // Constructor
    MyClass(int value) : data(new int(value)){
        cout << "Constructor called, value: " << *data << endl;
    }

    // Copy constructor
    MyClass(const MyClass &other) : data(new int(*(other.data))) {
        cout << "Copy constructor called, value: " << *data << endl;
    }

    // Copy assignment operator
    MyClass &operator=(const MyClass &other){
        if (this != &other) { 
          
          // Prevent self-assignment
            *data = *(other.data);
            cout << "Copy assignment operator called, value: " << *data << endl;
        }
      
      // Return reference to the current object
        return *this; 
    }

    // Destructor
    ~MyClass(){
        cout << "Destructor called, deleting value: " << *data << endl;
        delete data;
    }

    // Getter for the value
    int getValue() const{
        return *data;
    }
};

int main()
{
  // Constructor called
    MyClass obj1(10);  
  
   // Constructor called
    MyClass obj2(20);   
  
  // Copy constructor called
    MyClass obj3 = obj1; 
  
  // Copy assignment operator called
    obj2 = obj1;         

    cout << "obj1 value: " << obj1.getValue() << endl;
    cout << "obj2 value: " << obj2.getValue() << endl;
    cout << "obj3 value: " << obj3.getValue() << endl;

    return 0; 
}

Constructor called, value: 10
Constructor called, value: 20
Copy constructor called, value: 10
Copy assignment operator called, value: 10
obj1 value: 10
obj2 value: 10
obj3 value: 10
Destructor called, deleting value: 10
Destructor called, deleting value: 10
Destructor called, deleting value: 10

What Happens if Assignment Operator Does Not Return a Reference?

If we overload the assignment operator and return by value instead of by reference, several issues could arise:

• Chained assignments like a = b = c; would not work correctly.
• Returning by value would create temporary objects, leading to inefficiencies.
• The overloaded assignment operator would behave differently from the built-in assignment operator, which could confuse others of our class.

Conclusion

In C++, when overloading the assignment operator, we must return a reference to the current object (*this) as it allows for assignment chaining, maintains consistency with built-in types, and avoids unnecessary object copying. By following these best practice, we can ensure that our overloaded operators are efficient, intuitive, and behave as expected, making our C++ programs more robust and maintainable.

vinamrajha

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Article Tags :

• C++ Programs
• C++
• Operators
• C++-Operator Overloading

Practice Tags :

• CPP
• Operators