Operator Overloading In C++ | Detailed Explanation +Code Examples

Overloading is an important component of object-oriented programming concepts. It entails redefining an operator or function to have special execution behaviour or use. In this context, operator overloading in C++ programming is a powerful feature that allows developers to redefine the behavior of operators for user-defined types, such as classes and structures. This ability enhances code readability and enables intuitive manipulation of objects in a manner similar to primitive data types. 

By overloading operators, you can perform custom operations using familiar syntax, making your code more expressive and easier to understand. In this article, we will discuss everything about operator overloading in C++ with detailed code examples.

Types Of Overloading In C++

There are primarily two types of overloading in C++, including

1. Function overloading
2. Operator overloading

Both of these compile-time polymorphisms and the underlying mechanism remains the same. That is, we define two or more operators/ functions with the same name but different implementation is terms of operations to perform or parameters in intake. This difference in definition helps select the right behavioural implementation at execution time. 

What Is Operator Overloading In C++?

We can easily apply C++ operators to primitive data types (like integers, floating point numbers, characters, etc.). However, to make these operators available for use with user-defined datatypes (like complex numbers and fractions), we need to define their operations explicitly. 

• The concept of operator overloading in C++ allows us to extend the abilities of existing operators and give them special meaning while keeping their original meaning intact.
• In other words, operator overloading in C++ is the process of defining new behaviors for existing operators when they are applied to user-defined types.
• This feature allows you to define how operators like arithmetic addition (+), subtraction (-), multiplication (*) operators, and equality relational operators (==), etc., work with objects of your classes or structures.
• This enables you to perform operations on these objects in a natural and intuitive way.
• The three types of operators used for operator overloading in C++ are unary, binary, and special operators.

Let's consider the addition arithmetic operator, for example. In normal arithmetic terms, it adds two numbers. However, if you were to define a custom class representing a complex number, you might want the + operator to add two complex numbers.

• If you have two complex numbers (a+bi) and (c+di), then you would want the addition operator (+) to perform the operation (a+c)+(b+d)i on the numbers.
• For this, you will have to define the functionality, where the basic function is still the same, i.e., addition. But you are adding the real parts and imaginary parts of the numbers together. 

In a real-world context, think of operator overloading as customizing a tool for a specific job. Imagine you have a specialized screwdriver designed for a unique type of screw. While the screwdriver's primary function is to turn screws, overloading would be like modifying the screwdriver to fit and turn the unique screw properly.

How To Overload An Operator In C++?

For operator overloading in C++ language, we must define a special operator function inside a class or the structure whose objects/variables we want the overloaded operator to work on. This function can be a member function of a class or a friend function (non-member function).

We must use the operator keyword followed by the operator symbol that we want to overload. The syntax for the same is given below.

Syntax Of Operator Overloading In C++

class className {
public:
return_DataType operator symbol (parameter_list) {
//implementaion
}
//Lines of code
};

Here,

• The class keyword marks beginning of class definition with the name className, and public is an access specifier that controls the level of visibility for class elements.
• return_DataType is the data type of the return value that the function will generate.
• The operator keyword indicates that we are overloading an operator, and the symbol refers to the sign/ symbol of the respective operator we want to overload (i.e., +, <, -, ++, etc.).
• The parameter_list refers to the parameters that the function intakes, and the implementation of the operation is to be provided in place of the code comment.

An alternative method of operator overloading in C++ is to define the overloading special function as a friend function using the friend keyword. This is a special non-member function that has access to the class's private & protected members. However, the member function approach is more commonly used for operator overloading in C++.

Operators Overloading In C++ Example

Let's look at a C++ program example illustrating the overloading of the addition arithmetic operator. 

Code Example:

Output:

Complex number: 3.7+2.2i

Explanation:

In the C++ code example, we first include the <iostream> header file for input/ output operations and use the namespace std. 

1. We then define a class called complex with private float data type member variables real and imag, representing the real and imaginary parts of a complex number, respectively.
2. Then, we have a parameterized class constructor to initialize the real and imag members with default values of 0.0 if no arguments are provided.
3. Next, we define a member function to overload the addition operator (+) that takes a constant reference to a complex object other as the parameter.
   • Inside, we first create a new complex object called sum.
   • It then adds two complex objects together by adding the real and imaginary parts of the current object with those of the other object.
   • The addition of the real and imaginary parts are stored in the real and imaginary parts of the sum object, which the function returns.
   • That is, the real parts of the two complex numbers (real of the current object and real of the other object) are added and assigned to the real member of the sum object.
   • Similarly, the imaginary parts of the two complex numbers (imaginary of the current object and imaginary of the other object) are added and assigned to the imaginary member of the sum object.
4. Then, we define a member function called display() to print the complex number in the format real+imaginary(i), where i represents the imaginary unit. 
5. This function uses the if-else statement to check if the imaginary part is negative and adjusts the output accordingly.
6. In the main() function, we create two objects of class complex called c1 and c2 with values (2.5, 3.0) and (1.2, -0.8), respectively. Note that since we have provided value, the constructor body is not implemented.
7. Next, we create another Complex class object sum and assign it the total of the other two objects. Here, we use the overload operator (+) to add c1 and c2. 
8. After that, we use the dot operator to call the display() function on the sum object to print the sum of the complex numbers.
9. Finally, the main() function terminates with a return 0 statement.

Overloadable & Non-overloadable Operators In C++

We now know what operator overloading in C++ is, but do you wonder if all operators can be overloaded? The answer to this question is that almost all operators can be overloaded except for a few shown in the table below:

 Nearly all operators except the ones listed above can be overloaded. We have compiled that information in the table below for convenience. 

In the sections ahead, we will discuss operator overloading for three different types of operators, i.e., unary, binary, and special. 

Unary Operator Overloading In C++

Operators that can operate only on a single operand are referred to as unary operators. For example, the increment operator (++) and the decrement operator (--) are unary operators since they have only one operand. We can overload these operators, as shown in the sample ahead.

Example Of Unary Operator Overloading In C++

In the example below, we have illustrated how to overload the pre-increment operator (++). We can follow the same process for the decrement operator and the suffix type of both operators.

Code Example:

Output:

Counter: 10

Explanation:

In the C++ sample code-

1. We create a class called Counter with a private data member num (integer type) and a public class constructor to initialize it with default value 9.
2. Then, we define a member function to overload the increment operator (++) as a prefix or pre-increment operator.
3. Inside this function, we first increment the value of variable num by 1 (since it is a prefix operator). That is, whenever this operator is used with an operand or variable, the value of the respective variable is increased by 1.
4. Then, we have a void function print(), which uses the cout statement to print the value of the num data member/ variable of the class.
5. In the main() function, we first create a Counter class object called counter without initialization. Since we do not assign a value, the class constructor assigns the default value 9 to it.
6. Next, we use the overloaded pre-increment operator on the counter object. This calls/ invokes the overloaded operator ++ function, which increments the value of the count object.
7. Then, we call the print() function on the counter object using the dot operator to output its value to the console.

Example 2: Return Value Of Overloaded Operator Function (++ Operator)

In this example, we again overload the increment operator, but this time, we use both its prefix and suffix forms. The same can be applied to the prefix decrement operator and its suffix form. 

Code Example:

Output:

Output after pre-increment:Count: 10
Output when applying post-increment:Count: 10
Output after post-increment has been applied:Count: 11

Explanation:

In the example C++ code-

1. Just like in the previous example, we create the Counter class with private data member num and a class constructor that initializes it with the default value 9.
2. We define two member functions to overload the increment operator (++), but the difference is one uses it as a prefix and the other as a suffix.
3. The first operator++() function overloads the pre-increment operator as follows-
   • First, we define the function to create a class object temp and then increment the value of num by 1.
   • After that,  the function returns the temp object with the incremented value of num.
4. The second operator++() function overloads the post-increment operator-
   • We define the function to first create a Counter object called temp with the current value of num.
   • Since we are using the post-increment operator, the value of num is incremented by 1 afterwards, and the function returns the temporary object with this value.
5. Then, we have a void print() function, which displays the value of the class data member num using the cout statements.
6. Moving on, in the main() function, we create two objects of the Counter class called count and output without initialization.
7. Since we did not assign a value, the class constructor initializes both with the data member value 9.
8. Note that whenever we use the increment operator on class objects, the Counter class will implement the functions defined in its scope according to the argument or parameter matching.
   • If the prefix operator is used, the value is increased before assigning to the output variable.
   • If postfix is used, then the value of the count is increased after assigning to the output. Therefore, the change is seen in the next assignment.
9. Next, we use the pre-increment operator on the count object and assign that value to the output object.
   • As per the function definition in the class, the value of the count object is incremented by 1 and then assigned to the output object. 
   • Then, we call the print() function on the output object to display the value after pre-increment is applied.
10. After that, we apply the post-increment operator on count object and assign the value to output.
11. As per the definition, the value of count is assigned to output before increment. So, when we display this value using the print(), the value has not been increased.
12. However, when we call the print() function on the output object after that, the value as shown in the output console has increased by 1. 

Binary Operator Overloading In C++

The operators that work with two operands to carry out an operation are referred to as binary operators. For example, addition, subtraction, and so on. Look at the example below where we overload the less than and greater than relational operators, to compare the area of squares.

Code Example:

Output:

The area of square S1 is larger than area of square S2
The area of square S1 is smaller than area of square S2

Explanation:

 In the example-

1. We first define a class Square representing a square with a single public member len which represents the length of a side of the square.

2. Inside the class, we have a constructor, i.e., Square (int _len), which initializes the len member variable with the value passed to the constructor.

3. Then, we have a member function area() that multiplies the len variable by itself to calculate the areas of the square and return the same.
4. After that, we define a function to overload the greater than operator (>).
   • Inside the function, we use > to compare the area of the current square with another square a.
   • It returns true if the current square's area is greater than the area of a.
   • If the condition is false, then it returns false.
5. Similarly, we define a function to overload the less than operator (<) where it compares the area of the current square with the area of a as follows:
   • It returns true if the condition area() < a.area() is met.
   • If the condition is not met, it returns false. 
6. In the main() function, we create two objects of the Square class, s1 with side length 4 and object s2 with side length 3.
7. Then, we use an if-else ladder conditional statement to compare the areas of these two squares. 
   • The program flow first moves to the if condition, s1 > s2 to check if area of s1 is greater than s2. If this is true, the function prints a string message- "The area of square S1 is larger than area of square S2".
   • If the condition is false, we move to the next if-condition, s1 < s2, to check if the area of s1 is smaller than s2. If true, the function prints the corresponding message.
   • If both conditions are false, the function prints the message in the last else block- "The area of square S1 is equal to the area of square S2".
8. After that, we modify the side length of s2 object and assign 5 to it. Now the area os s1 is still 16, but that of s2 is 25.
9. We again repeated the comparison process. Since the area of s1 is smaller than the area of s2, the function prints the message - "The area of square S1 is smaller than the area of square S2".

Special Operator Overloading In C++

Besides the unary and binary operators, there are some special operators which may not be used in the traditional sense. For example, the new and delete operators are used for dynamic memory allocation.

The act of overloading these operators can extensively increase their use case. Listed in the table below are special operators that can be overloaded in C++, followed an example.

In the sample C++ program below, we have illustrated the process of overload the subscript operator []. This example shows how the [] operator can be overloaded to allow intuitive access to elements within a custom array-like class.

Code Example:

Output:

Element at index 2: 10

Explanation:

We begin the example C++ program by including the <iostream> header file for input and output operations, and the <vector> header to use the std::vector class for storing elements.

1. Then, we define a class IntArray with a private member arr, which is an vector (i.e., std::vector<int>) used to store integer elements.
2. We then have a constructor that initialises the class object, i.e., arr vector, with a list of integers.
3. Next, we define two member functions to overload the subscript operator, once as a non-const, and then as a constant.
4. The non-const overloaded subscript operator allows non-const access to the elements of the arr vector. It returns a reference to the element at the specified index, allowing modification of the element.
5. The const overloaded subscript operator allows const access to the elements of the arr vector. It returns a const reference to the element at the specified index, ensuring the element cannot be modified through this reference.
6. This overload is necessary to support the use of the subscript operator on const instances of IntArray.
7. In the main() function, we create an object of IntArray class called myArray and initialize it with the list of integers {1, 2, 3, 4, 5}.
8. Then, we use the overloaded operator [] to modify the value at index position 2, and assign 10 to it.
9. Next, we use cout statements to print this value, and the output shows that the value has infact changed.
10. This indictes that we can overload the subscript operator to alter its behaviour such that it can help us access the elements of the vector in the same intuitive ways as in case of arrays.

Rules For Operator Overloading In C++

1. Operators (=) and (&) are overloaded by default in C++. For example, if we want to copy objects of the same class, we directly use the assignment operator (=) instead of creating an operator function.
2. Operator overloading in C++ does not change the precedence or the associativity of operators, but we can control the order of evaluation with the use of parenthesis.
3. Only existing operators can be overloaded, and some of them have to be overloaded using the member function since it can not be done using the friend function.
4. Operator overloading in C++ can only be done for user-defined types ( classes and objects) and not for built-in types(char, int, float, etc.).
5. At least one operand must be a user-defined class object.

Advantages And Disadvantages Of Operator Overloading In C++

Listed in the table below are some advantages and disadvantages of operator overloading:

Function Overloading In C++

Function overloading is a concept in which different functions can have the same name but with different function prototype, i.e., arguments, parameters, datatype, order, and use.

• Just like operator overloading in C++, this is also a compile-time polymorphism and is one of the salient features of the language.
• Having the same name function with different arguments, etc. increases the readability of the program.

For example, suppose we have to add two given numbers which can be of any data type. Then, writing different functions for different types, like- add_int( int a, int b), add_float(float a, float b), etc., can be tedious and lead to errors or unexpected behavior. Here,  function overloading can help as it lets us use the same name for the functions with different parameters, like data types. 

As discussed before, the compiler can differentiate between the functions based on the parameters/ arguments. Note that for function overloading to work, one or all of the following conditions must be met:

Parameters should have a different type. For example:

add(int a, int b)
add(float a, float b)

Parameters should have a different number of arguments. For example:

add(int a, int b)
add(int a, int b, int c)

Parameters should have a different order of parameters. For example:

add(int a ,float b)
add(float a, int b)

For more, read: Function Overloading In C++ With Code Examples & Explanation

What Is the Difference Between Operator Functions and Normal Functions?

Although operator functions are similar to normal functions, there are some differences between them. The main difference between the two functions is the keyword operator and the operator symbol that is written in operator functions in place of a function name in normal functions.

Here's a table summarizing the differences between operator functions and normal functions:

Take a look at the piece of code below to gain a better understanding of the same:

Normal Function -

return_DataType function_name(arguments){
// Do something.
:::::::::::::::::::::::::
// Return something.
}

Operator Function -

return_DataType operator + (arguments){
// Do something.
:::::::::::::::::::::::::::
// Return type something.
}

Note that addition arithmetic operator (+) can be replaced by other valid operators, i.e., not non-overlodable operators.

Conclusion

Operator overloading in C++ is a runtime polymorphism concept that helps increase the use case or functionality of a built-in operator by redefining their behaviour.

• However, it is important to note that only the built-in operator can be overloaded, and no new operator can be overloaded.
• Overloading operators in C++ or any other language can largely improve the readability of the code.
• Almost all the operators can be overloaded in C++ except for the scope operator (::), pointer operator (.*), dot operator notation (.), and ternary operator (?:).
• Operator overloading can only define an operator's operation; its precedence can't be changed.
• We must make use of the parentheses to change the associativity or precedence between operators.

Frequently Asked Questions

Q. What is operator overloading in C++? Explain with an example.

Operator ovelroading is the process of redefining the behavior of operators when they are to be applied on user-defined data types.

• Say for example, you want to perform addition on complex numbers, the regular addition arithmetic operator cannot work in the traditional sense.
• You will have to define its behavior to add real and imaginary parts individually.
• This process of redefining the implementation behaviour of an operator is referred to as operator overloading in C++ programming.
• It is a compile-time polymorphism that allows us to extend the abilities of existing operators.

Code Example:

Output:

Complex number: 2.5+1.7i

Explanation:

In the given program-

1. The Complex class is defined with private member variables real and imag, representing the real and imaginary parts of a complex number, respectively.
2. The constructor of the Complex class initializes the real and imag members with default values of 0.0 if no arguments are provided.
3. The operator- function is overloaded as a member function to subtract one Complex object from another. It takes a constant reference to another Complex object (other) as the parameter.
4. Inside the operator- function, a new Complex object diff is created to store the difference of the complex numbers. The real parts of the two complex numbers (real of the current object and real of the other object) are subtracted, and the result is assigned to the real member of the diff object.
5. Similarly, the imaginary parts of the two complex numbers (imag of the current object and imag of the other object) are subtracted, and the result is assigned to the imag member of the diff object.
6. The diff object is then returned.
7. The display function is defined to print the complex number in the format real+imaginaryi, where i represents the imaginary unit.
8. In the main function, two Complex objects, c1 and c2, are created and initialized with different real and imaginary values.
9. The - operator is used to subtract c2 from c1, and the result is stored in the diff object.
10. Finally, the display function is called on the diff object to print the difference of the complex numbers.

Q. What are logical operators in C++ with examples?

Logical operators are those that compare and connect various expressions such that the value of the whole expression is dependent on the original expression, variable, and values. While relational operators can only check the relation between two particular variables or values, with logical operators, we can check various conditions simultaneously.

In other words, logical operators provide us with the ability to test multiple conditions at the same time. The general syntax for the use of logical operators is given below. 

Syntax:

Operand1 logical_operator operand2 ;

Here, operand1 and operand2 are the two expression which can be any boolean conditions made up using other operators (like relational). And the logical_operator is the respective operator we are using to connect and compararative expressions. The various types of logical operators are listed in the following table:

Q. Is operator overloading a type of polymorphism?

Yes. Operator Overloading in C++ is a compile-time polymorphism that allows us to extend the abilities of existing operators and give special meaning to them while keeping their original meaning intact. Here the name or symbol of the operator remains the same, but the parameters required and implementation/ behaviour change.

Q. Is operator overloading a type of method overloading?

No, they are two distinct concepts. Operator overloading and method overloading may seem similar, there is a prominent point of difference.

• Operator overloading overloads the built-in operators to provide the specific implementation for user-defined data types without changing their original meaning.
• Whereas method/ function overloading refers to the overloading of two or more regular functions (function operator) with the same name but different numbers, datatype, and order of the parameters.

Q. What are the types of operator overloading?

The three types of operators used for operator overloading in C++ are:

• Unary: These operators can operate only on one operand. For example, the increment (++) and decrement (--) operators. 
• Binary: These operators need two operands to carry out an operation. For example, arithmetic, logical relational, and other operators like addition, greater than, etc.
• Special operators: Some special operators in C++ are new, delete, [], (), and (-->) operators. 

Q. Provide an example of operator overloading in Cpp.

Let’s take an example of how can we overload the logical NOT binary operator (!) and extend its ability to help us find the conjugate of a complex number.

Code Example:

Output:

Enter first complex number:
Enter real and imaginary parts:
real:7
imaginary:3
Complex number: 7+3i
Conjugate Complex number: 7+-3i

Explanation: 

In the given program 

1. The Complex class is defined with private member variables r and i, representing the real and imaginary parts of a complex number, respectively.
2. The default constructor initializes the real and imaginary parts to 0.
3. The input function prompts the user to enter the real and imaginary parts of the complex number and stores them in the corresponding member variables.
4. The operator! function is overloaded as a member function to find the conjugate of a complex number. It creates a new Complex object, copies the real part (r) of the current object, and assigns the negation of the imaginary part (i) to the new object's imaginary part.
5. The print function is defined to display the complex number in the format real+imaginaryi.
6. In the main function, three Complex objects (comp1, comp2, and result) are created. The user is prompted to enter the first complex number using the input function, and the print function is called on the result object to display the complex number.
7. The operator! is used to find the conjugate of comp1, and the result is assigned to the result object.
8. Finally, the print function is called on the result object to display the conjugate of the complex number.

Q. What is the list of operators in C++ programming language? 

The most common forms of operators in C++ programming are

• Arithmetic Operators: They are used to perform common arithmetic operations.
• Assignment Operators: They perform variable initialization or value assignment to other variables.
• Comparison Operators: Perform common relational operations generating Boolean-type values.
• Logical Operators: They are used to perform logical calculations or special operations on one or more operands. For example, AND, OR, and NOT.
• Bitwise operators: They perform operations on the bit level of the operands, i.e., they perform Bitwise operations.

Q. What are the two types of overloading in C++?

The two types of overloading are function overloading and operator overloading in C++. Both of these overloadings are a type of compile-time polymorphism wherein the name of the function/ operator remains the same, but the parameters required, implementation, and the use case change.

• Function Overloading is when different functions have the same name but different arguments, parameters, datatype, order, and use.
• Operator Overloading in C++ allows us to extend the abilities of the existing operators to be applied with user defined data types like complex numbers, arrays, structures, class objects, etc.

Do check out the following:

1. 1. C++ 2D Array & Multi-Dimensional Arrays Explained (+Examples)
   2. Typedef In C++ | Syntax, Application & How To Use (+Code Examples)
   3. C++ Templates | Types, Usage, Overloading & More (+Code Examples)
   4. Static Member Function In C++: How to Use Them, Properties, & More
   5. 25+ Difference Between C And C++ | Similarities, Features & More