What is Function Overloading ?

A function name having several definitions that are differentiable by the number or types of their arguments in known as overloading of function.

We know that a function consists of a prototype and a body. The protype consists of the function’s return type, function’s name and its argument list (also known as function signature). Any two functions that happen to have the same name but differ in their function signature, are said to be overloaded. This difference in their signature may be due to changed types of arguments or by changed number of arguments in the argument list or both.

Function overloading implements polymorphism, which is a key feature of Object Oriented Programming (OOP). The ability of a function to have same name but different behaviours depending upon the situation is considered as polymorphism.

Also, using overloaded functions reduces number of comparisons in a program and thereby makes the program execute faster.

 

Declaring & Defining Overloaded  Functions

The key to function overloading is a function’s argument list which is also known as the function signature. It is the signature, not the function return type that enables function overloading.

If two functions are having same number and types of arguments in the same order, they are said to have the same signature. Even if they are using distinct variable names, it doesn’t matter. For instance, following two functions have same signature.

void squar (int a, float b);

void squar (int x, float y);

C++ lets you overload a function name provided the functions with same name are having different signatures. The signature can differ in the number of arguments or in the type of arguments, or both.

To overload a function name, all you need to do is – declare and define all the functions with the same name but with different signatures, separately. For instance, following code fragment overloads a function name square().

void square (int i);

void square (char c);

void square (float f);

void square (double d);

After declaring, you must define them separately. This defining can also be done directly at the time of declaration.

When a function name is declared more than once in a program, the compiler will interpret the second (and the rest) declarations as :

  • if the signatures of subsequent functions match the previous function’s, then the second is treated as a re-declaration of the first. In that case the compiler raises error.

         void sum (int a, int b);

         void sum (int x, int y);  // error;

  • if the signatures of two functions match exactly but the return types differ, the second declaration is still treated as an erroneous re-declaration of the first and is flagged at compile time as an error.

         int square (float f);

         float square (float x);   // error;

Functions with the same signature but different return types are not allowed in C++. You can have different return types, but only when the function signatures are different.

  • if the signatures of two functions (having same name) differ, the two functions are considered to be overloaded.


NOTE : While considering the function signature, int and unsigned int or int and const int, etc are considered different datatypes and so functions with arguments with such differences are considered overloaded.

Read more >> Restrictions on Overloaded Functions.

 

Calling of Overloaded Functions

Overloaded functions are called just like other functions. The number and type of arguments determine which function should be invoked. A function call first matches the prototypes available with the number and type of arguments provided with the function call and calls the appropriate function for execution.

Read more >> Steps involved in finding the Best Match.

 

Let us see a demo program in C++

Output screen :

Function 2 called

Function 1 called

Function 3 called

 

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