FUNCTIONS IN C++

What is a Function?

A function is a set of statements that perform a particular task. When a particular operation needs to be done time and again in a program, we can define a function to perform that operation. The function can then be used anywhere in the program any number of times. Functions are one of the most important parts of any program language. In this article we will learn about basics of functions.

The main() function

Technically, when we compile a program, there should be a starting point from where our program starts its execution. This starting point is the function main(). Every C++ program must have one and only one main() function. The main() function is an interesting aspect of C++ and we will read in details about it in some later articles. But almost always, it has an int return type and no arguments.

Types of functions

In C++, there are 2 types of functions.

In-built library functions :

There are many functions that are already defined under some specific header files and are already present in the installation file for C++. All these header files form the Standard Template Library (STL) of C++. To use these pre-defined functions, we need to include the specific header file in our program using the syntax :

                        #include<header_name>

 To read in details about STL see other articles on our website.

User-defined functions :

These functions are declared and defined by a programmer according to his need and convenience. In this article and the articles that follow, we will be focusing on user-defined functions only.

 

Function Definition

A function definition consists of the function prototype and its body.

 

Function Prototype : The entire function declaration, i.e. the return type, function name and the parameter list constitute a function prototype.

Function signature : It is a subset of function prototype. The parameter list is termed as a function’s signature. The use of this term arrives while discussing Function Overloading.

return_type : It is a valid C++ datatype. A function may or may not return any value. In case if a function doesn’t return any value, the return_type is set as void. Otherwise, the return_type is any valid C++ datatype (primitive, derived or user-defined) depending upon the type of value being returned.

function_name : This the name of the function. The function is known in the program by this name. While naming a function, keep the following points in mind :

First, it can only consist of alphabets, digits and underscore(_).

Second, it cannot contain any special character (not even space).

Third, it cannot start with a digit.

Fourth, it must not match any keyword in C++.

Fifth, As C++ is a case dependent language, using different case letters means different names. Example, func & Func are two different names.

Argument List : It is a collection of the type of values that the function will take whenever it is called. For example,

               (int a, float b)

This list will take 2 values whenever it is called, one of int type and other of float type. These values will be stored in a and b respectively.

Body : It is the set of statements that perform the desired operation. They may call other functions as well.

Let us see a demo function definition :

//function to take 2 integers as argument

//and return the maximum of the two.

int max (int a, int b)

{

    int result;

    if(a>b)

        result=a;

    else

        result=b;

    return result;

} 

Here the function named max is created which takes two integer values as a & b and then returns an int value. See the following statement :

int great = max(5 , 10);

cout << great;

Output : 10

Here, the function puts 5 in a and 10 in b and then executes all its body statements. As b is greater than a, 10 is stored in result and it is returned. This return value is caught by the int variable great. So now great stores 10 (i.e. the larger value among a & b).

 

Function Declaration

A function declaration tells the compiler about a function name and how to call that function. Basically, a function declaration is the function prototype. The actual body of the function can be defined separately.

Also, in declaration without body, there is no need to name the arguments. Only the type name is sufficient. Example :

int max(int a, int b);

int max(int, int);

both are correct.

There are a few things you must keep in mind while declaring and defining a function :

First, the functions that are directly called into main() must be at least declared  before main(). Their full definition may be made before or after main(). This is important as the compiler must know that such a function that is being called in main() actually exists.

Second, any other function, that is not being directly called into main(), can be declared and defined anywhere in the program.

Third, a function must be declared and defined outside of all functions.

 

A demo program in C++

Output Screen :

Enter 2 integers : 25   34

Larger integer is 34

 

 

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