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Learn to Define and Use Functions in C++

Functions can be thought of as separate programs that are designed to perform specific tasks. For example, in a program, separate functions can be so designed to perform specific tasks such as taking input, doing calculation, giving output etc.

In this article we will be discussing about what functions are and how they are designed in C++.

Generally, programs have many logically different parts (input, output etc.) which can be coded separately as function. This makes programs easily manageable and understandable.

Suppose we have to design an invoice program that takes input, calculates and at last displays the output. We have two options to do so, we can either program the whole thing linearly or we can make separate functions for each of the tasks (input, output etc.), this would make the program far more manageable and upgradeable. (In the future, if you need to change certain things, you only need to alter the functions)

In this way if you divide various activities of the program into different function, then it would be easy for you to code and check each of the function besides making the program easier to understand and manage.

Have a look at this program:

   //C++ program
   int sqr(int x);//declaration of the function
   void main(void)
   int a=9;
   cout<<"square of "<<a<<" is ";
   cout<<sqr(a)<<endl;//function is being called here

   int sqr(int x)//no ;(semicolon) here
   return (x);//return keyword is used
   //to return values, in this case
   //the value of x is returned

Few points about the program:

  • In C++, it is necessary to declare a function before defining it as we have done in the line: int sqr(int x);//declaration of the function

  • One function cannot be defined inside another. Therefore sqr(int x) function cannot be defined inside main () which itself is a function.

  • A function can return any type of values except arrays. The value is returned in the line: return (x); and the value got accepted and printed in the line: cout<<sqr(a);

Now that you understand how to declare and use functions, let me give you a slightly more complex program:

   //C++ program
   //declaration of functions
   int take_input(void);
   void play(void);//it is optional to use the void keyword
   void help(void);
   void quit(void);
   //main program starts from here
   void main(void)
   int choice;
   choice=take_input();//take input
   //perform action as per the value returned
   if(choice==1) play();
   else if(choice==2) help();
   else if(choice==3) quit();
   //functions are defined below
   int take_input()
   int ch=0;
   cout<<" MAIN MENU";
   cout<<"1> PLAY"<<endl;
   cout<<"2> HELP"<<endl;
   cout<<"3> QUIT"<<endl;
   cout<<"enter choice and press enter"<<endl;

   while (ch!=1 && ch!=2 && ch!=3)

   return (ch);
   void play(void)
   cout<<"write code for PLAY"<<endl;
   void help(void)
   cout<<"write code for HELP"<<endl;
   void quit(void)
   cout<<"write code for QUIT"<<endl;

This is a simple program to illustrate how the various activities of a program can be divided into separate functions. Notice how easy it is to understand the program. Even if the functions play (), help () etc. had 100’s of lines of code, the program would have been easily understandable.

Hope this helps…

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