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  • Function is defined as a named group of statements.
  • A function can be called from any point in a program.
  • If a problem can be divided into small modules, then each module can be represented as a function.
  • There are two type of functions in C++. They are
    • Library function
    • User defined function.

Library Function

  • These are functions which are not required to be written by the programmer.
  • But these are available in separate files and the programmer has to include it in the appropriate places.
  • Example
    • ​​cin >>, cout<< etc.

User Defined Function

  • A user defined function or simply functions have to be written by the programmer to carry out some specific well defined task.

  • All C++ programs consist of one or more functions.

  • Out of this one function should contain main.

  • This function shows from where the program execution begins.

Note: Library function is also called as Built in function

Function Definition

  • A function should be defined before it is used.
  • A function has two parts. They are
    • Function header
    • Statement body

The general format of defining a function is given below

Function type function name (list of arguments)


Local variable declaration;

Executable statement1;



return (expression); }


Function type - represents the data type of the value returned by the function
Function name - valid C++ variable name
List of arguments- formal argument
Local variable - declaration of variables in the statement body
Return - keyword to return the value of the function to other functions.
Rules for defining a function:
  • Function header should not terminate with semicolon
  • List of arguments and argument declaration are optional
  • if the function has no list of arguments an empty parenthesis is a must.
  • The expression in the return statement is optional
  • The parentheses around the expression in the return statement is optional

For example

int abc(int I, int j)


int k;

k = I + j;

return (k);



Declaring Function Type

  • The function type in the function header is optional.

  • If there is no function type in the function header the return statement by default returns a integer value.

  • But with the help of the option function type in the function header we can declare the type of the value returned by the function.

  • General format

Type function_ name (parameter list);

Use of void Statement

  • void statement does not return any value.

  • It can be used to declare any function.

  • It specifies an empty set of values and it is used as the return type for function that does not return a value.

  • Thus a function that does not return a value is declared as follows

  • The general format is

void function_name(parameter list);

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