2.5 Functions and pointers

Function and parameter declarations

In C++ all subprograms are referred to as functions . A function allows you to treat a related group of C++ statements as a single unit. The programmer can write functions to define specific tasks that could be used at many points in a program.

Functions allow the programmer to modulize a program. All variables declared in function definitions are local variables – they are known only in the function in which they are defined. Most functions have a list of parameters that provide the means for communicating information between functions.

There are several motivations for dividing a program into functions. The divide-and-conquer approach makes program development more manageable. Another motivation is software reusability – using existing functions as building blocks to create new programs. Software reusability is a major factor in object-oriented programming. With good function naming and definition, programs can be created from standardized functions that accomplish specific tasks. A third motivation is to avoid repeating code in a program. Packing code as a function allows the code to be executed from several location in a program simply by calling the function.

Defining a function

The lines that compose a function within a C++ program are called a function definition. The syntax for defining a function is

data_type name_of_function (parameters){

statements;

}

A function definition consists of four parts:

- A reserved word indicating the return data type of the function’s return value.

- The function name

- Any parameters required by the function, contained within parentheses.

- The function’s statements enclosed in curly braces { }.

Example: The following function determines the largest integer among three parameters passed to it.

int maximum( int x, int y, int z )

{

int max = x;

if ( y>max )

max = y;

if ( z>max )

max = z;

return max;

}

You designate a data type for function since it is common to return a value from a function after it executes.

General format of a function is described below.

Variable names that will be used in the function header line are called formal parameters.

How to call functions

To execute a function, you must invoke, or call, it from the main () function.

The values or variables that you place within the parentheses of a function call statement are called arguments or actual parameters.