Lesson 12: User-Defined Functions
Functions are the building blocks of a C/C++ program. A function is a group of statements that together performs a specific task.
Why do we need functions?
- Functions help us in reducing code redundancy. If functionality is performed at multiple places in software, then rather than writing the same code, again and again, we create a function and call it everywhere. This also helps in maintenance as we have to change at one place if we make future changes to the functionality.
- Functions make code modular. Consider a big file having many lines of code. It becomes easier to read and use the code if the code is divided into functions.
- Functions provide abstraction. For example, we can use library functions without worrying about their internal work.
12.1 Functions
The basic syntax of a function in C/C++ has 2 parts:
- Function Definition:
A function definition provides the actual body of the function. - Function Declaration:
A function declaration tells the compiler about a function's name, return data type, and parameters.
Function Definition
Function Definition
At the heart of a C/C++ program is the function. It is the place in which all program activity occurs. The general form of a function definition is as below
return_type function_name (parameter_list)
{
// body of function
}
A C++ function definition consists of a function header and a function body. Here are all the parts of a function −
- Return Type
A function may return a value. The type of data returned by a function is specified by return_type. Some functions perform the desired operations without returning a value. In this case, the return_type is the keyword void. - Function Name
This is the actual name of the function. The function's name can be any valid identifier. The function name and the parameter list together constitute the function declaration. - Parameters
The parameter list is a comma-separated (,) list of variables that will receive any arguments passed to the function. The parameter_list refers to the data type, order, and the number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters. - Function Body
The function body contains a collection of statements that define what the function does.
For example, the following function has two integer parameters called i and j, and a double parameter called count, and no return value:
void fn(int i, int j, double count)
{
…
}
Notice, you must declare each parameter separately.
Function Declaration
Function Declaration
Function declarations are also called function prototypes. A function declaration tells the compiler about a function name and how to call the function. The actual body of the function can be defined separately. In C++, all functions must be declared. In C, declarations are technically optional but strongly recommended. The syntax of a function declaration is shown here:
ret_type function_name (parameter_list) ;
The function declaration is actually similar to the function definition except in two aspects:
- The parameters' names may not be specified in the function declaration.
- The function declaration must be ended by a semicolon.
For example:
Parameter names are not important in the function declaration, only their data type is required, so the following is also a valid declaration —
int max2 (int, int);
Return Values
Functions terminate and return automatically to the calling procedure when the last brace (}) is encountered. You may force a return before that by using the return statement.
All functions, except those declared as void, return a value. The type of the return value must match the type declaration of the function. Values are returned via the return statement.
Multiple or No Parameters
Multiple Parameters
A function definition may have multiple parameters, separated by commas. Parameters are assigned with argument values by position: The first parameter with the first argument, second with second, etc.
To specify the declaration for a function that takes a variable number of arguments, use three periods (...) at the point at which the variable number of parameters begin. For example, the printf() function could be declaration like this:
int printf(const char *format, ...);
No Parameters
In C, to specifying use declaration for a function that has no parameters, use void in its parameter list. For example:
int fn(void);
In C++, a function definition with no parameters must still have the parentheses with an empty parameter list; the void is optional. Thus, in C++, the preceding declaration can be written like this:
int fn();
The call must include parentheses with no argument, as in: fn().
In C++, you can include void in the parameter list, but doing so is redundant.
12.2 Function Calling Methods
Parameters and Arguments
The programmer can influence the behavior of the function through inputs.
- Parameter
A parameter is a function input specified in a function definition. - Argument
An argument is a value provided to a function's parameter during a function call. An argument can be a variable, a constant value, or an expression.
A parameter is like a variable declaration. Upon a call, the parameter's memory location is allocated, and the parameter is assigned with the argument's value. Upon returning to the original call location, the parameter is deleted from memory.
Function Calling
A function call is an invocation of a function's name, causing the function's statements to execute.