Function in c++

Introduction
Functions allow a sequence of statement to be referred to by a
name and parameterized.
•Calling a function causes statement sequence to be executed and
a value may be returned when the function terminates.
•Parameterization allows the same function to be applied to many
different values without changing the statements.
•sqrt is function that computes square root of number, sqrt(4)
computes square root of 4, sqrt(9) computer square root of 9!
Same sequence of statements inside function, with different
parameter returns different value.
•

Declaration
Introduces the function name, function return type,
and function parameters to the program.
•The function body (statements)is not part of the
declaration
•A function must be declared before it is used
Format: return_type function_name(parameter_list);
parameter_list: type param1,type param2,type
param3, etc
•

Some function declarations:

•

double sqrt(double);
int func1();
void func2(char,int);

Function Definition
Introduces the function name, function return type, and function
parameters as well as the function body to the program Function
body (the implementation)is a compound statement
return_type function_name(parameter_list)
{
// Function body
}
• Function Definition Example:
•

void print_func()
{
cout << “hello world” << endl;
}

A function can be defined in any part of the program text or
within a library
•

Functions And Program
Structure
Functions must be declared before they are used, e.g:
l// preprocessor statements
lDefine function 1
lDefine function 2
lDefine function 3
l{ call function 2}
lmain()
l{
lCall function 1
lCall function 3
l}

•

Void Functions
These are functions that do not return a value
They can still cause a side effect by modifying a global variable
Example:
l int num = 1;
lvoid print_func()
l{
lcout << “hello world” << num << endl;
lnum = num + 1;
l}
•Note: It is not possible to declare variables of type void because
type void has no values, i.e. it has no structure
•

Parameterized Functions
A function declaration must include the list of parameter types
(with an optional name):
int func(int x)
•Each parameter type specifies the type of the value that should
appear as a function argument when the function is called.
•Therefore the function func takes one parameter which is an
integer func can be called using: int x =func(42);
•The function name is followed by a bracketed list of function
arguments
•Note: A function definition also requires a parameter list with in
the function body the parameters are accessible as variables
which are named in the parameter list with their types:
•

Int func(int x,int y)
{
// place function body here
// variables x and y can be used here
}

x and y are the parameter variables that can be used in the
function body
• Note that the types of the parameters must be the same in the
function declaration and definition
•Formal parameters: the parameters used in the function
definition, e.g. x and y in
int func(int x, int y) …
•

Parameterized Functions
Actual parameters or arguments: the parameters used in the
function call, e.g. 10 and 20 in
int x = func(10, 20);
•Note that the actual parameters are evaluated before being
passed to
the function, e.g.
•

int a = 10, b =20;
int x =func(a/2, a+b);
•

The actual parameters passed to func are 5 and 30
The order of evaluation of arguments is undefined.

Functions and Scope
Functions may only be referred to within the scope of the
declaration.
•Declarations made in a function body or compound statement
are local to that scope.
•Function parameters are variables in the scope of the function
body.
•A new variable is created for each parameter when ever the
function is called.
•Each parameter variable is destroyed when the function
terminates.
•Hence parameters are local to the function they are part of.
•

Default Parameters
Function parameters may be given default values

•

int func(int x = 10);

If func is called with an empty parameter list

•

func();

then the parameter variable x will be initialized to 10
•If func is called with a parameter
func(15);
then the default is NOT used and the parameter variable x is
initialized to the actual parameter value,I.e.15

Default Parameters
Note that default parameters must appear at the END of the
parameter list
•

int func1(int x = 1);
int func2(int x, int y = 1);
int func3(int x = 1, int y);
•

// this is ok
// this is ok
// this is not ok

Also several default parameters can be given

int func4(char c, int x = 1, int y = 1); this is ok

Parameter Passing In
Functions

Arguments/Parameters are a way to share data between two
functions.
•In all the programs we have seen so far, the arguments in a
function call provide data needed by the called function
•

they pass data in to the function
There are times when a function wants to use its parameters to
pass data back to the calling program.
•

examples are when functions must return more than one value or must
modify the arguments passed to them.

We need two types of parameter passing to handle these two
situations.
•

Call By Value
Call by value means the value of i is passed as a parameter to
func
The local variable I of func is initialized to that value
The local variable I of main that appears in the call is left
untouched -only a copy of its value is passed to the function
func.
•

Call By Reference
References allow a second parameter passing mechanism to be
used - call by reference
•If a formal parameter type is a reference type
•

int func(int &x)

Then the parameter variable x will be a reference to the actual
parameter.
•

int y = 1;
func(y);
x will be initialized to a reference to y when the function func is called
So x and y will both denote the SAME object

Contrast this to call by value where x will be initialized to a copy
of the value of y.
•

Inline Function
Inline functions are functions where the call is made to
inline functions. The actual code then gets placed in the
calling program.
Once you define an inline function, using the 'inline'
keyword, whenever you call that function the compiler
will replace the function call with the actual code from
the function.
General form
inline datatype function_name(arguments)

Inline Function
#include <iostream>
using namespace std;
int func(int);
void main( )
{
int x;
cout << "n Enter the Input Value: ";
cin>>x;
cout << "n The Output is: " << func(x);
}
inline int func(int x1)
{
return 5*x1;
}
Note:-Inline functions will save time and are useful if the function is
very small. If the function is large, use of inline functions must be
avoided.

Function in c++

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Function in c++