Functions in c++

 Like c, c++ also contain a special function called
main().
 The main() function is an entry point of program
execution, it contain the code that tell computer what
to do as a programme execution.
 Because main() is function, you can write other
function within your source file and main() can call
them.
c++ lecture note by hansa halai

 Executable program
main()
{
// call functions..
someFunction1();
someFunction2();
}
someFunction2();
someFunction1();
Operating system
runs program
Return to OS

 In C++ , the main() returns value of type int to
operating system, so in c++ the main() function is
written as ,
int ()
int main(int arg1,char *argv[])
 So , if we write a return type int , we have to use return
statement inside the main() function, which are
written as:
int main()
{
…
return 0;
}

 Advantage of using functions:
o It help in reducing both physical and executable file size
in large program.
o Save memory, because all the calls to a function tell the
compiler to execute the same block of code.
o Enhances the program’s readability, understandability,
and maintainability.
o It helps in the reusability of the code.

 A function prototype tells the compiler the name of
function, the type of data returned by the
function, the number of parameter the function
expect to receive ,the type of parameters, and the
order in which these parameters are expected.
 The compiler use function prototypes to validate
function call.
 Syntax:
return_type function_name(argument_list);

#include<iostream>
using namespace std;
int max(int n1,int n2);
int main()
{
int n1;
int n2;
int a;
cout<<"Enter Number1: "<<"n";
cin>>n1;
cout<<"Enter Number2: "<<"n";
cin>>n2;
a=max(n1,n2);
cout<<"max value is "<<a<<"n";
return 0;
}

int max(int n1,int n2)
{
int result;
if(n1>n2)
result=n1;
else
result=n2;
return result;
}

 The c++ provides easy and effective use of reference
variable.
 When function is called, the caller creates a reference
for each argument and using references the actual
argument are accessed.
 This method of passing the arguments or parameters
to the function is called call by reference.

 To use the call by reference , function is written as:
int exch(int& a,int& b)
{
int t;
t=a;
a=b;
b=t;
}
When this function is called as :
exch(x , y);
It passes the arguments as
int & a=x;
int & b=y

#include<iostream>
int exch(int&,int&);
int main()
{
int num1,num2;
cout<<"Enter Number1: ";
cin>>num1;
cin>>num2;
cout<<"nBefore Swap:"<<"nn";
cout<<"Number1 is:"<<num1<<"n";

exch(num1,num2);
cout<<"nAfter Swap:"<<"nn";
return 0;
}
int exch(int& a,int& b)
{
int t;
t=a;
a=b;
b=t;
}

 Not only c++ function accepts the arguments as a
reference , but it can also return a reference.
 Like,
int& min(int& a, int& b)
{
return(a<b ? a:b);
}

#include<iostream>
int& max(int&,int&);
int main()
{
int num1,num2,m;
cin>>num1;
cin>>num2;

m= max(num1,num2);
cout<<"max number is "<<m;
return 0;
}
int& max(int& a,int& b)
{
return(a>b?a:b);
}

 Values of actual
argument are passed to
dummy argument.
 Need extra memory to
store copy of value.
 Called function can not
access actual values in
caller function
 Reference are created for
actual argument.
 No need for extra
memory as only aliases
are created.
 Called function can
access the actual values
in caller function using
reference.
Call By Value Call By Reference

 Provide only one way
communication from
caller to called function.
 Slower as values are to be
copied.
 Simple and easy to
understand.
 Provide two way
communication between
caller and called function.
 Faster as no memory
operation required.
 Simple and easy to
understand

 We know that functions save memory space because all
calls to function cause the same code to be executed,
the function body need not to be duplicate in memory.
 When a compiler see a function call, it normally
generate a jump to the function, at end of the function,
it jump back to the instruction after the call.
 But when every time a function is called, it takes a lot of
extra time in executing a series of instructions for task
as jumping to the instruction.

 One solution for speedy execution and saving memory
in c++ is use inline function , it is used only for short
function.
 When use inline function , the function code in the
inline function body is inserted into the calling
function, instead of the control going to the function
definition and coming back.
 So function can not jump from its calling place. So it
will save the jumping time program execution is faster
compare to normal function. But it is used only for
short function, not use any loop inside it.

 You can declare an inline function before main() by
just placing the keyword inline before the normal
function.
 Syntax:
inline function_name(argument_list)
{
function_body
}

#include<iostream>
inline float mul(float x, float y)
{
return(x*y);
}
inline float div(float a, float b)
{
return(a/b);
}

int main()
{
float a,b;
cout<<"Enter number1:";
cin>>a;
cout<<"Enter number2:";
cin>>b;
cout<<"Answer of multiplication is "<<mul(a,b)<<"n";
cout<<"Answer of Division is "<<div(a,b)<<"n";
}

 Normally function is called with same number of
argument listed in function header.
 C++ differ in this matter and allows calling of function
with less number of argument then listed in its header.
 C++ make it possible by allowing default argument.
 Ex:
int max(int a,int b, int c=10);

#include<iostream>
int max(int,int,int c=10);
int main()
{
int a,b,p;
cout<<"Enter value:";
cin>>a>>b;
p=max(a,b);
cout<<"Max num is: "<<p<<endl;
return 0;
}

int max(int x,int y,int z)
{
int max =x;
if(y>max)
max=y;
if(z>max)
max=z;
return max;
}

 We can use const to make the arguments to the
function constant so that function can not change the
value of that argument.
 Like,
int fun(const int x)
{
int y;
x= x+1; // invalid
y=x; // valid
Return(y);
}

#include<iostream>
float circle(float,const float pi=3.14);
int main()
{
float r,area;
cout<<"Enter r:";
cin>>r;
area=circle(r);
cout<<"Area of circle is "<<area<<endl;
}
float circle(float r,float pi)
{
float c_area;
c_area=pi*r*r;
return c_area;
}

 The function overloading is ,same function name is
used for different task or different purpose it is also
called function polymorphism in oop.
 Using this concept, we can design a family of function
with same function name but it have different number
of arguments, different types of arguments and
different return type.
 All the function perform different operation
depending on matching of argument list in the
function call.

#include<iostream>
const float pi=3.14;
float area(float r)
{
return(pi*r*r);
}
float area( float l, float b)
{
return(l*b);
}
int main()
{
float radius;
cout<<"Enter radius: ";
cin>>radius;

cout<<"Area of circle is : "<<area(radius)<<endl;
float length,breadth;
cout<<"Enter length: ";
cin>>length;
cout<<"Enter breadth: ";
cin>>breadth;
cout<<"Area of rectangle is: "<<area(length,breadth)<<endl;
return 0;
}

Functions in c++

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