Functions in C++

1.Function declaration(prototype)

• Function declaration tells the compiler the return type of the functions, number of arguments, function  return type of the arguments and order of arguments
• A function declaration  can be inside or outside the main() but the before  function call;

return_type func_name(arg1,arg2..argn);

int sum(int,int);

2.Function definition

• The set of statements which defines the task of the function
• The return type must be mentioned in function definition which is the same given in function prototype

int sum(int x,int y)// function defination
{
return x+y; 
}

3.Function call

• The function call is a request to execute the function defined , till then function will not execute
• No return-type should be specified at the function call
• A user can call function any number of time and anywhere

int add(2,3);//invalid
add(2,3);//valid

4.Formal Parameters

• The parameters present in the function definition are formal parameters
• Formal parameters will receive value from the actual parameter
• Formal parameters must be variable type only

int sum(int x,int y)//func_defination
{
return x+y;
// x and y are formal parameters
}

5.Actual parameters

• Parameters passed during function call are called formal parameters
• Actual parameters make a communication to formal parameters
• They can be the variable, constant or expression type

add(2,3);//constant type actual parametres
add(x,y);//variable type actual parametres
add(2*3,7-2);//expression type actual parametres

6.Return statement

• Return type specifies the function output and allocates some space to function
• Return type also depends on arguments type

int sum(int,int);
float sum(int,float);
long fact(int);

• Generally, it is used at the function definition end, because it makes to exit the function module
• Another major purpose of the return statement, it exits from the function module and gives control back to the caller

#include <iostream>
using namespace std;
int demo()//fun definition
{
cout<<"control acquired from main\n";
return 1;
//return0;error:multiple return types not allowed
}
int main()
{
cout<<"before call control under main\n";
cout<<"controltranmsferreed from main to demo\n"<<demo();
cout<<"control return back from demo to main()\n";
return 0;
}

before call control under main
controltranmsferreed from main to demo
control acquired from main
control return back from demo to main()

• here function call at line demo() transfers function call to from main() to demo(), “return 1” transfers control back to main()
• Only one return statement is allowed in a function definition
• void return type function cannot use the return statement

void add()
{
return a+b;//error:void cannot return
}

7.Function call

main()//fun call
{
add(2,3);
}

• here main() is calle for add() function, main is requesting add() to execute

8.Function signature

int add(int x,int y)
{
return a+b;
//add(int x,int y) is function signature
}