Functions in C++

In a C++ programming language a function is a group of statements that together perform a special task.

Every C++ program has at least one function, which is main().

In a C++ library has a numerous predefined or built-in functions.

For example, strcat() to concatenate two strings, main() from where program execution starts.

In a C++ programming function has a 3 part:

Function declaration
Function definition
Function call

Note: In a C++ program we can remove the function definition but we must place the function definition before the main() function.

Function declaration : function declaration tells the compiler about a function’s name, return type, and parameters.

A syntax of function declaration –

return_type   function_name( parameter list );

Example of the function declaration is as follows −

int max(int num1, int num2);

here int is a return type of function which tells function will return integer value and max is a name of the function.

“ int num1, int num2” is a parameter list which tells max function will receive two integer arguments.

Arguments are optional which depends on program requirements.

Function definition : function definition is an actual body of the function.

Syntax of function definition:

return_type function_name( parameter list ) 
{
   body of the function
}

function definition consist a return type, function name, parameter list and body of the function.

Function call : whenever program calls a function, the flow of control is transferred to the called function definition.

A called function performs a defined task and when flow of control reached to function-ending closing brace, flow of control returns to back to the main program.

Example of a user defined function.

#include 

using namespace std;

void DemoFunction(); // Function declaration

int main() // The main method
{
        DemoFunction(); // call the function
        return 0;
}

void DemoFunction() // Function definition
{
        cout << "Hello Aditya";
}

Output

Hello Aditya

Description: Program execution start from main() function.

In main() when DemoFunction(); executes it calls and transfer control to the function definition.

Then statements of DemoFunction() execute and print “Hello Aditya”.

Note: In a C++ function can be called multiple times.

#include 

using namespace std;

void DemoFunction(); // Function declaration

int main() // The main method
{
        DemoFunction(); // call the function 
        DemoFunction(); // call the function 
        DemoFunction(); // call the function
        return 0;
}

void DemoFunction() // Function definition
{
        cout << "Hello Aditya \n";
}

Output

Hello Aditya 
Hello Aditya 
Hello Aditya

Note: In a C++ program we can remove the function definition but we must place the function definition before the main() function.

#include 

using namespace std;

void DemoFunction() // Function definition
{
        cout << "Hello Aditya";
}

int main() // The main method
{
        DemoFunction(); // call the function
        return 0;
}

Output

Hello Aditya

User-defined function can’t declared after the main()

In a C++ program we can remove the function definition but we must place the function definition before the main() function.

If we place the function definition after main() then we will get the error message.

Because in a C++ execution occurs from top-to-bottom and if the function is not declared above main(), the program is unaware of it.

#include 

using namespace std;

int main() // The main method
{
        DemoFunction(); // call the function
        return 0;
}
void DemoFunction() // Function definition
{
        cout << "Hello Aditya";
}

 error: ‘DemoFunction’ was not declared in this scope

Categories of function

We can categories the function as:

Function with no argument, no return value
Function with argument but no return value
Function with argument and return value
Function with no argument, but return value

Function with no argument & no return value

Example: Write a program to find the area of circle using function with no argument, no return value.

The formula for the area of the circle is :

Area_circle = Π * r * r

Program: To calculate Area of Circle

#include 

using namespace std;
void areaOfcircle(); //Function declaration

int main() {
        areaOfcircle(); // Function Call
        return 0;
}

void areaOfcircle() // function definition to calculate area of circle		          
{
        float area, radius;
        cout << "Enter the radius of circle : ";
        cin >> radius; // take radius as input

        area = 3.14 * radius * radius;
        cout << "\n Area of circle= : " << area;
}

Output

Enter the radius of circle
5		// enter by user
Area of circle=78.50

Function with argument but no return value

Example: Write a program to find the area of circle using function with argument but no return value.

#include 

using namespace std;
void areaOfcircle(float r); //Function declaration

int main() {
        float radius;
        cout << "Enter the radius of circle : ";
        cin >> radius; // take radius as input
        areaOfcircle(radius); // Function Call
        return 0;
}

void areaOfcircle(float r) // function definition to calculate area of circle		          
{
        float area;
        area = 3.14 * r * r;
        cout << "\n Area of circle= : " << area;
}

Output

Enter the radius of circle
5		// enter by user
Area of circle=78.50

Function with argument & return value

Example: Write a program to find the area of circle using function with argument but return value.

#include 

using namespace std;
float areaOfcircle(float r); //Function declaration

int main() {
        float radius, a;

        cout << "Enter the radius of circle : ";
        cin >> radius; // take radius as input

        a = areaOfcircle(radius); // Function Call
        cout << "\n Area of circle= : " << a;
        return 0;
}

float areaOfcircle(float r) // function definition to calculate area of circle		          
{
        float area;
        area = 3.14 * r * r;
        return area;
}

Output

Enter the radius of circle
5		// enter by user
Area of circle=78.50

Function with no argument but return value

Example: Write a program to find the area of circle using function with no argument but return value.

#include 

using namespace std;
float areaOfcircle(); //Function declaration

int main() {
        float a;
        a = areaOfcircle(); //	 Function Call
        cout << "\n Area of circle= : " << a;
        return 0;
}

float areaOfcircle() // function definition to calculate area of circle		          
{
        float radius, area;
        cout << "Enter the radius of circle : ";
        cin >> radius; // take radius as input

        area = 3.14 * radius * radius;
        return area;
}

Output

Enter the radius of circle
5		// enter by user
Area of circle=78.50

Call by value & Call by reference

Call by value : Whenever we call a function and passes the value of variable to the called function is called call by value.

#include 

using namespace std;

void swap(int a, int b) {
        int temp;
        temp = a;
        a = b;
        b = temp;
}

int main() {
        int a = 10, b = 20;
        cout << "Value of a & b before swap=" << a <<" "<< b << endl;
        swap(a, b); // passing value to function
        cout << "Value of a & b after swap=" << a <<" "<< b;
        return 0;
}

Output

Value of a & b before swap=10   20
Value of a & b after swap=10   20

Call by reference : Whenever we call a function and passes the reference of variable to the called function is called call by reference.

In the call by reference, to pass the reference of the variable we use pointer.

#include 

using namespace std;

void swap(int * x, int * y) {
        int swap;
        swap = * x;
        * x = * y;
        * y = swap;
}

int main() {
        int x = 10, y = 20;
        cout << "Value of x & y before swap=: " << x <<" "<< y << endl;
        swap( & x, & y); // passing reference/ address of variable to function  
        cout << "Value of x & y after swap=: " << x <<" "<< y;

        return 0;
}

Output

Value of x & y before swap=: 10  20
Value of x & y after swap=: 20  10