Constructors and Destructors

Constructors and Destructors:

Constructors: A constructor is a member function of a class which initializes objects of a class. In C++, Constructor is

automatically called when object (instance of class) create. It is special

member function of the class.

class A

{

    public:

    int x;

    // constructor

    A()

    {

       

    }

};

Destructors: Destructor is a member function which destructs or deletes an object.

A destructor works opposite to constructor; it destructs the objects of classes. It can be defined only once in a class. Like constructors, it is invoked automatically.

class A

{

    public:

    // defining destructor for class

    ~A() 

    {

        // statement

    }   

};

                            Types of Constructors:

Constructors are of three types:

1. Default Constructor
2. Parametrized Constructor
3. Copy COnstructor

1.Default Constructors: Default constructor is the constructor which doesn’t take any argument. It has no parameters.

Syntax:

class_name(parameter1, parameter2, ...)

{ 

    // constructor Definition 

}

2.Parameterized Constructors: It is possible to pass arguments to constructors. Typically, these arguments help initialize an object when it is created. To create a parameterized constructor, simply add parameters to it the way you would to any other function. When you define the constructor’s body, use the parameters to initialize the object.

class Cube

{

    public:

    int side;

    Cube(int x)

    {

        side=x;

    }

};

int main()

{

    Cube c1(10);  Cube c2(20);

    Cube c3(30);

    cout << c1.side;

    cout << c2.side;

    cout << c3.side;

}

3.Copy Constructor: A copy constructor is a member function which initializes an object using another object of the same class. Detailed article on Copy Constructor.

1.    class A  

2.    {  

3.        A(A &x) //  copy constructor.  

4.       {  

5.           // copyconstructor.  

6.       }  

7.    }   

Function:

A function is a block of code which only runs when it is called.

You can pass data, known as parameters, into a function.

Functions are used to perform certain actions, and they are important for reusing code: Define the code once, and use it many times.

---

Create a Function

C++ provides some pre-defined functions, such as main(), which is used to execute code. But you can also create your own functions to perform certain actions.

To create (often referred to as declare) a function, specify the name of the function, followed by parentheses ():

Call a Function

Declared functions are not executed immediately. They are "saved for later use", and will be executed later, when they are called.

To call a function, write the function's name followed by two parentheses () and a semicolon ;

In the following example, myFunction() is used to print a text (the action), when it is called:

Example

Inside main, call myFunction():

// Create a function
void myFunction() {
  cout << "I just got executed!";
}

int main() {
  myFunction(); // call the function
  return 0;
}

// Outputs "I just got executed!"

Function Declaration and Definition

A C++ function consist of two parts:

• Declaration: the function's name, return type, and parameters (if any)
• Definition: the body of the function (code to be executed)

Parameters and Arguments

Information can be passed to functions as a parameter. Parameters act as variables inside the function.

Parameters are specified after the function name, inside the parentheses. You can add as many parameters as you want, just separate them with a comma:

Syntax

void functionName(parameter1, parameter2, parameter3) {
  // code to be executed
}

Example

void myFunction(string fname) {
  cout << fname << " Refsnes\n";
}

int main() {
  myFunction("Liam");
  myFunction("Jenny");
  myFunction("Anja");
  return 0;
}

output:

Liam Refsnes
Jenny Refsnes
Anja Refsnes

Default Parameter Value

You can also use a default parameter value, by using the equals sign (=).

If we call the function without an argument, it uses the default value ("Norway"):

Example

void myFunction(string country = "Norway") {
  cout << country << "\n";
}

int main() {
  myFunction("Sweden");
  myFunction("India");
  myFunction();
  myFunction("USA");
  return 0;
}

output:

Sweden
India
Norway
US

Types of function:

1.Library Function

Library functions are the built-in function in C++ programming.

Programmer can use library function by invoking function directly; they don't need to write it themselves.

Example 1: Library Function

#include <iostream>

#include <cmath>

using namespace std;

int main()

{

    double number, squareRoot;

    cout << "Enter a number: ";

    cin >> number;

    // sqrt() is a library function to calculate square root

    squareRoot = sqrt(number);

    cout << "Square root of " << number << " = " << squareRoot;

    return 0;

}

Output

Enter a number: 26

Square root of 26 = 5.09902

2.User-defined Function:

C++ allows programmer to define their own function.

A user-defined function groups code to perform a specific task and that group of code is given a name(identifier).

When the function is invoked from any part of program, it all executes the codes defined in the body of function

Example 2: User Defined Function

C++ program to add two integers. Make a function add() to add integers and display sum in main() function.

#include <iostream>

using namespace std;

// Function prototype (declaration)

int add(int, int);

int main()

{

    int num1, num2, sum;

    cout<<"Enters two numbers to add: ";

    cin >> num1 >> num2;

    // Function call

    sum = add(num1, num2);

    cout << "Sum = " << sum;

    return 0;

}

Output

Enters two integers: 8

-4

Sum = 4

.