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
.