#constructor
define constructor in english and hindi
#constructor Constructors in C++
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What is constructor?
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.
How constructors are different from a normal member function?
A constructor is different from normal functions in following ways:
Constructor has same name as the class itself
Constructors don’t have return type
A constructor is automatically called when an object is created.
If we do not specify a constructor, C++ compiler generates a default constructor for us (expects no parameters and has an empty body).
Constructor Parameters
Constructors can also take parameters (just like regular functions), which can be useful for setting initial values for attributes.
The following class have brand, model and year attributes, and a constructor with different parameters. Inside the constructor we set the attributes equal to the constructor parameters (brand=x, etc). When we call the constructor (by creating an object of the class), we pass parameters to the constructor, which will set the value of the corresponding attributes to the same:
Just like functions, constructors can also be defined outside the class. First, declare the constructor inside the class, and then define it outside of the class by specifying the name of the class, followed by the scope resolution :: operator, followed by the name of the constructor (which is the same as the class):
Use of Constructor in C++
Suppose you are working on 100's of Person objects and the default value of a data member age is 0. Initialising all objects manually will be a very tedious task.
Instead, you can define a constructor that initialises age to 0. Then, all you have to do is create a Person object and the constructor will automatically initialise the age.
These situations arise frequently while handling array of objects.
Also, if you want to execute some code immediately after an object is created, you can place the code inside the body of the constructor.
A constructor is defined which initialises length to 5 and breadth to 2.
We also have three additional member functions GetLength(), AreaCalculation() and DisplayArea() to get length from the user, calculate the area and display the area respectively.
When, objects A1 and A2 are created, the length and breadth of both objects are initialized to 5 and 2 respectively, because of the constructor.
Then, the member function GetLength() is invoked which takes the value of length and breadth from the user for object A1. This changes the length and breadth of the object A1.
Then, the area for the object A1 is calculated and stored in variable temp by calling AreaCalculation() function and finally, it is displayed.
For object A2, no data is asked from the user. So, the length and breadth remains 5 and 2 respectively.
Then, the area for A2 is calculated and displayed which is 10.
Output
Enter length and breadth respectively: 6
7
Area: 42
Default Area when value is not taken from user
Area: 10
Constructor Overloading
Constructor can be overloaded in a similar way as function overloading.
Overloaded constructors have the same name (name of the class) but different number of arguments.
Depending upon the number and type of arguments passed, specific constructor is called.
Since, there are multiple constructors present, argument to the constructor should also be passed while creating an object.
Default Copy Constructor
An object can be initialized with another object of same type. This is same as copying the contents of a class to another class.
In the above program, if you want to initialise an object A3 so that it contains same values as A2, this can be performed as:
....
int main()
{
Area A1, A2(2, 1);
// Copies the content of A2 to A3
Area A3(A2);
OR,
Area A3 = A2;
}
You might think, you need to create a new constructor to perform this task. But, no additional constructor is needed. This is because the copy constructor is already built into all classes by default.