What are constructors and deconstructors in C programming language?

Answer 1

Constructors are functions that you use to define (initialize) the properties and methods of a class. By definition, constructors are functions within a class definition that have the same name as the class. For example, the following code defines a Circle class and implements a constructor function: // file Circle.as class Circle { private var circumference:Number; // constructor function Circle(radius:Number){ this.circumference = 2 * Math.PI * radius; } } The term constructor is also used when you create (instantiate) an object based on a particular class. The following statements are calls to the constructor functions for the built-in Array class and the custom Circle class: var my_array:Array = new Array(); var my_circle:Circle = new Circle(9);

Answer 2

C is not an object oriented programming language. Even if it were, you cannot have a virtual constructor. Only destructors can be virtual.

In C++, which is object oriented, a constructor is used to instantiate an object of a class. All objects must be constructed from the ground up, from the least-derived class to the most-derived class. This is done automatically.

Declaring a method virtual forces the most-derived override of that method to take precedence. This is exactly what we want during destruction because we cannot destroy a base class object until we have destroyed all its derivatives -- the exact opposite of construction. Thus when designing base classes we must declare the destructor virtual. If we don't, we will end up with an invalid derivative should a base class object be forced out of scope before its derivatives, as can happen if we delete a pointer to the base class object. With a virtual destructor, the most-derived class would be destroyed first, even if we did not know its exact type at compile time. The object's virtual table ensures the correct method is invoked at runtime.