What is virtual constructors/destructors?

Virtual destructors: If an object (with a non-virtual destructor) is destroyed explicitly by applying the delete operator to a base-class pointer to the object, the base-class destructor function (matching the pointer type) is called on the object.   There is a simple solution to this problem – declare a virtual base-class destructor. This makes all derived-class destructors virtual even though they don’t have the same name as the base-class destructor. Now, if the object in the hierarchy is destroyed explicitly by applying the delete operator to a base-class pointer to a derived-class object, the destructor for the appropriate class is called.  Virtual constructor: Constructors cannot be virtual. Declaring a constructor as a virtual function is a syntax error.  Does c++ support multilevel and multiple inheritance? Yes.   What are the advantages of inheritance? • It permits code reusability. • Reusability saves time in program development. • It encourages the reuse of proven and debugged high-quality software, thus reducing problem after a system becomes functional.   What is the difference between declaration and definition?  The declaration tells the compiler that at some later point we plan to present the definition of this declaration.  E.g.:  void stars () //function declaration  The definition contains the actual implementation. E.g.:  void stars () // declarator  {   for(int j=10; j>=0; j--) //function body    cout<<”*”;   cout<<endl;  }  
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deepak

  • Jul 4th, 2006
 

Constructor can't be virtual but u can have the feature of virtual construct which is also called factory pattern.

SomGollakota

  • Jun 13th, 2007
 

Constructors cannot be virtual. This is a given.
Distructors, on the other hand, can be virtual and help in destroying child class objects using a base class pointer. Consider the following example.

class A {
public:
B();
~B();
}
class B : public A {
B();
~B();
}

When you create an instance (object) of class B, the way the object is created is, first Object A is created and then Object B (thus concluding the object of B - Object B contains both the parent and child objects). The destruction of the object B is in reverse order (first object B is destroyed, and then object A).
If, on the other hand, you were to have a pointer to object A and assign it a dynamic object B, when you destroy it, only object A's destructor is called and object B part of the object is never destroyed. Virtual destructors are used to solve this problem and destroy the objects in the right order.

krisgroup

  • Oct 22nd, 2007
 

hi,

we can not have the virtual constructors the basic reason is that, it turns out to be a syntax error because when you create some virtual function it must go into a VTable kind of thing that keeps track of these virtual functions, since its the constructor we dont have the table created at that time so declaring a constructor virtual can throw an execption or error...

the prime reason to declare the destructors virtual is that in large scale programs we may forget to deallocate all the virtual functions we delcared and declaring the destructor virtual can ensure that all objects created get deallocated at the termination of the program,

hope it makes sense,

chaitanya gudipati

berezleon

  • Dec 1st, 2007
 

Virtual constructor is not build-in C++ feature but it doesn't mean its not used by devs in code and in conversations. There are many other things that doesn't exist in particular language yet, people find ways around to solve it (SingleTon, Virtual Constructor, Double-Dispatching etc). What means by virtual ctr is a static member function that returns pointer to a heap object (whether new or existing) or it might return reference to static object declared in this function. It would be really wrong to say that "virtual ctr" doesn't exist --it means you are not aware of such thing and absolutely nothing else. The nice thing about virt ctr each derived "virt ctr" can return pointer to a type of the class it's declare in --thus we can say virtual functions have different signatures depending where they implemented:

class base                     {public: virtual base * virtCtr(){return new base;}
class deriv:public base  {public: virtual deriv * virtCtr(){retun new deriv;}

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Tausif

  • Nov 29th, 2014
 

Constructor can be virtual function because constructor get invoked whenever an object is created, which take place at compile time. But in the case of virtual function binding takes place at the run time.

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Eric Nantel

  • Apr 23rd, 2015
 

There is no virtual constructor !

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