virtual

This is a discussion on virtual within the C++ Programming forums, part of the General Programming Boards category; can any body please say me about virtual constructor and destructor...

  1. #1
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    Cool virtual

    can any body please say me about virtual constructor and destructor

  2. #2
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    "me about virtual constructor and destructor"

    No, really, what do you want to know ? I'm not going to write a tutorial for you but I can help you on a specific question though.

  3. #3
    and the hat of sweating
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    Constructors can't be virtual.

    Destructors of a base class should almost always be virtual to ensure that the proper destructor is called. Eg.
    Code:
    Base* pBase = new Derived;
    ...
    delete pBase;
    In the code above, if the destructor in Base is virtual, then the delete would call Derived::~Derived(); but if the destructor was not virtual, it would call Base::~Base(). Since pBase is actually a pointer to Derived, the wrong destructor would be called and bad things could happen.

  4. #4
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    Quote Originally Posted by cpjust View Post
    Constructors can't be virtual.
    This is true, but there is an idiom known as the "virtual copy constructor". In this code;
    Code:
    class Base
    {
        public:
    
            virtual ~X() {};    // virtual destructor
    
            virtual Base *Clone() const = 0;
    };
    
    class Derived: public Base
    {
        public:
    
             virtual Base *Clone() const {return new Derived(*this);};
    }
    
    int main()
    {
        Base *x, *y;
        x = new Derived;
        y = x->Clone();
        //   x and y both point at an instance of Derived, and *y is a copy of *x
    }
    the Clone() member function is referred to as a "virtual copy constructor". It must be supplied by every class derived from Base, and is used to create a copy of an object (assuming that both base and derived classes have properly functioning copy constructors).

  5. #5
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    a *virtual* constructor is not really a constructor, it's a member function that returns a pointer to the class it's declared in.

    virtual constructor is really an oxymoron, they don't really exist. though its purpose may seem to be a constructor, it's just a member function.

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