TITLE: Why use virtual destructors (let me count the ways)


PROBLEM: khan@San-Jose.ate.slb.com (Furqan Khan)

How is the virtual destructor definition used in the
case of an abstract class (class that cannot be
instantitated directly).


RESPONSE: rmartin@rcmcon.com (Robert Martin), 20 Sep 94

Folks: Using virtual destructors is very very important.  You need an
extremely good reason for not using one.

There are three (count 'em, three) reasons to use virtual destructors.

1.  Without a virtual destructor, the proper destructor may not be
called:

struct B {~B();};
struct D : B {~D();};
B* b = new D;
delete b; // <--------- Will not call D::~D() !!!!!

2. Without a virtual destructor, operator delete(void*, size_t) may
not be called with the correct size.

struct B {~B(); operator delete(void*, size_t);};
struct D : B {~D();};
B* b = new D;
delete b; // <--------- Will call operator delete(void*, size_t) with
          //            the size of B not the size of D!!!

3.  Without a virtual destructor, and when MI is used, operator
delete(void*) or operator delete (void*, size_t) may be called with
the wrong address.

struct B {~B();};
struct A {};
struct D : A, B {~D();};
B* b = new D;
delete b; // <--------- May not pass to operator delete the address
          //            that was returned by operator new!!!


All of these conditions are very deadly.  It does not matter if B is
an abstract base or not.  The same issues apply.  So ALWAYS use a
virtual destructor unless you have a very very good reason.

What is a good reason?  Well, you have a class like:

    struct TinyPoint
    {
      char x,y;
    };

This class takes up two bytes.  A virtual destructor will probably add
4 bytes to this for the vtbl pointer.  If you are going to allocate a
million of them, then you will have 2meg taken up by data, and 4meg
taken up by pointers, that all point to the same thing.  Thus, this is
probably a good case for not declaring a virtual destructor.