Overriding Destructors

Grzegorz Kraszewski

The only task of a destructor is freeing resources allocated by the constructor and other methods (some resources may be allocated on-demand only). In any case the destructor must leave the object in the same state as right after DoSuperMethod()/DoSuperNew() in the constructor. After that the destructor calls a super class destructor. The destructor receives an empty message.

IPTR MyClassDispose(Class *cl, Object *obj, Msg msg)
{
  struct MyClassData *d = (struct MyClassData*)INST_DATA(cl, obj);

  if (d->ResourceA) FreeResourceA();
  if (d->ResourceB) FreeResourceB();
  if (d->ResourceC) FreeResourceC();
  return DoSuperMethodA(cl, obj, msg);
}

The example destructor follows the example of the constructor in the Overriding Constructors article. Three resources obtained in the constructor are freed here. The destructor is also prepared for a partially constructed object, every resource is checked against NULL before freeing. If for some type of resource NULL is a valid handle, an additional flag may be added to the object instance data area.