[Insight-developers] Circular smart pointer references causing memory leaks

[Williams, Norman K]

Vince figured this out and solved one leak, but it points to something
that I didn't think about that needs to be guarded against.


class A contains a SmartPointer to instance of class B
class B contains a SmartPointer to instance of class A

Whoever wrote A & B thought that the destructors of instances of A & B
would in turn call the destructors for the smart pointers, and eventually,
the instances of A & B would all be freed when no smart pointers to them
went out of scope.

But this isn't the case and here's why: Even after all third party
SmartPointers to A & B go out of scope, the instances of A & B will still
have a reference count of 1, so the destructors will never get called, and
the smart pointer members will live forever.

The only solution, apparently, is that there needs to be a Pre-destructor
that clears all smart pointers in the class, that gets called before all
the smart pointers to an object go out of scope or get re-assigned.

Now, in general, this isn't a problem, but the FEM code is a multiple
interconnected cyclic graphs of smart pointer references. Knowing that I
can begin to try and fix it.

But it points up a weakness in ITK's dependence on Smart Pointers.  Even
though in most cases, it solves memory allocation issues; in others it's a
nightmare, where letting a SmartPointer to an object go out of scope
actually makes the object pointed to immortal.

Anyone have any ideas how to fix this other than "don't do that!"?



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