RE: Strange crashes

Hi Steve,

You wrote:

> if (cachedDir)
> return m_mostRecentStatus = cachedDir->GetStatusForMember(...);
>
> The crash happens on the second line, because 'cachedDir' is NULL!
> Now, how can it crash there? The if() statement above should
> make sure
> that 'cachedDir' isn't NULL when the second line is executed.

This can happen if cachedDir points to an object, that has been destroyed
somewhere else. It can become a real bitch to track down, because depending
on your 'luck' the place that cachedDir points to can still be a valid
object of the proper type for some time until the memory really gets
overwritten.

Do you (for example) have a cachedDirList somewhere? Assume cachedDir points
to a member of the list. The list gets freed at some other place and
cachedDir never gets to know that this has happened.

A good way to get around this is to implement the observer pattern. Short
version: Each object being watched has a list of its observers. In its
destructor it notifies the observers of its destruction. It can also notify
the observers of other changes if need be. Each observer adds/removes itself
to/from the objects list of observers when he starts/stops watching an
object.

I am working on a measurement application, where lots of windows have to
monitor changes in a lot of objects, which can show up, change and
dissappear at any time (from the windows point of view) and the observer
pattern really helps a lot.

Cheers
- Lübbe

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