Please see below ... Joe.

-----Original Message----- From: Frank Heckenbach [SMTP:frank@g-n-u.de] Sent: Monday, February 18, 2002 8:27 AM To: gpc@gnu.de Subject: Re: gcc-3+

Markus Gerwinski wrote:

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Frank Heckenbach wrote:

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Though I'm not sure why the distinction between store and reference pointers is necessary. Why not make all pointers the same, and when one pointer to an object is disposed of, all the other ones are invalidated?

This is usually not, what you do... is it? If the pointer p1 is the one used to allocate p1^, and it is also referenced by p2..pn, you normally take care to use p1 again to deallocate, don't you?

I don't think so. It's not uncommon, e.g., for a function to allocate and return (copy) something in a pointer, or to pass (copy) a pointer to a procedure to clean it up and dispose it. Or to allocate a pointer in a local variable, do something with it, then put it in a global list from where it's later disposed, etc.

Or do you have any arguments why these are less common situations than what you describe?

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Besides, when you really want to evaluate things, you have to describe all the hidden costs first. AFAICS, for every object you need an implicit list containing all the reference pointers, so they can be invalidated later. Depends on the way how to implement it... Now I'm interested how else you'd implement it. An alternative I could think of are double-pointers, but these are slower to dereference and cause another kind of memory leak: Each second-level pointer allocated once can never be freed if you don't know if there's somewhere a first-level pointer pointing to it. I suppose that's not what you have it mind.

To be honest, I don't quite get the idea nor the problem... What exactly do you mean by double-pointers?

The problem is, as Marco said, how do you find all the existing reference pointers to invalidate when disposing of one store pointer.

With double pointers I mean: When you allocate some memory, you allocate another pointer to it, and let the real pointers point to that. So if you dispose of the memory, you set the other pointer to nil (or something), and you don't have to iterate over all reference pointers. The problem then would be that you can never dispose of the other pointer if you don't keep track of all existing reference pointers, so it's not a good solution.

Or how do you implement it?

[Joe da Silva] FWIW, I see the "store pointer" being what you are describing here as the "other pointer" (I think). It points to the real data, and the "reference pointers" just point to it (the "store pointer"). Since you are not allowed to "re-new" a "store pointer" without "disposing" first, the "reference pointers" can't become "stale". I'm not sure that affects how you keep track of the "reference pointers" though, you probably still need to point back at them as you suggest ("double pointers"), since searching for them would generally be inefficient.

Frank

-- Frank Heckenbach, frank@g-n-u.de, http://fjf.gnu.de/, 7977168E GPC To-Do list, latest features, fixed bugs: http://agnes.dida.physik.uni-essen.de/~gnu-pascal/todo.html