langley@yoyodyne.rtp.dg.com (03/19/90)
How do I write a class so that a routine will get called every time the object is forgotten? In particular, whenever the object becomes unreachable, or is subjected to an explict forget operation, I want a class routine to get called. Stated another way, if I have a function that is called whenever something is created, it seems like I should have a function called when it is deleted too. Alternatively, if creation is implicit then destruction should be implicit too. Being allowed an explicit constructor, but no destructor, seems inconsistent. People who have used C++ will know what I am talking about when I ask "how do you define a destructor?" Is it the case that for some reason this sort of functionality isn't useful in Eiffel? It seems to me that even in a GC-ed environment I might like to have some way of keeping references to my own objects... Mark Langley langley@dg-rtp.dg.com
jos@cs.vu.nl (Jos Warmer) (03/19/90)
langley@yoyodyne.rtp.dg.com writes: >How do I write a class so that a routine will get called >every time the object is forgotten? The class MEMORY from the support library has a function `dispose'. Whenever an object is destroyed by the garbage collector this function is called. To get the behaviour you desire, inherit from class MEMORY and redefine the `dispose' function. The function is not exported, so it does not show up in the class interface and therefore it is not described in the library manual. The only place where it is documented is in the `RELEASE 2.2 OVERVIEW'. Jos Warmer jos@cs.vu.nl -- Jos Warmer jos@cs.vu.nl ...uunet!mcvax!cs.vu.nl!jos
greg@ait.trl.oz (Greg Aumann) (03/20/90)
From article <1586@xyzzy.UUCP>, by langley@yoyodyne.rtp.dg.com: > > How do I write a class so that a routine will get called > every time the object is forgotten? > > I ask "how do you define a destructor?" Is it the case that > for some reason this sort of functionality isn't useful in Eiffel? I would very much like to have this sort of facility available for an application that I am writing. The application is written in Eiffel and C and uses X windows. There are two reasons that I would like to have a destructor function. One is to free memory that was malloc'ed in the C part of the application but is linked to an Eiffel object. The second is to free X resources in the X server which are also similarly linked to an Eiffel object. What I would like to be able to do is to write a destructor which simply frees the non-Eiffel resources when the corresponging Eiffel object is destroyed. I realise that such a facility can be dangerous but I think it is better to have it than to try and free stuff explicitly (which is why there is a GC in the first place). ------------------------------------------------------------------------- Artificial Intelligence Systems ACSnet: greg@trlamct.trl.oz Telecom Research Laboratories Internet: greg@trlamct.trl.oz.au Melbourne, AUSTRALIA Voice: +61 3 541 6222
plogan@mentor.com (Patrick Logan) (03/21/90)
In article <6025@star.cs.vu.nl> jos@cs.vu.nl (Jos Warmer) writes:
To get the behaviour you desire, inherit from class MEMORY and
redefine the `dispose' function.
--
Jos Warmer
jos@cs.vu.nl
...uunet!mcvax!cs.vu.nl!jos
Here's what I was missing from my previous posting, i.e. multiple
inheritance so that the "dispose" procedure is only called on objects
that need to use it.
Does Eiffel use this in the window system classes to protect X's C
allocated structures?
Where (else) does Eiffel use this? Is it always to deallocate C data?
--
Patrick Logan uunet!mntgfx!plogan |
Mentor Graphics Corporation |
Beaverton, Oregon 97005-7191 |jos@cs.vu.nl (Jos Warmer) (03/22/90)
plogan@mentor.com (Patrick Logan) writes: >In article <6025@star.cs.vu.nl> jos@cs.vu.nl (Jos Warmer) writes: > To get the behaviour you desire, inherit from class MEMORY and > redefine the `dispose' function. >Does Eiffel use this in the window system classes to protect X's C >allocated structures? I checked the window system classes and found no dispose function anywhere. So the answer is no. In my opinion this is shortcoming of the library classes. I think each class that allocates C structures should use the dispose function to deallocate this structures. Otherwise C data garbage will never be deallocated. >Where (else) does Eiffel use this? Is it always to deallocate C data? It can be used to deallocate C data, but also for other purposes. E.g: if an object is associated with a file, the dispose routine could close that file (to avoid too many opened files). You might also use it to trace the behaviour of the garbage collector for one or more classes. Jos Warmer jos@cs.vu.nl