[fpc-pascal]ReAlloc Question

James Mills prologic at prologitech.com
Fri May 16 15:35:58 CEST 2003

On Fri, May 16, 2003 at 01:25:22PM +0000, memsom at interalpha.co.uk wrote:
> James,
> didn't I send you a revised version of this ages ago... even if you dumped the 
> whole of it I would have pointed out why not to use Init and Done over Create 
> and Destroy. I'll tell you one good reason why you may have memory 
> leaks... 'done' will never get called if you 'free' an instance of TStrings. 
> That alone will cause a memory leak. 
> You did see TList too, right? TStringList and TList will do everything you 
> want, and TObjectList is another good one (if fpc has it implemented.) For 1.1 
> also look out for TInterfaceList (one of my favourites in Delphi) and there's 
> also the heavy handed but usefull TThreadList in multithreaded environments ;-)
> In fact, looking at your code, your biggest problem is that you missed how the 
> destructor works in Object Pascal. TObject has a virtual destructor 
> called 'Destroy' that gets called when you call 'Free'. Calling the destructor 
> directly is simply wrong. If you are using Turbo Pascal mode, you should be 
> calling 'dispose( instance, destructor );' and in Object Pascal (Delphi like) 
> mode it has to be 'instance.free' which will indirectly call the destructor. If 
> you name your destructor something random and call it directly, it will leak 
> memory like a sieve. 

Okay, I didn't know this. Thanks. Sorry if you've tried helping me
before, too much uni work I guess and it was a long time ago...

> Also it would be worthwhile take a look how inheritence works. Look, for 
> example, at the definition of TObject. The destructor is defined as 'destructor 
> Destroy; virtual;'. The 'virtual' is the clue here. You must 'override' this 
> method to get things to work propperly!! Your destructor needs to 
> call 'inherited destructor' to have any chance at not leaking memory. 
> destructor TStrings.Destroy;
> begin
>   //whatever deallocation
>   inherited; //also valid : 'inherited Destroy;'
>   //like 'super' in java 
> end;
> Naming your constructor as Init, whilst not technically incorrect, breaks 
> naming conventions and will cause you endless problems if you should ever wish 
> to derrive your class from somewhere else other than TObject. So, once 
> interfaces come into play in 1.1, you would want to descend from 
> TInterfacedObject, and not calling the inherited constructor could be a real 
> problem. Another classic problem is TThread. Try creating a TThread descendent 
> and overriding the constructor but not calling the inherited version. The 
> thread never starts ;-) Got caught by that one a few times.

Thanks Matt :)
Be a lot of re-writing...


> Matt
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