[fpc-devel] Generics.Collections as package for Lazarus or package for FPC RTL

[Sven Barth]

Am 27.01.2016 23:41 schrieb "Maciej Izak" <hnb.code at gmail.com>:
>
> 2016-01-27 17:24 GMT+01:00 Sven Barth <pascaldragon at googlemail.com>:
>>
>> I'll need to look at your code again, but even for class functions the
same rules (and worries) as for normal methods apply.
>
> I hope that your research in Generics.Defaults will be successful for
manual interfaces :P
>
> Most important about manual interfaces, and why can't be used any other
solution without painful consequences:
>
> 1. we don't want to use ARC interfaces (for singletons created at startup
in Generics.Defaults)
> side note: in Generics.Defaults is introduced TSingletonImplementation
for non ARC COM interfaces

Why not? This would solve the whole problem below. Also you can create them
lazily, so not every instance would need to be available from the start
on...

> 2. even if point 1. is simple to achieve, it brings a lot of problems
because we need to keep somewhere our singletons and that means:
>
> SIGSEGV exception is guaranteed, during closing application in many
scenarios
>
> program X;
> uses
>   A, B;
> begin end.
>
> A source code:
>
> unit A; interface uses Contnrs;
>   var GarbageCollector: Contnrs.TObjectList;
> implementation initialization
>   GarbageCollector := Contnrs.TObjectList.Create(true);
> finalization
>   GarbageCollector.Free;
> end.
>
> in B is used A and Generics.Collections.
>
> unit B; interface uses A, Generics.Collections; implementation
> begin
>   GarbageCollector.Add(TDictionary.Create);
> end.
>
> Generics.Collections (which is using Generics.Defaults) and our
Generics.Defaults is released before A. It will cause a memory violation in
Generics.Defaults for our singleton O__o.
>
> To avoid this memory violation is necessary to omit point 1.
>
> 3. To avoid memory violation, any instance of collection should be owner
of instance of IComparer / IEqualityComparer and that is performance and
memory killer in comparison to manual interfaces.

Why would each collection instance need to contain an instance of the
comparer? They don't contain state and are reentrant, so they can be easily
shared with ARC singletons.

Note: there would either need to be a global variable for each instance or
a list that gets automatically finalized as well (e.g. as a stupid
implementation an array of IInterface). Then if Generics.Defaults is
finalized before the other unit using it only the reference count will be
decreased and only when the other unit is finalized the comparer will be
destroyed.

Regards,
Sven
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