From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2007-09-19 14:56:22
Tobias Schwinger wrote:
> JOAQUIN LOPEZ MU?Z wrote:
>> ----- Mensaje original -----
>> De: Tobias Schwinger <tschwinger_at_[hidden]>
>> Fecha: Sábado, Agosto 25, 2007 1:06 am
>> Asunto: Re: [boost] Threadsafe singleton - questions
>> Para: boost_at_[hidden]
>>
>>> Robert Ramey wrote:
>> [...]
>>>> g) what happens if same interface used by different DLLS.
>>>> Is one instance still created?
>>> Yes.
>> Are you sure? For instance, in Windows globals aren't merged
>> on DLL loading, so if you've got something like:
Currently, not even symbol merging can keep us from potential breakage.
It can even make things worse, at cases (see below).
<code>
>>
>> you'll get different t1 and t2 values.
>
> Right.
...which (after some more thinking about how Singletons should work with
across dynamic library boundaries) is in fact a good thing since a
dynamic library shouldn't make any assumptions about when it gets unloaded.
>
>> I don't see how
>> you're able to prevent this behavior for the local static data
>> of singleton_manager.
The data is initialized (and inserted into the manager's list) by the
Singleton itself, while the manager just handles destruction. Still
problematic, however.
>
> Not at all: As client Singletons are user classes it's the users
> responsibility to eventually add declspec stuff for windows.
>
> It should probably be mentioned in the docs. Thanks for the heads up.
Unfortunately, it's not that simple...
A Singleton that is exposed in the header will currently get inlined
into the client (including the manager) which can cause serious trouble:
A Singleton might end up to exist multiple times and a destructor
located in already-unloaded code.
This situation can even occur if a dynamic library uses a Singleton
internally and symbols get merged (unless merging is #pragmaTically
disabled, which might not be possible for all ABIs).
The good news: The next version will introduce means for full control
over what happens in all sorts of shared library scenarios.
Thanks again.
Regards,
Tobias