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From: Terence Wilson (tez_at_[hidden])
Date: 2005-05-08 16:21:02


> -----Original Message-----
> From: boost-bounces_at_[hidden]
> [mailto:boost-bounces_at_[hidden]] On Behalf Of Pavel
> Antokolsky aka Zigmar
> Sent: Friday, May 06, 2005 7:48 AM
> To: boost_at_[hidden]
> Subject: Re: [boost] Re: [Formal review] Singleton library
> On 5/5/05, Neal Becker <ndbecker2_at_[hidden]> wrote:
> > I'm curious as to how singleton behaves with respect to
> shared libraries.
> > Is this tested? I'd imagine the results are platform-dependent.
> I've tested Singleton library for various aspects, including
> this. On my test platform (windows, VC7.1) the singleton in
> shared libraries
> (dlls) does not work as expected.
> I've made the following test: I've create two shared
> libraries and a test program.
> The first shared library (my_manager.dll) contained the
> singleton class itself The second (other_lib.dll) contained a
> class that uses the singleton (from my_manager.dll) The test
> program was linked with both libraries and also uses the
> singleton (from my_manager.dll) and test class from other_lib.dll
> The test shown that the singleton actually creates _twise_
> once from test program, and once from other_lib.dll, therefor
> resulting two independent instances, instead of one instance
> shared for entire process.
> I was a very disappointed with those results. I liked a
> Singleton library design very much, but this aspect makes it
> nearly useless for my work, because our typically application
> are split across multiple shared libraries, where many of the
> have to access some shared resource. Our current
> implementation is very simplistic (GOF-style), but it least
> it works across multiple shared libraries.
> Hope there is a way to work around this problem.

__declspec(dllexport)/__declspec(dllimport) needs to be specified in order
that the variable can be exported from the DLL it is defined in and imported
in client DLL's & EXE's. This will give you a per-process singleton, if you
want a singleton instance shared by *all* clients of the DLL of which it is
defined use #pragma data_seg(".SHARED").

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