RE: [boost] Re: [Formal review] Singleton library

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").