Getting Loki Singleton to work in DLLs in VS 2008 C++

Question

I'm pretty sure this problem isn't new, and pretty sure it's hard to solve. Hopefully I'm wrong about the latter.

I'm trying to use the Loki::Singleton from Modern C++ Design in a program of mine.

However, I can't seem to get it to work across DLLs. I think I know why this is happening: the templated code gets instantiated in every source module, so instead of there being one global variable, each module has its own.

Obviously, this makes the Singleton very much non-single.

Is there any way to get around this behavior?

Answer 1

You are probably correct that each DLL has its own instance of the singleton. I'm not that familiar with the Loki implementation and the source code isn't a lot of fun to figure out.

Possible solutions are:

• Not using singletons. This is actually my usual preference because you can avoid whole classes of issues by changing your design to not need them. For a long rant on why Singletons may be harmful see this Yegge post. I'm not THAT vehemently against them but 95% of the time Singletons cause many more problems than they solve (if they actually solve any)
• Copying static members across DLL boundaries. I've also done this as a hack, where the DLL gets a pointer from an application or another DLL, and resets its own copy of the static class member to the pointer passed from outside. It's evil, it's dirty, you can't clean up after it, but it does work.

Answer 2

Note this is not going to address the question. An explicitly instantiated and exported singleton should do the trick...

-Rick

Check out #pragma data_seg here basically, you need to declare an instance of the singleton in a shared section of your code. By default statics are scoped to the dll.

It may get tricky with templates, but this is the path to success here that doesn't involve passing / copying static data around.

Answer 3

I see in the Loki source directory that they have a specific SingletonDLL directory under test, looks like they use an exported, explicitly instantiated template (which would work). Hopefully that contains the code you want.