Mithun R K wrote:
...However, my question had more to do with separate instantiations of Singleton<MyClass> in different shared libraries. Since each lib has a separate Singleton<MyClass> definition, won't each lib also have its own singleton-instance? I'm pretty sure the loader won't be stripping out multiple instantiations across shared libraries (well, not on HPUX, anyway). Won't this pose a problem?
Hum. I honestly don't know. Whats interesting is that this may be another reason that the CRTP model is better. The final singleton class used by client code would (probably) not be a templated class (it would just derive from one), and would (most likely) have its implementation compiled into its own discrete unit.
<quote> Perhaps the lifetime of the dlls themselves could be managed with singletons? When the dll's dtor is called, it could unregister the relevant classes and unload the dll. </quote>
The loading/unloading of the dll's isn't really in our control, is it? It's the dynamic-library-loader of the OS that does it. (/usr/lib/libdld.so, on HPUX). This problem is bound to happen on any app with multiple dlls, right? I've to use a hack on HPUX fix this... I'm open to ideas.
Sorry for my naivety, I haven't really done a lot of work involving multiple dlls in C++. My primary experience with such scenarios has been with C# in .NET, where one has the option of using the classes in the System.Reflection namespace to load other assemblies on command.
Thank you for looking at my approach... I really appreciate it.
Ok, so I shouldn't be introducing a virtual-base for the singletons... If I were to register only Destroy functors with the SingletonRegistry (each knowing how to destroy its Singleton), would that do? That would move the virtual-base out of the Singleton, and into the "SingletonDestroyer" (I'll still need a virtual base, because the SingletonRegistry will need to hold all the Destroyer functors).
Actually, you don't need a virtual base... because of the very nature of a singleton (single instance) you can use plain old static functions. Those functions would then be responsible for tracking down and destroying the single instance. -Jason