Sharing resources between game states

What is the best way to share resources between game states? To the make the game engine flexible, the resources should be able to be loaded on demand and unloaded only when no states no longer need them. This also includes functions that will be shared by the states.

Would it be ugly to have one set of functions, shared_init, shared_cleanup in a base class, and then inherit that class and specify custom init/cleanup functions for resources that only the derived state uses? The shared_init/shared_cleanup functions would then need reference counting in order not to cleanup when there are still states around that use the resources.

You could make a resource manager class that counts references to the resources loaded.

So, pseudo example

Code:

texture* myTex;
resMgr->GetResource(myTex,  "C:\\Test.png");

This would load the the texture from the disk ( if the resource does not yet exist, else it would simply increment a reference count )
Then return a state code ( success, error, file_not_found, etc ), and create a reference to the resource.

Code:

myTex->Release();

This would decrement the texture reference, and null the myTex pointer.

Code:

resMgr->CleanUp();

This would only free/remove resources with a 0 reference count.

Note:
For this to be safe, you will need to implement copy functionality for the resource classes to handle the reference count to insure a valid count at all times. Or you can disable copying, but this may hinder some things you may want to do.

Some of the information I have provided may be inaccurate, as I am running purely off memory. If you would like, I can formulate a short code example to demonstrate what I am proposing.