I want to use a library that makes heavy use of singletons, but I actually need some of the manager classes to have multiple instances. The code is open source, but changing the code myself would make updating the lib to a newer version hard.
What tricks are there, if any, to force creation of a new instance of a singleton or even the whole library?
Remember that the library also depends on the singleton nature of the singleton. If you want to update the library you need to verify that your changes don't violate library functioning. That can only be done manually each time.
Find out who wrote the library in the first place, visit their home address and beat them to a bloody pulp, preferrably with a book about software design. :)
Apart from that: maintain your changes to the original library as a patch set so you can (more or less) easily apply it to each new version. Also, try to get your changes into the library so you don’t have to maintain the patches yourself. ;)
The answer is "it depends". A good example is the C++ heap used by Microsofts C runtime. This is implemented as a singleton, of course. Now, when you statically link the CRT into multiple DLLs, you end up with multiple copies. The newer implementations have a single heap, whereas the older CRTs created one heap per library linked in.