Thread: making a class only get constructed once

Isn't a key to MT to minimize the amount of shared data between the threads? As such wouldn't anything of which only one instance can exist be rather unsuitable to be used across threads?

Cannot the same problem occur (more easily perhaps) with a single object passed by reference? What's to stop a programmer from constructing one where he thinks he needs it?

At least the dependencies are more visible. As to constructing more than one (CWorld), you'd probably notice and you can limit construction...

And of course, if there is and should only ever ONE, there are two obvious solutions to avoid it being created in multiple threads:
1. Create it before threads are started, in the main thread.
2. Create it on the stack in the main function.
Both or either can be used.

Assuming that the class object isn't large, this should work fine, and avoids all sorts of problems.

I agree that sharing should be avoided in a MT environment, as all shared data that is ever updated by any thread will be subject to locks to ensure that no other thread is getting inconsistent data.

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Mats

For one thing this lazy instantiation is a bit of a problem. If you create a single instance normally or through Elysia's factory before other threads start, you won't have this problem.

Using the singleton in multiple threads might be quite problematic though.

I think that it also tends to be more difficult to design unit tests involving singletons.