Interop and Integration

This release of sol2 includes the addition of a new userdata checker/getter paradigm that allows non-sol2 usertypes to be integrated with sol2's mechanisms. There are a good chunk of examples from popular external frameworks, showcasing ways to do it. You can add your own custom logic to enable sol2 in your already millions-of-lines codebase and slowly ease out other libraries -- or not, and have the two play together perfectly nicely!

Find your framework here, or just take a peek to see how it's done: https://github.com/ThePhD/sol2/tree/develop/examples/interop
Read about it in the stack API: http://sol2.readthedocs.io/en/latest/api/stack.html#objects-extension-points

Thanks to @feltech for bringing this to my attention and helping me work on it.

Have fun, everyone!