Refcounting Smart Pointers

[insertinterestingnamehere]

So, as this is, things still segfault, and there's a compilation error with MSVC that I need to track down. I'll put this up here anyway so we can discuss the proposed API while I track down the remaining errors. I also still need to add an assignment operator that allows moving from an rvalue reference.

Ultimately, I'd like to see a class like this used to interface between our binding code that's written in C++ and Cython. It'd be ideal if we could also use classes like this to interact with many of the built in Python C API functions. The goal there would be to make it so that compile-time errors can be thrown when a function is used with incorrect reference semantics or null handling. Encoding thse semantics into the type system seems like a good way to do that.

[insertinterestingnamehere]

I think this is good to go now. The test failures are not related to this PR.

Here are my future plans with this (in no particular order):

• Build a Cython interface for the new reference counting pointers. Eventually this should help make it so that we don't have to deal with raw PyObject* pointers anywhere, even when defining interfaces for Cython to use.
• Wrap various Python and NumPy C API functions using this interface so that they can be called without having to manually manage the reference counting and null checking so much.
• Add support for single inheritance to these smart pointer types. This would be useful for wrapping NumPy C API functions since there are functions that should only operate on the PyArrayObject type, but there are also functions that can operate on PyObjects. These smart pointer types should be able to encode those semantics as well.
• Convert all of the existing mixed C++/Python C API code to use these smart pointers. Aside from making the semantics for a function a part of its type signature, this should also help to make sure that references are cleaned up safely when exceptions are thrown.
• Use these types to create corresponding DyND types that can be used to manage the lifetimes of Python objects in DyND arrays.

[mwiebe]

One high level question - is there a description of how they work and why they're designed the way they are, so that people using them can refer to that to develop their mental model of correct vs incorrect code? Would be good to link to that description from the README/getting started.

[insertinterestingnamehere]

Thus far I've just been doing that via thorough commenting. I can put together a design doc if you'd like though.

[mwiebe]

That would be awesome, yeah, When I first used the python C API, I found https://docs.python.org/2/c-api/intro.html#reference-counts very useful to orient myself about reference counts, you might call out specific things from there and describe how the types you've created make it easier to produce correct code.

[insertinterestingnamehere]

Sure, that sounds like a great idea. I'll add that in here as well. Thanks!