Support "in" operator for STL containers

[cmpute]

This is a feature request for supporting python-like "in" operator for STL containers (vector, map, etc.)

For example to test whether a set contains a value in python:

a = set([1, 2])
assert 1 in a

But right now similar operator in Cython is not supported:

from libcpp.set cimport set
cdef set a = [1, 2]
assert 1 in a

The compiler will complain Invalid types for 'in' or Invalid types for 'not_in'

This is not critically necessary but it's useful

[da-woods]

The one thing against it is that it's wrapping an operation that doesn't quite exist in the C++ STL. Obviously you can do it with something like std::find(a.begin(), a.end(), 1) != a.end(), but the there isn't an std::contains for example.

[cmpute]

Yes, but actually almost all containers support find function, which can be used to implement this feature which is common in python side

[scoder]

We support for … in <c++ iterable>, so I think supporting in via the same iteration mechanism is reasonable. PR welcome.

This can be implemented via a generated C++ function, one per container type. See the C++ container conversion functions in CppConvert.pyx, for example. This wouldn't be done through the type instance (like the container conversion), but through a function generated in PrimaryCmpNode, which handles the in and not in operators (and others). It can be implemented in Cython code.

[jhelgert]

Just stumbled upon this. I think it's worth mentioning that C++20 added .contains() methods for all the associated STL containers (map, multimap, set, multiset and the _unordered ones) making this less verbose.

[da-woods]

Thanks @jhelgert - I suspect practically we'd want to support C++97 still for this (it seems like kind of a basic feature to require C++20 for).

I actually think this is easier than Scoder says and it's a very small number of template functions rather than needing generated code. I'll have a look at this sometime fairly soon.