Support "in" operator for STL containers

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]

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.

[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.