[C++-sig] returning list of class pointers that compare equal

[Jim Bosch]

On Thu, Feb 6, 2014 at 2:43 PM, Gary Oberbrunner <garyo at genarts.com> wrote:
> ----- Original Message -----
>> From: "Jim Bosch" <talljimbo at gmail.com>
> ...
>>  - Add a __eq__/__ne__ overrides to the class of things you're
>>  putting
>> in the list, with an implementation that compares C++ pointers, not
>> Python pointers.  This is almost certainly much easier, but it's a
>> little less "complete" as a solution, in the sense that you'll still
>> be getting different Python objects back for the same C++ object (so
>> while "param2 == param1" will succeed, "param1 is param2" will still
>> fail).
>
> This is very helpful!  That's OK about "is" not working, I think (though true, a little odd.)
>
> Can I override __hash__ too, so set membership works correctly?  Also do I need to override __ne__; I think not?
>
> For the record, here's what I did.  Defined a comparison function:
>
> static bool params_equal(Param &left, Param &right)
> {
>   return &left == &right;
> }
>
> and then in the exposer:
>
>     Param_exposer.def("__eq__", &params_equal);
>

That should be fine, as long as you don't care about what happens when
you try to compare a Param to something else entirely (idiomatic
Python behavior would be to return False; this implementation will
likely raise an exception).

You should be able to override __hash__ in much the same way, and I'd
recommend checking to see if __ne__ works as expected before assuming
you don't need to implement it yourself; I don't recall Python's
behavior in this case, but I think it has a tendency not to define
most operators implicitly.

Jim