[filters] Add a filter accepting a functor to reduce boiler plate code for simple filters

[kunaltyagi]

Currently modification of filter needs sub-classing of Filter or FilterIndices and quite some boiler plate code which can be reduced by working with a simple functor.

Please check the PR for a WIP implementation of a class to reduce code needed for a custom class to a simple functor which checks point-by-point.

If all goes well, we can reduce a lot of existing classes to instantiations of this class with a functor, as well as more functionality easily (eg: frustum-culling, passthrough, crop-hull)

I'd have preferred to pass in the function to the applyFilter but that's not possible.

TODO:

• Add a check for is_derived_from on the impl class
• Move impl to namespace detail
• Proper year on License
• Add a check on lambda for is_invocable from the same arguments as the std::function based appraoch

[Morwenn]

Shouldn't is_incokable and remove_cvref_t be in type_traits.h? They are useful enough; if I was looking for them that's where I would expect to find them.

[kunaltyagi]

PTAL

[kunaltyagi]

PTAL. More tests needed though

[kunaltyagi]

What about removing the const

Kind of doesn't work. The return type of getLambda function also needs to be const in order to keep the function as const.

If we drop all the 3 consts (variable, return type, function signature), then this works.

If we force the lambda to be copyable (lambdas might be an issue here), then we can return a copy (instead of a reference) of the stored functor and still keep the method as const

[SergioRAgostinho]

Kind of doesn't work. The return type of getLambda function also needs to be const in order to keep the function as const.

I suggested to not store the lambda as const, getLamda can stay and should stay as is. I'm not really getting what use case you have in mind but take my code snippet and modify it to illustrate the issue. https://godbolt.org/z/ErKbyn

[kunaltyagi]

Pushed a diff which is giving me errors

[SergioRAgostinho]

Managed to trigger your error once I started invoking

filter.getLambda()(/*arguments*/);

If I did

auto l = filter.getLambda();
l(/*arguments*/);

No errors we're being triggered. I'm unsure why. (Double checked. It's implicitly creating a copy of the object stored inside)

Multiple edits from here down
Nevertheless, what you suggested works

    const FunctorT&
    getLambda() const noexcept
    {
      return lambda_;
    }

    /* The non-const version of the method */
    FunctorT&
    getLambda() noexcept
    {
      return lambda_;
    }

But basically it means we're allowing the contained object to be fully modified and rewritten once initialized. Is this something we want? I don't really care to be honest. The class does not rely on what object it holds. The user has full control over it.

Alternatively we don't define the non-const method and always force the user to make a copy if it wants to modify the object.

I don't mind both approaches.

[kunaltyagi]

PTAL

[SergioRAgostinho]

It feels like removed_size should be 0 here. If true, why not explicitly setting 0 here?

[kunaltyagi]

It should be unmodified, not equal to 0 (based on code in base classes)

[kunaltyagi]

Merge if no other issues (and CI is green)

EDIT:
Ignore MSVC errors. CI OOMed