Re: [boost] Review Request: poly_collection

El 01/03/2017 a las 23:15, Vicente J. Botet Escriba escribió:

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Le 01/03/2017 à 09:32, Joaquin M López Muñoz via Boost a écrit :

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Hello,

I'd like to ask for formal review of (candidate) Boost.PolyCollection [...]

Hi,

thanks for proposing this library Joaquîn.

Hi Vicente, thanks for your interest.

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I've surely missed why base_collection is named this way. Is it because the parameter is a base class. Could you confirm?

Exactly. base_collection<Base> pretty much behaves as a container of Base objects.

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wondering if a single class with specializations wouldn't help

collection collection> collection

In this way we could have

collection

BTW, I see that you have in the implementation a detail::poly_collection that is the base of the 3 collections.

This is mostly a naming issue. Indeed the three collections derive from the same implementation and could be publicly provided as specializations of the same class, if this is what reviewers lean towards. Personally I like it better to keep names separated as in the current proposal. Ok, no problem.

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In your blog there were some comments about a collection of variants. Have you considered adding a variant_collection

I've thought about it. Such a collection would deviate from the others both in terms of its interface (no type registration, for instance) and implementation --the data structure detail::poly_collection relies on could be used to support variants, but more efficient implementations exist for this particular case. Ok, I see. Even if it needs a specific implementation I believe it is worth having it. I say that because more and more people are using variant as an alternative way of polymorphism (See Andrzej blog - Another polymorphism) https://akrzemi1.wordpress.com/2016/02/27/another-polymorphism/

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As the collections are closer to multisets (unordered/not-hash), maybe it is worth using this name.

I'd say similarities with unordered_multiset are superficial (segments resemble buckets) and the interfaces ultimately are too different. I chose "collection" because the term is not overloaded in the STL or Boost. My intention was to confirm we have something close to a multiset. Anyway, it would be great to see a comparison table. Aside construction, what is provided by std::unordered_multiset that is not provided by the

Le 02/03/2017 à 10:41, Joaquin M López Muñoz via Boost a écrit : poly_collections (or it is provided in a different way) ?

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Have you think about using the same design for maps/unordered maps of polymorphic objet (where the key is not polymorphic)?

Same-type contiguity is an essential feature of PolyCollection: I fail to see how this could be preserved for an associative container. Care to elaborate?

I was thinking in having the a map of (keys,poly_collection.::iterator) and store the values using your poly_collection. After thinking a little bit more on that maybe there is a problem with the iterators stability.

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There is a C++ proposal for a polymorphic_value (https://github.com/jbcoe/polymorphic_value/blob/master/draft.md). Your base_collection<T> corresponds quite well to this polymorphic_value<T>, isn't it? Maybe polymorphic_collection<T> or collection The main difference is the memory management, polymorphic_value can be adapted to use an Allocator.

base_collection<T> behaves approximately as a std::vector, the crucial difference being that the former groups elements by concrete type in contiguous chunks of memory, while the latter holds indirection pointers behind the scenes, so there is no real memory contiguity.

I talked of polymorphic_value as it close to std::function and boost::type_erasure::any. At the end all of them are type erasures for some concept. IMO, you base_collection<T> is closer to vector than to ptr_vector<T>. Of course the layout is not the same, and this makes the difference of your poly_collections.

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What are the function types passed to for_each whee we have a function_collection or a any_collection? Is a reference to the |value_type? |Do we need generic lambdas?

These functions are passed a (const) value_type&, where

value_type=function_collection_value_type<Sig> for function_collection<Sig>, value_type=any_collection_value_type<Concept> for any_collection<Concept>

(see http://tinyurl.com/hmywnea , http://tinyurl.com/zh9ugl8 ). Now, when type restitution is used (http://tinyurl.com/gopdpuz ), and only then, the functions are passed a (const) Ti& for each of the restituted types Ti: in this case, a generic lambda is a very convenient way of taking advantage of this, but you can also resort to a polymorphic functor if you wish.

Let me see if I understood. For the normal algorithms, the function has as parameter the const value_type and needs to use dynamic polymorphism. When we use the “restituted" algorithm overload, the algorithm "restitutes" the specific types and call to an heterogeneous function, an overload set for the reconstituted types, as if it were a visitor, isn't it? hana::overload function should be useful here. Vicente