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

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.

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.

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.

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.

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.

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?

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.

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. Best, Joaquín M López Muñoz

El 02/03/2017 a las 18:46, Vicente J. Botet Escriba via Boost escribió:

Le 02/03/2017 à 10:41, Joaquin M López Muñoz via Boost a écrit :

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

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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/

This can be defintely included in the lib roadmap. A variant_collection won't fit 100% within the current concepts in Boost.PolyCollection (e.g. type registration makes no sense, type restitution shoul be provided through a different interface as there's no need for the user to specify the restituted types), but I guess we could work it out. A variant-specific implementation could be very fast as no virtual calls are needed and segment visitation can be resolved statically (detail::poly_collection uses a virtual-powered backend).

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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 poly_collections (or it is provided in a different way) ?

poly collections do not behave like multisets in many respects: * There is no predicate-induced ordering of elements: order in a poly collection is free for the user to change *within a segment* (much like in a regular std::vector<T>). * No binary-search lookup interface. * segments might resemble buckets, but they're completely different beasts: segments are dedicated vector-like structures for same-type elements, whereas elements in an unordered multiset migrate from bucket to bucket as the container grows (rehashes). All in all, I really don't see much connection between both data structures.

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.

Exactly. Both structures hide pointer indirections away.

For the normal algorithms, the function has as parameter the const value_type and needs to use dynamic polymorphism.

Right. (Parameter is (const) value_type&, note the '&'.)

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?

Exactly. The nice thing about generic lambdas is that code such as this [](auto& x){ //use common interface of all types } automatically generates the necessary overloads. For instance: struct base { virtual void f(); }; struct derived1:base{...}; struct derived2:base{...}; struct derived3:base{...}; poly_collection::for_each( c.begin(),c.end(), [](auto& x){x.f();}}; generates overloads in the lambda function for derived1, derived2 and derived3, which sufficiently smart compilers (GCC , Clang) can use to completely eliminate virtual machinery in calling x.f().

hana::overload function should be useful here.

Can be useful when you want to use a different interface for each type: struct base { virtual void f(); }; struct derived1:base{void d1();...}; struct derived2:base{void d2();...}; struct derived3:base{void d3();...}; poly_collection::for_each( c.begin(),c.end(), hana::overload( [](derived1& x){x.f();x.d1();}, [](derived2& x){x.f();x.d2();}, [](derived3& x){x.f();x.d3();})}; Joaquín M López Muñoz

El 02/03/2017 a las 7:41, Ronald Garcia via Boost escribió:

Hi,

I have added PolyCollection to the review schedule.

Hi Ron, Please note we already have a Review Manager volunteer for this (Ion Gaztañaga). Thank you, Joaquín M López Muñoz