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Subject: Re: [boost] [cpo-proposal] presentation of the idea
From: Santiago Tapia (santiago.tapia_at_[hidden])
Date: 2013-08-16 09:21:30

> Date: Tue, 13 Aug 2013 18:33:41 +0200
> From: Klaim - Jo?l Lamotte <mjklaim_at_[hidden]>
> Maybe a classic implementation example would make a clearer point on which
> case it solves.

I will try, actually I have to write the full documentation.

Anyway, let us suppose you have something as simple as a class hierarchy of
shapes. Base class: shape, and derived classes: square and triangle. You might
like to store shape objects in one container but you might not want to use:
vector <shape*>, nor vector< shared_ptr< shape> >, nor boost pointer container

Then the classifier could be an alternative.

>> What if the class hierarchy is used to model a graph that
>> contains cycles. For example, something like a spirit grammar
>> where a non-terminal on the rhs of a non-terminals definition?
>> Wouldn't this require some sort of smart pointer or other
>> garbage collection method with the ability to collect cycles?
> My understanding of the proposed container is that it's mostly like having
> a collection
> of vectors, one for each final types, but exposing only one base interface
> in container interface.
> If I'm correct, I don't see how the problem you describe might be a problem.
> Joel.

Yes, that is the idea. Actually, whether the implementation uses a collection
of vectors or other mechanism is an implementation detail. I use a map
of vectors in the classifier container, but I will try other implementations.

> Date: Tue, 13 Aug 2013 13:54:09 -0500
> From: Larry Evans <cppljevans_at_[hidden]>
> Rereading the OP, I see:
> collection.insert( derived_A(21) );
> which means a completely new object is created during the insert; hence,
> no cycles in pointer graph can occur.

Yes, the interface of classifier<base> (for instance: classifier<shape>)
should be similar to vector<triangle> but with the ability of inserting
any object that derives from shape. Moreover, you can not have a
std::vector of a pure abstract class, but you could have a classifier of
an abstract class.

> Date: Tue, 13 Aug 2013 23:33:25 +0400
> From: Andrey Semashev <andrey.semashev_at_[hidden]>
>> - The container classifier is _not_ a sequence.
> Why? Is this intentional?

Both intentional and accidental, because I want to allocate objects
contiguously in memory I have to store them in a collection of
vectors, the container is not a sequence but a collection of sequences,
one sequence for every class in the classifier.
I will implement a sequence, but later.

>> - The allocator argument in the classifier will be applied to every object
>> added to the classifier. Thus, the allocator is provided without its
>> argument. The std::allocator is the default argument.
> It's better not to use template template parameters in the container interface
> as it limits its usefullness because users won't be able to specify non-
> template allocators or template allocators with different template arguments.
> You can use the standard interface for allocator rebinding:
> typedef allocator<T> allocator_T;
> typedef typename allocator_T::template rebind<U>::other allocator_U;
> You can use std::allocator<void> by default then.

I am not sure how to do this. I need a allocator with a parameter in order
to get: allocator<triangle>, allocator<square>, ... this allocator must be
instantiated when an object of a new class is inserted in the classifier.

> All in all the basic idea looks interesting, although I'm not sure about the
> particular classifier container. It looks very specific to me, the same
> functionality could be achieved with a multimap or unordered_multimap of
> polymorphic elements.
> ...or I should have said "multiset or unordered_multiset".

As far as I know it is not possible to get a multimap of a pure abstract class,
therefore, for example, you can not use a multimap<string, shape> or
multiset<shape> but you can use a classifier<shape> to store triangles,
squares, or any derived class. I plan to implement an associate container
later, thus you could have classifier<string, shape> and you could look for
an object in the classifier using a key.

Best regards,

   Santiago Tapia

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