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Subject: Re: [boost] [cpo-proposal] presentation of the idea
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2013-08-13 15:33:25

On Tuesday 13 August 2013 02:07:15 Santiago Tapia wrote:
> Hi everyone,
> I would like to make a proposal for a new library. I only have a draft
> without documentation but I would like to know people’s opinion
> about the idea before making a formal proposal.
> The library will be named CPO (containers for polymorphic objects)
> and it will provide containers to store dynamic allocated objects from
> a class hierarchy. I have drafted one container and I have two more
> in process. The cpo library could be an alternative solution for
> boost pointer container library or some other solutions that use pointers.
> In order to understand the idea, I think that the following example of the
> use of the library might be better than a long explanation:


I created containers of dynamically typed elements with Boost.Intrusive a few
times. It was a bit more verbose than your example but the resulting solution
was quite efficient and it had all the properties of a container. The drawback
was that the solution was intrusive w.r.t. the stored types. Do you think you
could create a set of containers (e.g. list, set, map) of polymorphic objects
based on Boost.Intrusive without requiring to modify the types of the

> Some details and advantages must be remarked in the provided
> example:
> - The container classifier is _not_ a sequence.

Why? Is this intentional?

> - 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.

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.

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