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Subject: Re: [boost] [density] new library
From: Klaim - Joël Lamotte (mjklaim_at_[hidden])
Date: 2016-06-13 20:40:47

On 14 June 2016 at 00:14, Giuseppe Campana <giu.campana_at_[hidden]> wrote:

> Il 13/06/2016 19:56, Klaim - Joël Lamotte ha scritto:
>> You might want to clarify the differences with this library as colony is
>> well advanced in it's production.
> The main difference is that plf::colony is an homogeneous container
> (elements have all the same type), while density deals with heterogeneous
> containers, in which elements may have a common base class (i.e.
> polymorphic containers like dense_list<Widget>), or may be completely
> type-erased (for example: dense_list<void> or
> dense_function_list<int(float)>). The core idea of colony is avoiding
> element reallocation and moving. The core idea of density is avoiding
> wastes of memory and sparse data structures.
Ah yes I forgot the the polymorphic part mixed up colony with some other
library discussed recently, certainly related to entity-component systems
(like EntityX or another library proposed to boost that I forgot the name
which imply polymorphic storage. I can't remember which library it was
Sorry about that.

> Another difference is in the memory management: plf::colony allocates
> increasingly big memory blocks while the container get filled, while
> density paged containers allocate fixed-size pages. Fixed size allocation
> is extremely fast and straightforward. Furthermore, a page allocator may
> use a thread-local temporary depot of free pages to get a new page without
> having to synchronize with the other threads. This optimization is not
> still implemented, but I believe that it will be beneficial for all the
> paged containers (that is paged_queue, paged_function_queue, and all the
> lifo classes). Anyway density is already performing better than the
> alternatives, especially with queues (
> density deals with lifo allocation as well. This may may at first seem
> unrelated to heterogeneous containers, but actually it's not. For example
> you may need to copy or move on the stack an element from an heterogeneous
> container, but you don't know how big it is, so you use a lifo_buffer, like
> in the producer-consumer sample (
> As explained in the comments, the drawback of using lifo_buffer in this way
> is that the code is not automatically exception safe. Very soon density
> will provide a type-erased lifo_object that will fix this.
> I care a lot about exception safeness: the test of density performs an
> exhaustive test on the strong exception guarantee on all the testcases,
> trying to raise an exception in every place in which an exception may occur
> and checking in a catch block that the container has not changed.
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