Subject: Re: [boost] Interest check: container with preallocated buffer
From: Dmitry Vinogradov (sraider_at_[hidden])
Date: 2009-05-26 09:33:35
>> Thorsten Ottosen wrote:
>>> Dmitry Vinogradov skrev:
>>>> Boost.Array offers constant size container with preallocated buffer,
>>>> Boost.Optional provides from zero to one element. There are no STL container
>>>> with preallocated buffer and it seems custom allocator does not help to do
>>>> this with existing containers.
>>>> Does any container exist to offer functionality like Boost.Array but
>>>> allowing to store from 0 to N elements? Is there any interest in such
>>>> PS. It's similar to fixed_string but as a generic container.
>>> Please see
> I am curious. Would it not be possible to implement a class that
> handles memory allocations of the standard STL style and just pass an
> instance of that allocator (after allocated on the stack of course) to
> the stl::vector template parameter, or does std::vector require a
> class type? I have not read the standard in this area, and from above
> I guess the C++0x is more clear on this?
Writing custom allocator can help to solve problem, but preformance will
fall. Std::vector does not have GrowPolicy template parameter, so there
is no way to tell it about pre-allocated buffer size.
Every time vector think it needs to grow, it does reallocation by
copying data from previous location to a new one. It's not nessesary
when a new size() is less than size of buffer. Then if std::vector will
grow by twiceing its size from 32 (for example) to 64 to store 50
objects, it will force the allocator to use heap - but imagine that the
buffer buffer size is 60, so heap allocating is not required at all!
Boost list run by bdawes at acm.org, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk