Re: [Boost-bugs] [Boost C++ Libraries] #6575: copy_remaining_to and copy_some_and_update don't use allocator_traits for construction

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #6575: copy_remaining_to and copy_some_and_update don't use allocator_traits for construction
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2012-02-19 15:59:38

#6575: copy_remaining_to and copy_some_and_update don't use allocator_traits for
construction
-------------------------------------------------+--------------------------
  Reporter:  Erik Jensen <Erik.Jensen@…>         |       Owner:  igaztanaga
      Type:  Bugs                                |      Status:  new       
 Milestone:  To Be Determined                    |   Component:  container 
   Version:  Boost 1.49.0                        |    Severity:  Problem   
Resolution:                                      |    Keywords:            
-------------------------------------------------+--------------------------

Comment (by igaztanaga):

 Both libc++ (clang) and libstdc++ (g++) implement the same approach as
 Boost.Container. The inserted element gets the container allocator as it
 is moved to an existing object built with with allocator_traits. I think
 the issue is not clear, and only happens in vector and deque (node-based
 containers allocates some memory and build the object directly there).

 Instead of aligned storage, or a temporary, which might use some undefined
 stack space (we don't know how big T is), another alternative is to
 reserve additional capacity for two new elements at the end of the
 vector/deque before emplacing (instead of just one, it's really a
 negligible cost, as the vector capacity grows exponentially). Then built
 the object with allocator_traits in the end() + 1 position (with a
 rollback operation to destroy the element at the end of the scope, as it
 should be destroyed anyway after being moved to the right position).

 This is not simple to do in Boost.Container as all [uninitialized]copy
 operations are hidden behind an abstract interface to avoid instantiating
 insertion bookeeing functions for each iterator type, but maybe we need
 some interface extension.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/6575#comment:4>
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