[Bug libstdc++/54075] [4.7.1] unordered_map insert still slower than 4.6.2

François Dumont frs.dumont@gmail.com
Sun Nov 25 17:01:00 GMT 2012

On 11/23/2012 11:22 PM, Jonathan Wakely wrote:
> On 23 November 2012 21:59, François Dumont wrote:
>> - To create a node I need to request the allocator twice, once for the node
>> and an other time for the stored value. This is quite unusual so I don't
>> know if it is a real problem
> That's normal for node-based containers.
> The correct behaviour (see 23.2.1  [container.requirements.general]
> p3) is to use one allocator type to allocate memory for a node, but
> *not* use that type to construct the node (instead use placement new),
> then use a different allocator type to construct the element in the
> node.  See _M_create_node in <bits/forward_list.h> for more details.
     I indeed considered your recent modification of forward_list but in 
my patch it is different. I not only build the value with its allocator 
construct method but I also allocate it separately because the node 
contains a pointer to the value and not the value directly. This was 
necessary to represent empty nodes with a nullptr. Doing so also 
increase memory consumption so yet an other reason not to adopt this design.


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