Re: [boost] pool allocator question

From: Michael Marcin (mmarcin_at_[hidden])
Date: 2007-08-15 14:00:47

Alexander Vushkan wrote:
> Hi Steve.
> 
> On 8/15/07, Steve Kettle <skettle2000_at_[hidden]> wrote:
>> I have a question about the memory pool allocation that boost
>> implements.  Suppose we do the following:
>>    std::vector<int, boost::pool_allocator<int> > v;
>>    for (int i = 0; i < 10; ++i)
>>      v.push_back(-1);
>>
>> And the std::vector<int> is implemented so that it starts out with
>> reserved space of 1 and doubles space every time.  So At the end of the
>> above loop v will have size 10 and reserved size 16.
>>
>> Does the boost allocator mean that there will memory arrays lieing
>> around of size 1,2,4,8 that can be reused ?
>>
>> Eg if you do this:
>>
>>   std::vector<int, boost::pool_allocator<int> > v;
>>    for (int i = 0; i < 10; ++i)
>>      v.push_back(-1);
>>
>>   std::vector<int, boost::pool_allocator<int> > v2;
>>    for (int i = 0; i < 8; ++i)
>>      v2.push_back(-1);
>>
>> There will be no system call needed to get more memory -
>> v2 will just reuse the memory in the pool_allocator<int> ?
> 
> 
> 
> No, it seems to me each allocator holds own piece.
> 

Are you sure about that?

IIRC the pool allocator is a singleton per size i.e.

boost::singleton_pool< boost::pool_allocator_tag, sizeof(int) >

- Michael Marcin