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NNTP-Posting-Date: Fri, 10 Jun 2011 15:32:31 -0500
From: Pete Becker <pete@versatilecoding.com>
Organization: Roundhouse Consulting, Ltd.
Newsgroups: comp.lang.c++
Date: Fri, 10 Jun 2011 10:32:30 -1000
Message-ID: <2011061010323081406-pete@versatilecodingcom>
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Subject: Re: std::set preallocate?
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On 2011-06-10 10:01:02 -1000, Christopher said:

> Got a complaint that allocation is costly for 2000 items to be
> inserted into a set one by one. I need to preallocate, and depend on
> the set growing, if needed, after the number of items exceeds the
> preallocation.

Sets typically use a separate data block for each element, so 
allocating a bunch of blocks up front doesn't really accomplish 
anything. It's not like a vector, which uses a single data block and 
has to resize that block as elements are added.

> 
> Do I do so like this?

No, definitely not.

> 
> class A
> {
> public:
>    A()
>    {
>        m_values.get_allocator.allocate(2000);

This allocates a data block that can hold 2000 elements and throws away 
its address.

-- 
  Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The 
Standard C++ Library Extensions: a Tutorial and Reference 
(www.petebecker.com/tr1book)