[boost] Re: Challenge: pool allocator

From: Boris Fomitchev (fbp_at_[hidden])
Date: 2000-02-28 00:38:34

Dave Abrahams wrote:

> Hi everyone,
>
> I spent a considerable amount of effort recently trying to come up with a
> pool allocator for boost, but failed. Here's what I'm trying to accomplish:
>
> A general system for dynamically allocating and constructing instances of a
> (possibly non-POD) type. Any outstanding instances in a pool are destroyed
> and deallocated when the pool is destroyed.
>
> I'm thinking of usage something like this:
>
> class Foo { ... };
> boost::pool<Foo> foo_pool;
> Foo *foo1 = foo_pool.create(<args>);
> [One could also use placement new above, but it seems harder to control: you
> have to force the correct placement delete to be defined, and some compilers
> don't support that correctly anyway]
>

Well, how do you suppose to handle variable arguments here ? I believe placement
new
is the only right solution. It also allows for important generalization - then
the pool
allocation manage only allocation itself, not initialization. More good news :
SGI STL/STLport
already use that generalized pool allocator ;). And, you may just use
allocator<T> w/o
redefined placement new or delete.

>
> This is harder than it looks - remember that if an exception is thrown
> during construction the the memory for the instance should be returned to
> the pool *and more importantly* the instance should not be double-destroyed
> when the pool is destroyed.
>

Yep, and it's trivial for pool allocator<>, as those issues are decoupled then.

>
> Some of the hard issues are:
> 1. Memory alignment of instances in the pool
> 2. You can't make unions of non-POD types, so it's hard to thread a
> free-list through memory for instances in the pool
>
> Any suggestions?
> Anyone already have a good solution?
>
> -Dave
>
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