Re: [boost] Boost memory management guidelines

From: Peter Dimov (pdimov_at_[hidden])
Date: 2003-08-30 08:31:22

Gregory Colvin wrote:
> On Friday, Aug 29, 2003, at 18:05 America/Denver, David Abrahams
> wrote:
>> Gregory Colvin <gregory.colvin_at_[hidden]> writes:
>>
>>> * use the standard parameterization mechanisms (Allocator) when
>>>    choosing to parameterize
>>
>> I'm not sure about this one.  std::allocator are a mess, and almost
>> everyone knows it.  They were designed for containers, and they only
>> barely work there, IMO.
>
> IMO they work just fine, and are not so messy as "everyone" thinks.
> The only real ugliness is rebind, but without template typedefs there
> isn't much choice.  And the only really missing functionality is
> reallocation, but one can usually do without that.  They can be
> difficult to implement well, but I think that is separate issue.

They work, but they arguably aren't well designed. Containers manage memory
in different ways.

vector<T> allocates, reallocates, and deallocates contiguous blocks of T
elements, and it can put a "how much did I just allocate" feature and a "how
big is this allocation" feature to good use.

Node-based containers allocate single nodes, but not of value_type.

Deque allocates fixed size pages (and some additional housekeeping data).

I'm not sure that lumping these together in a single Allocator concept has
any benefits.

The stateful allocator problem is a thorny issue too, since to allow
stateful allocators, one needs to specify whether the stored allocator is
part of the logical state of the object. IE

vector<T, A> v1(a1);
vector<T, A> v2(a2);

v1 = v2; // which allocator does v1 hold now?

Extending this already overstretched concept to function<> is a bad idea
IMHO. function<> has its own memory management pattern that does not even
resemble a container. In particular

function<void(), A> f(a);

is not required since f never allocates except when passed an object.

function<void(), A> f(x, a);

can be made to work without the A template parameter since the constructor
is already a template, and x and a can be treated as a whole:

function<void()> f(x, a); // allocate pair<X, A> with a, copy x and a into
it.