Requirements on standard library allocator pointer types

问题

I am trying to write a quadtree sparse matrix class. In short, a quadtree_matrix<T> is either the zero matrix or a quadruple (ne, nw, se, sw) of quadtree_matrix<T>.

I'd like eventually to test different allocation schemes since this will probably impact the performance of linear algebra operations. So I will also template quadtree_matrix on a standard allocator type, so that I can reuse existing allocators.

I will have to allocate two different kind of data: either a T, or a node, which contains four pointers (to either T or node). For all the algorithms I will consider, I know for sure what kind of data to expect because I know what are the sizes of the submatrices I am facing at any point of the algorithm (I don't even need to store these sizes).

I will of course be using two different allocators: this is ok, since allocator types provide the rebind template and a template copy constructor (and are intended to be used as value types, as the get_allocator members of standard containers suggest by returning a copy).

The problem is that allocator member functions use a certain pointer type, which is not required to be a vanilla pointer. Some allocators (boost interprocess allocators) use this feature extensively.

If the allocator pointer types were garden variety pointers, I would have no problems: at the very least, I could use pointers to void and reinterpret_cast them to the right type (either node* or T*). I could also use a union (probably better).

As far as I know, there is no requirement on the PODness of the allocator::pointer types. They are only required to be random access iterators.

Now, my question is:

Given an allocator class template A<T> (or its equivalent A::rebind<T>::other), is there any guarantee on:

1. The ability to static cast A<T>::pointer to A<U>::pointer provided U is an accessible base of T ?
2. The ability to static cast A<T>::pointer to A<U>::pointer provided T is an accessible base of U and the "runtime type" (whatever this means in this context) of the castee is U ?
3. The type A<void>::pointer (if this makes sense) ?

Or is there a solution to my problem I didn't think about ?

回答1:

From the tables in 20.1.5/2 it clearly indicates that the type of A<T>::pointer must be "pointer to T". Since those pointer types are normally convertible your 1 and 2 are true. It follows then that A<void>::pointer must be void*.

EDIT: There's also explicit wording in 20.1.5/4 (it applies to what standard containers may assume about allocators):

The typedef members pointer, const_pointer, size_type, and difference_type are required to be T*,T const*, size_t, and ptrdiff_t, respectively.

回答2:

No, not really.

There is a requirement that A<T>::pointer is convertible to A<T>::const_pointer and A<T>::void_pointer but that is about all I can find.

A<void>::pointer is likely to be void*, unless you have some fancy special memory.

回答3:

Note that even if a union is usable, I would still not use one, especially because here you could probably benefit from some form of automatic memory management (in order for your contain not to leak), which requires fancy classes.

I would therefore recommend a two-steps approach:

• Write a small smart pointer that use a given allocator to perform the destruction (instead of delete)
• Use boost::variant on your pointers

This way you have both automatic memory management and compacity.

来源：https://stackoverflow.com/questions/5434737/requirements-on-standard-library-allocator-pointer-types