On 09/22/07 08:00, Achilleas Margaritis wrote:
O/H Sebastian Redl έγραψε: [snip]
Then first you need the node allocator:
typedef typename Alloc::template rebind<node<T> >::other node_allocator;
Then you can get the new allocator's pointer type:
typedef typename node_allocator::pointer node_ptr;
class node { T elem; node_ptr next; node_ptr prev; };
Don't forget to construct the node_allocator member you'll hold from the allocator you're passed in. All allocators must have a templated constructor for this purpose. Something most implementations do, by the way, is that they are written in such a way that EBO applies to the allocator if it has no state.
But STL containers do not use the rebind struct for their internal allocations or for their member pointers. They just use T* instead of allocator<T>::rebind<U>::pointer.
The main_stl.zip in boost-vault/memory uses Sebastian's suggestion and it seems to work. It runs a little std_list test, then a GCBench with std:pair, then GCBench with std_list<., gc_allocator>. As (I think) you're pointing out above, the std_list<T,gc_allocator<T> > would define gc_allocator<T>::pointer as gc_ptr<T>, which is never used in std_list at all because the std_list stores the complete T (not T*) in std_list<...>::node. I can see where this would be confusing and is a shortcoming of the allocator model. It just happens to work in this case.