STL container library - Is it legal to call allocate/deallocate on different instances of the allocator class?

Question

First of all, I don't think it is. But, I've observed such a behavior with MSVC 10.0 in Debug mode. I'm using a custom allocator class which relies on the user to pass only pointers allocated on the same instance to deallocate. However, in Release mode, my code is working.

Is this a bug or am I mistaken?

Answer 1

The standard requires that any allocator be able to deallocate memory produced by any other allocator of the same type, even if it's a totally different instance. This is required to get list::splice working correctly. It's largely considered a design flaw in the C++ spec, and in C++0x they're introducing a set of fixups to allocators to rememdy this. In the meantime, any allocator you use in the STL containers must not have its own local state.

EDIT: For those of you who want the original language on this, here's §20.1.5/4 of the C++ ISO spec:

Implementations of containers described in this International Standard are permitted to assume that their Allocator template parameter meets the following two additional requirements beyond those in Table 32.

— All instances of a given allocator type are required to be interchangeable and always compare equal to each other.

In the latest ISO draft of the C++0x standard, this requirement is no longer present. The default std::allocator will still maintain this invariant as required, but it doesn't look like you'll have to constrain yourself this way in the future.