How can I write a stateful allocator in C++11, given requirements on copy construction?

Question

As far as I can tell, the requirements on an allocator to be used with STL containers are laid out in Table 28 of section 17.6.3.5 of the C++11 standard.

I\'m a bit

Answer 1

1) Are my interpretations above correct?

You are right that your free-list might not be a good fit for allocators, it need be able to handle multiple sizes (and alignments) to fit. That's a problem for the free-list to solve.

2) I've read in a few places that C++11 improved support for "stateful allocators". How is that the case, given these restrictions?

It is not so much improved, than born. In C++03 the standard only nudged implementers toward providing allocators which could support non-equal instances and implementers, effectively making stateful allocators non-portable.

3) Do you have any suggestions for how to do the sort of thing I'm trying to do? That is, how do I include allocated-type-specific state in my allocator?

Your allocator may have to be flexible, because you are not supposed to know exactly what memory (and what types) it is supposed to allocate. This requirement is necessary to insulate you (the user) from the internals of some of the container that uses the allocator such as std::list, std::set or std::map.

You can still use such allocators with simple containers such as std::vector or std::deque.

Yes, it is a costly requirement.

4) In general, the language around allocators seems sloppy. (For example, the prologue to Table 28 says to assume that a is of type X&, but some of the expressions redefine a.) Also, at least GCC's support is non-conformant. What accounts for this weirdness around allocators? Is it just an infrequently used feature?

The Standard in general is not exactly easy to read, not only allocators. You do have to be careful.

To be pedant, gcc does not support allocators (it's a compiler). I surmise that you are speaking about libstdc++ (the Standard Library implementation shipped with gcc). libstdc++ is old, and thus it was tailored to C++03. It has been adapted toward C++11, but is not fully conformant yet (still uses Copy-On-Write for strings, for example). The reason is that libstdc++ has a huge focus on binary compatibility, and a number of changes required by C++11 would break this compatibility; they must therefore be introduced carefully.