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In reading through 20.1 Allocator requirements [lib.allocator.requirements]

I have come across some wording which I find inconsistent with what is
in the rest of the standard as I understand it.

X::difference_type 
the type that can represent the difference between any two pointers in
the memory model.

Which seems to say that the operation 
X::pointer a,b;
  // initialize a & b to point into different objects
X::difference t = a - b; // seems to be well defined.

later in 
20.4.1 The default allocator [lib.default.allocator]
in the definition of the default allocator,

class allocator {
public:
  typedef size_t size_type
  typedef ptrdiff_t difference_type

If I understand the definition of ptrdiff_t from C the result is
undefined if you take the difference of two pointers which do not
point into the same object.

So is it undefined or not, if you are using the types supplied with
the allocators?  What is the intent here?

Eric



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