I wrote:

Yes quite. i.e. What you believe is not true about boost::allocate_unique.

i.e. For auto p = boost::allocate_unique(a, n); assert(p.get().size() == n);

This is because the unique_ptr stores a pointer adaptor around the Allocator::pointer which also stores the size.

So the pointer and size are stored together in the unique_ptr result.

Note, that the size is only stored for allocate_unique(a,n) but not for allocate_unique(a) since there the N is known at compile time. For auto p = allocate_unique(a); your p.get().size() will return N, but there's no need to store a size_t for N, so we don't.

To get the plain Allocator::pointer out of it you can use p.get().ptr()

Or, for example, p.release().ptr() for someone who wants to manage it themselves.

Glen