On Mon, 3 Oct 2011, Krzysztof Żelechowski wrote:
Dave Abrahams wrote:
on Sun Oct 02 2011, Andrew Sutton
wrote: I would take that to mean that singularity is independent of type.
Although many types have singular values with respect to certain operations (NULL is singular with respect to pointer dereference),
- Not every type has singular values - Some types have multiple singular values - In general, one type's singular values are distinct from another type's singular values
I don't know what you and Chris mean by "independent of type," but to me this sounds like singularity is highly dependent on type.
It means iterators of the same type can be singular but need not, whereas all (valid) iterators of the same type either must support random access operations or must not, depending on the type in question.
Of course, the result of subtraction may be undefined, but it will not fail to compile. If you insist that random access iterators are things you can universally subtract, it will be a concept that applies to pairs, not to individual objects.
Another complicating issue is that an iterator is singular or non-singular with respect to a particular container. For example, given the following declarations: int a[5], b[5]; and assuming a and b are contiguous in memory, &a[5] is a singular iterator for the array a, while &b[0] is non-singular for the array b, even though those two pointers will have exactly the same value and are not distinguishable by C++ code. -- Jeremiah Willcock