On Thu, Sep 29, 2011 at 9:50 AM, Dave Abrahams
on Thu Sep 29 2011, Krzysztof Żelechowski
wrote: Dave Abrahams wrote:
on Wed Sep 28 2011, Nathan Ridge
wrote: 2. The concept mechanism used by Boost should not require singular iterators to exist; the standard is obnoxious and misguided here and promotes sloppy coding.
I think the Boost authors are unlikely to decide to ignore a part of
the
standard just because one person believes it is obnoxious and misguided and promotes sloppy coding.
I would suggest making your case about the default constructibility of forward iterators at comp.std.c++. If you gain consensus there that this requirement is indeed misguided, then your request will carry more weight here.
I'd also like to point out that there's no rule saying default-constructed iterators must be singular.
A default-constructed iterator must be singular not because the government says so but because of logic.
No. Well, we have to say what we mean by "is singular." Because the concept "singular" only contains two operations (assign and destroy), every valid iterator is-a singular iterator in some sense. But I suspect you mean "minimally singular," and there's no reason at all that a default-constructed iterator need be minimally singular. You usually can't make it usefully dereferenceable (would you want to?) but you can make it copyable, for example. A wrapper over a plain pointer could initialize the pointer to 0. Now it's a valid past-the-end iterator into an array of length zero. Such an iterator is also comparable with other iterators into the same sequence. That's actually far from being minimally singular.
I'm sorry - but regarding the statement "every valid iterator is-a singular iterator in some sense"... I thought the Standard (2003) explicitly stated that *singular *values for an iterator had all but one operation as Undefined Behavior, and the only defined operation on an iterator with a singular value was to assign it a non-singular value. (24.1, para 5) *Results of most expressions are undefined for singular values; the only exception is an assignment of a non-singular value to an iterator that holds a singular value. In this case the singular value is overwritten the same way as any other value. Dereferenceable values are always nonsingular.* Doesn't the last sentence make it true that "every valid iterator is-a *non*singular iterator"? I also thought it was the case that the latest standard defines *singular *to be the same as *uninitialized *with respect to iterators? If that's the case then we cannot call a valid iterator a singular iterator in any sense. "I am but an egg", but it seem that there's no need to create a new term when the old one is well-defined and sufficient. regards, Brian