Re: [boost] [range] Proposal: addition of front(), back(), at(), operator[]

On Mar 24, 2014, at 10:36 AM, Marshall Clow wrote:

On Mar 24, 2014, at 2:14 AM, Valentin Ziegler wrote:

...

Hi Adam,

...

1. front() returning the reference to the first element precondition (assert): !empty(rng)

...

return *begin(rng);

...

2. back() returning the reference to the last element precondition (assert): !empty(rng)

...

For BidirectionalRange return *(--end(rng));

In 99% of all cases above implementations will work just fine. However, there may be rare cases where the lifetime of the reference is bound to the lifetime of the iterator:

[iterator.requirements.general] 9. Destruction of an iterator may invalidate pointers and references previously obtained from that iterator.

Interesting. I think that in C++14, the committee put that possibility to bed. See LWG issue 2360 http://cplusplus.github.io/LWG/lwg-defects.html#2360

I’ll open an issue that gets that cleared up.

Thinking about this some more, I think that “may” is the key there. [forward.iterators] says: • 6 If a and b are both dereferenceable, then a == b if and only if *a and *b are bound to the same object. which disallows “stashing iterators” for forward (or greater) iterators. So, I think that [iterator.requirements.general] 9 can only really be true for input iterators. — Marshall