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1079. UK-265:RandomAccessIterator'soperator- has nonsensical effects clause

Section: 24.3.5.7[random.access.iterators]Status:C++11Submitter: Doug GregorOpened: 2009-03-20Last modified: 2016-01-28

Priority:Not Prioritized

View all otherissues in [random.access.iterators].

View all issues withC++11 status.

Discussion:

Addresses UK 265

UK-265:

This effects clause is nonesense. It looks more like an axiom statingequivalence, and certainly an effects clause cannot change the state oftwo arguments passed by const reference

[2009-09-18 Alisdair adds:]

For random access iterators, the definitions of(b-a) and(a<b) are circular:

From table Table 104 -- Random access iterator requirements:

b - a :==>  (a < b) ? distance(a,b) : -distance(b,a)a < b :==>  b - a > 0

[2009-10 Santa Cruz:]

Moved to Ready.

[2010-02-13 Alisdair opens.]

Looking again at LWG1079⁽ⁱ⁾, the wording in the issue no longer exists, andappears to be entirely an artefact of the concepts wording.

This issue is currently on our Ready list (not even Tentative!) but I think ithas to be pulled as there is no way to apply the resolution.

Looking at the current paper, I think this issue is now "NAD, solved by theremoval of concepts". Unfortunately it is too late to poll again, so we willhave to perform that review in Pittsburgh.

[2010-02-13 Daniel updates the wording to address the circularity problem.]

[The previous wording is preserved here:]

Modify 24.3.5.7[random.access.iterators]p7-9 as follows:

difference_type operator-(const X& a, const X& b);

-7-Precondition: there exists a valuen ofdifference_type such thata == b + n.

-8-Effects:b == a + (b - a)

-9-Returns:(a < b) ? distance(a,b) : -distance(b,a)n

[2010 Pittsburgh:]

Moved to Ready for Pittsburgh.

Proposed resolution:

Modify Table 105 in 24.3.5.7[random.access.iterators]:

Table 105 — Random access iterator requirements (in addition tobidirectional iterator)

Expression	Return type	Operational semantics	Assertion/note pre-/post-condition
b - a	Distance	distance(a,b)returnn	pre: there exists a valuen ofDistance such thata +n == b.b == a + (b - a).