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### 1079. UK-265: `RandomAccessIterator`'s `operator-` has nonsensical effects clause

Section: 25.3.5.7 [random.access.iterators] Status: C++11 Submitter: Doug Gregor Opened: 2009-03-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 265

UK-265:

This effects clause is nonesense. It looks more like an axiom stating equivalence, and certainly an effects clause cannot change the state of two 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 LWG 1079, the wording in the issue no longer exists, and appears to be entirely an artefact of the concepts wording.

> This issue is currently on our Ready list (not even Tentative!) but I think it has 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 the removal of concepts". Unfortunately it is too late to poll again, so we will have 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 25.3.5.7 [random.access.iterators]p7-9 as follows:

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

> > > > -7- Precondition: there exists a value `n` of `difference_type` such that `a == 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 25.3.5.7 [random.access.iterators]:

> Table 105 — Random access iterator requirements (in addition to bidirectional iterator)
> Expression | Return type | Operational semantics | Assertion/note pre-/post-condition
> --- | --- | --- | ---
> `b - a` | `Distance` | `distance(a,b)` return `n` | pre: there exists a value `n` of `Distance` such that `a + n == b`. `b == a + (b - a)`.