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### 2106. `move_iterator` wrapping iterators returning prvalues

Section: 23.5.3 [move.iterators] Status: C++17 Submitter: Dave Abrahams Opened: 2011-11-30 Last modified: 2017-07-30

Priority: 3

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Discussion:

Shouldn't `move_iterator` be specialized so that if the iterator it wraps returns a prvalue when dereferenced, the `move_iterator` also returns by value? Otherwise, it creates a dangling reference.

Howard: I believe just changing `move_iterator<I>::reference` would do. A direction might be testing on `is_reference<iterator_traits<I>::reference>`, or `is_reference<decltype(*declval<I>())>`.

Daniel: I would prefer to use a consistent style among the iterator adaptors, so I suggest to keep with the `iterator_traits` typedefs if possible.

>     using reference = typename conditional<
>       is_reference<typename iterator_traits<Iterator>::reference>::value,
>       value_type&&,
>       value_type
>     >::type;

We might also want to ensure that if `Iterator`'s `reference` type is a reference, the referent is equal to `value_type` (after removal of cv-qualifiers). In general we have no such guarantee.

Marc: In the default case where we don't return `value_type&&`, should we use `value_type` or should we keep the `reference` type of the wrapped iterator?

Daniel: This suggestion looks appealing at first, but the problem here is that using this typedef can make it impossible for `move_iterator` to satisfy its contract, which means returning an rvalue of the value type (Currently it says rvalue-reference, but this must be fixed as of this issue anyway). I think that user-code can reasonably expect that when it has constructed an object `m` of `move_iterator<It>`, where `It` is a valid mutable iterator type, the expression

>     It::value_type&& rv = *m;

is well-formed.

Let's set `R` equal to `iterator_traits<Iterator>::reference` in the following. We can discuss the following situations:

1. `R` is a reference type: We can only return the corresponding xvalue of `R`, if `value_type` is reference-related to the referent type, else this is presumably no forward iterator and we cannot say much about it, except that it must be convertible to `value_type`, so it better should return a prvalue.
2. `R` is not a reference type: In this case we can rely on a conversion to `value_type` again, but not much more. Assume we would return `R` directly, this might turn out to have a conversion to an lvalue-reference type of the value type (for example). If that is the case, this would indirectly violate the contract of `move_iterator`.

In regard to the first scenario I suggest that implementations are simply required to check that `V2 = remove_cv<remove_reference<R>::type>::type` is equal to the value type `V1` as a criterion to return this reference as an xvalue, in all other cases it should return the value type directly as prvalue.

The additional advantage of this strategy is, that we always ensure that `reference` has the correct cv-qualification, if `R` is a real reference.

It is possible to improve this a bit by indeed supporting reference-related types, this would require to test `is_same<V1, V2>::value || is_base_of<V1, V2>::value` instead. I'm unsure whether (a) this additional effort is worth it and (b) a strict reading of the forward iterator requirements seems not to allow to return a reference-related type (Whether this is a defect or not is another question).

[2011-12-05: Marc Glisse comments and splits into two resolution alternatives]

I guess I am looking at the speed of:

>     value_type x;
>     x = *m;

(copy construction would likely trigger copy elision and thus be neutral) instead of the validity of:

>     value_type&& x = *m;

In this sense, Daniels earlier proposition that ignored `value_type` and just did `switch_lvalue_ref_to_rvalue_ref<reference>` was easier to understand (and it didn't require thinking about reference related types).

The currently proposed resolution has been split into two alternatives.

[2012, Kona]

Move to Review.

Alisdair: This only applies to input iterators, so keep that in mind when thinking about this.

STL: I see what B is doing, but not A.

Howard: I agree.

Alisdair: Should we use `add_rvalue_reference`?

STL: No, we do not want reference collapsing.

STL: Re A, messing with the CV qualification scares me.

Alisdair: Agree. That would break my intent.

STL: Actually I don't think it's actually wrong, but I still don't see what it's doing.

Alisdair: A is picking the value type, B is picking the proxy type.

Howard: I like returning the proxy type.

STL: Returning a reference (B) seems right, because the requirements say "reference". I suspect that B works correctly if you have a move iterator wrapping a move iterator wrapping a thing. I think that A would mess up the type in the middle.

Considerable discussion about which version is correct, checking various examples.

STL: Still think B is right. Still don't understand A. In A we are losing the proxyness.

Howard: Agree 100%. We don't want to lose the proxy. If it's const, so be it.

STL: B is also understandable by mortals.

Howard: Remove to review, keep A but move it out of the proposed resolution area (but keep it for rational).

Alisdair: Adding an explanatory note might be a good idea, if someone wants to write one.

Walter: Concerned about losing the word "reference" in p.1.

Howard: `move_iterator` will return an xvalue or a prvalue, both of which are rvalues.

[Proposed resolution A, rejected in preference to the currently proposed resolution (B)

1. Change 23.5.3 [move.iterators] p1 as indicated:

   > Class template `move_iterator` is an iterator adaptor with the same behavior as the underlying iterator except that its dereference operator implicitly converts the value returned by the underlying iterator's dereference operator to an rvalue reference of the value type. Some generic algorithms can be called with move iterators to replace copying with moving.

2. Change 23.5.3.2 [move.iterator], class template `move_iterator` synopsis, as indicated:

   >     namespace std {
   >       template <class Iterator>
   >       class move_iterator {
   >       public:
   >         typedef Iterator iterator_type;
   >         typedef typename iterator_traits<Iterator>::difference_type difference_type;
   >         typedef Iterator pointer;
   >         typedef typename iterator_traits<Iterator>::value_type value_type;
   >         typedef typename iterator_traits<Iterator>::iterator_category iterator_category;
   >         typedef value_type&&see below reference;
   >         […]
   >       };
   >     }

3. Immediately following the class template `move_iterator` synopsis in 23.5.3.2 [move.iterator] insert a new paragraph as indicated:

   > -?- Let `R` be `iterator_traits<Iterator>::reference` and let `V` be `iterator_traits<Iterator>::value_type`. If `is_reference<R>::value` is `true` and if `remove_cv<remove_reference<R>::type>::type` is the same type as `V`, the template instantiation `move_iterator<Iterator>` shall define the nested type named `reference` as a synonym for `remove_reference<R>::type&&`, otherwise as a synonym for `V`.

]

[2012, Portland: Move to Tentatively Ready]

AJM wonders if the implied trait might be useful elsewhere, and worth adding to type traits as a transformation type trait.

Suspicion that the Range SG might find such a trait useful, but wait until there is clear additional use of such a trait before standardizing.

Minor wording tweak to use `add_rvalue_reference` rather than manually adding the `&&`, then move to Tentatively Ready.

[2013-01-09 Howard Hinnant comments]

I believe the P/R for LWG 2106 is incorrect (item 3). The way it currently reads, `move_iterator<I>::reference` is always an lvalue reference. I.e. if `R` is an lvalue reference type, then reference becomes `add_rvalue_reference<R>::type` which is just `R`. And if `R` is not a reference type, then reference becomes `R` (which is also just `R` ;-)).

I believe the correct wording is what was there previously:

-?- Let `R` be `iterator_traits<Iterator>::reference`. If `is_reference<R>::value` is true, the template instantiation `move_iterator<Iterator>` shall define the nested type named `reference` as a synonym for `remove_reference<R>::type&&`, otherwise as a synonym for `R`.

Additionally Marc Glisse points out that `move_iterator<I>::operator*()` should return `static_cast<reference>(*current)`, not `std::move(*current)`.

Previous resolution:

> This wording is relative to the FDIS.

> 1. Change 23.5.3 [move.iterators] p1 as indicated:

>    > Class template `move_iterator` is an iterator adaptor with the same behavior as the underlying iterator except that its dereference operator implicitly converts the value returned by the underlying iterator's dereference operator to an rvalue reference. Some generic algorithms can be called with move iterators to replace copying with moving.

> 2. Change 23.5.3.2 [move.iterator], class template `move_iterator` synopsis, as indicated:

>    >     namespace std {
>    >       template <class Iterator>
>    >       class move_iterator {
>    >       public:
>    >         typedef Iterator iterator_type;
>    >         typedef typename iterator_traits<Iterator>::difference_type difference_type;
>    >         typedef Iterator pointer;
>    >         typedef typename iterator_traits<Iterator>::value_type value_type;
>    >         typedef typename iterator_traits<Iterator>::iterator_category iterator_category;
>    >         typedef value_type&&see below reference;
>    >         […]
>    >       };
>    >     }

> 3. Immediately following the class template `move_iterator` synopsis in 23.5.3.2 [move.iterator] insert a new paragraph as indicated:

>    > -?- Let `R` be `iterator_traits<Iterator>::reference`. If `is_reference<R>::value` is `true`, the template instantiation `move_iterator<Iterator>` shall define the nested type named `reference` as a synonym for `add_rvalue_reference<R>::type`, otherwise as a synonym for `R`.

[2014-05-19, Daniel comments]

The term instantiation has been changed to specialization in the newly added paragraph as suggested by STL and much preferred by myself.

[2014-05-19 Library reflector vote]

The issue has been identified as Tentatively Ready based on five votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Change 23.5.3 [move.iterators] p1 as indicated:

   > Class template `move_iterator` is an iterator adaptor with the same behavior as the underlying iterator except that its indirection operator implicitly converts the value returned by the underlying iterator's indirection operator to an rvalue reference. Some generic algorithms can be called with move iterators to replace copying with moving.

2. Change 23.5.3.2 [move.iterator], class template `move_iterator` synopsis, as indicated:

   >     namespace std {
   >       template <class Iterator>
   >       class move_iterator {
   >       public:
   >         typedef Iterator iterator_type;
   >         typedef typename iterator_traits<Iterator>::difference_type difference_type;
   >         typedef Iterator pointer;
   >         typedef typename iterator_traits<Iterator>::value_type value_type;
   >         typedef typename iterator_traits<Iterator>::iterator_category iterator_category;
   >         typedef value_type&&see below reference;
   >         […]
   >       };
   >     }

3. Immediately following the class template `move_iterator` synopsis in 23.5.3.2 [move.iterator] insert a new paragraph as indicated:

   > -?- Let `R` be `iterator_traits<Iterator>::reference`. If `is_reference<R>::value` is `true`, the template specialization `move_iterator<Iterator>` shall define the nested type named `reference` as a synonym for `remove_reference<R>::type&&`, otherwise as a synonym for `R`.

4. Edit [move.iter.op.star] p1 as indicated:

   >     reference operator*() const;

   > > -1- Returns: `std::movestatic_cast<reference>(*current)`.