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2127. Move-construction with raw_storage_iterator

Section: 99 [depr.storage.iterator] Status: C++17 Submitter: Jonathan Wakely Opened: 2012-01-23 Last modified: 2017-07-30

Priority: 3

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Discussion:

Aliaksandr Valialkin pointed out that raw_storage_iterator only supports constructing a new object from lvalues so cannot be used to construct move-only types:

template <typename InputIterator, typename T>
void move_to_raw_buffer(InputIterator first, InputIterator last, T *raw_buffer)
{
  std::move(first, last, std::raw_storage_iterator<T *, T>(raw_buffer));
}

This could easily be solved by overloading operator= for rvalues.

Dave Abrahams:

raw_storage_iterator causes exception-safety problems when used with any generic algorithm. I suggest leaving it alone and not encouraging its use.

[2014-11-11, Jonathan provides improved wording]

In Urbana LWG decided to explicitly say the value is constructed from an rvalue.

This wording is relative to N3337.

1. Add a new signature to the synopsis in [storage.iterator] p1:

   namespace std {
     template <class OutputIterator, class T>
     class raw_storage_iterator
       : public iterator<output_iterator_tag,void,void,void,void> {
     public:
       explicit raw_storage_iterator(OutputIterator x);
   
       raw_storage_iterator<OutputIterator,T>& operator*();
       raw_storage_iterator<OutputIterator,T>& operator=(const T& element);
       raw_storage_iterator<OutputIterator,T>& operator=(T&& element);
       raw_storage_iterator<OutputIterator,T>& operator++();
       raw_storage_iterator<OutputIterator,T> operator++(int);
   };
   }
   

2. Insert the new signature and a new paragraph before p4:

   raw_storage_iterator<OutputIterator,T>& operator=(const T& element);
   raw_storage_iterator<OutputIterator,T>& operator=(T&& element);
   

   -?- Requires: For the first signature T shall be CopyConstructible. For the second signature T shall be MoveConstructible.

   -4- Effects: Constructs a value from element at the location to which the iterator points.

   -5- Returns: A reference to the iterator.

[2015-05, Lenexa]

MC: Suggestion to move it to Ready for incorporation on Friday
MC: move to ready: in favor: 12, opposed: 0, abstain: 3

Proposed resolution:

This wording is relative to N4140.

1. Add a new signature to the synopsis in [storage.iterator] p1:

   namespace std {
     template <class OutputIterator, class T>
     class raw_storage_iterator
       : public iterator<output_iterator_tag,void,void,void,void> {
     public:
       explicit raw_storage_iterator(OutputIterator x);
   
       raw_storage_iterator<OutputIterator,T>& operator*();
       raw_storage_iterator<OutputIterator,T>& operator=(const T& element);
       raw_storage_iterator<OutputIterator,T>& operator=(T&& element);
       raw_storage_iterator<OutputIterator,T>& operator++();
       raw_storage_iterator<OutputIterator,T> operator++(int);
   };
   }
   

2. Insert a new paragraph before p4:

   raw_storage_iterator<OutputIterator,T>& operator=(const T& element);
   

   -?- Requires: T shall be CopyConstructible.

   -4- Effects: Constructs a value from element at the location to which the iterator points.

   -5- Returns: A reference to the iterator.

3. Insert the new signature and a new paragraph after p5:

   raw_storage_iterator<OutputIterator,T>& operator=(T&& element);
   

   -?- Requires: T shall be MoveConstructible.

   -?- Effects: Constructs a value from std::move(element) at the location to which the iterator points.

   -?- Returns: A reference to the iterator.