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1052. reverse_iterator::operator-> should also support smart pointers

Section: 25.5.1.6 [reverse.iter.elem] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2021-06-06

Priority: Not Prioritized

View all other issues in [reverse.iter.elem].

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Duplicate of: 2775

Discussion:

Addresses UK 281 [CD1]

The current specification for return value for reverse_iterator::operator-> will always be a true pointer type, but reverse_iterator supports proxy iterators where the pointer type may be some kind of 'smart pointer'.

[ Summit: ]

move_iterator avoids this problem by returning a value of the wrapped Iterator type. study group formed to come up with a suggested resolution.

move_iterator solution shown in proposed wording.

[ 2009-07 post-Frankfurt: ]

Howard to deconceptize. Move to Review after that happens.

[ 2009-08-01 Howard deconceptized: ]

[ 2009-10 Santa Cruz: ]

We can't think of any reason we can't just define reverse iterator's pointer types to be the same as the underlying iterator's pointer type, and get it by calling the right arrow directly.

Here is the proposed wording that was replaced: 

template <class Iterator>
class reverse_iterator {
  […]
  typedef typename iterator_traits<Iterator>::pointer pointer;

Change [reverse.iter.opref]: 

pointer operator->() const;

Returns: 

&(operator*());
this->tmp = current;
--this->tmp;
return this->tmp;

[ 2010-03-03 Daniel opens: ]

  1. There is a minor problem with the exposition-only declaration of the private member deref_tmp which is modified in a const member function (and the same problem occurs in the specification of operator*). The fix is to make it a mutable member.

  2. The more severe problem is that the resolution for some reasons does not explain in the rationale why it was decided to differ from the suggested fix (using deref_tmp instead of tmp) in the [ 2009-10 Santa Cruz] comment: 

    this->deref_tmp = current;
--this->deref_tmp;
return this->deref_tmp;

    combined with the change of 

    typedef typename iterator_traits<Iterator>::pointer pointer;

    to 

    typedef Iterator pointer;

    The problem of the agreed on wording is that the following rather typical example, that compiled with the wording before 1052 had been applied, won't compile anymore: 

    #include <iterator>
#include <utility>

int main() {
  typedef std::pair<int, double> P;
  P op;
  std::reverse_iterator<P*> ri(&op + 1);
  ri->first; // Error
}

    Comeau online returns (if a correspondingly changed reverse_iterator is used): 

    "error: expression must have class type
     return deref_tmp.operator->();
            ^
         detected during instantiation of "Iterator
                   reverse_iterator<Iterator>::operator->() const [with
                   Iterator=std::pair<int, double> *]""

    Thus the change will break valid, existing code based on std::reverse_iterator.

IMO the suggestion proposed in the comment is a necessary fix, which harmonizes with the similar specification of std::move_iterator and properly reflects the recursive nature of the evaluation of operator-> overloads.

Suggested resolution:

  1. In the class template reverse_iterator synopsis of 25.5.1.2 [reverse.iterator] change as indicated: 

    namespace std {
template <class Iterator>
class reverse_iterator : public
             iterator<typename iterator_traits<Iterator>::iterator_category,
             typename iterator_traits<Iterator>::value_type,
             typename iterator_traits<Iterator>::difference_type,
             typename iterator_traits<Iterator>::pointer,
             typename iterator_traits<Iterator>::reference> {
public:
  [..]
  typedef typename iterator_traits<Iterator>::pointer pointer;
  [..]
protected:
  Iterator current;
private:
  mutable Iterator deref_tmp; // exposition only
};
  2. Change [reverse.iter.opref]/1 as indicated: 
    pointer operator->() const;

    1 Returns Effects: &(operator*()). 

    deref_tmp = current;
--deref_tmp;
return deref_tmp;

[ 2010 Pittsburgh: ]

We prefer to make to use a local variable instead of deref_tmp within operator->(). And although this means that the mutable change is no longer needed, we prefer to keep it because it is needed for operator*() anyway.

Here is the proposed wording that was replaced:

Change [reverse.iter.opref]: 

pointer operator->() const;

Returns: 

&(operator*());
deref_tmp = current;
--deref_tmp;
return deref_tmp::operator->();

[ 2010-03-10 Howard adds: ]

Here are three tests that the current proposed wording passes, and no other solution I've seen passes all three:

  1. Proxy pointer support: 

    #include <iterator>
#include <cassert>

struct X { int m; };

X x;

struct IterX {
    typedef std::bidirectional_iterator_tag iterator_category;
    typedef X& reference;
    struct pointer
    {
        pointer(X& v) : value(v) {}
        X& value;
        X* operator->() const {return &value;}
    };
    typedef std::ptrdiff_t difference_type;
    typedef X value_type;
    // additional iterator requirements not important for this issue

    reference operator*() const { return x; }
    pointer operator->() const { return pointer(x); }
    IterX& operator--() {return *this;}

};

int main()
{
    std::reverse_iterator<IterX> ix;
    assert(&ix->m == &(*ix).m);
}

  2. Raw pointer support: 

    #include <iterator>
#include <utility>

int main() {
  typedef std::pair<int, double> P;
  P op;
  std::reverse_iterator<P*> ri(&op + 1);
  ri->first; // Error
}

  3. Caching iterator support: 

    #include <iterator>
#include <cassert>

struct X { int m; };

struct IterX {
    typedef std::bidirectional_iterator_tag iterator_category;
    typedef X& reference;
    typedef X* pointer;
    typedef std::ptrdiff_t difference_type;
    typedef X value_type;
    // additional iterator requirements not important for this issue

    reference operator*() const { return value; }
    pointer operator->() const { return &value; }
    IterX& operator--() {return *this;}

private:
    mutable X value;
};

int main()
{
    std::reverse_iterator<IterX> ix;
    assert(&ix->m == &(*ix).m);
}

[ 2010 Pittsburgh: ]

Moved to NAD Future, rationale added.

[LEWG Kona 2017]

Recommend that we accept the "Alternate Proposed Resolution" from 2775.

[ Original rationale: ]

The LWG did not reach a consensus for a change to the WP.

Previous resolution [SUPERSEDED]:

  1. In the class template reverse_iterator synopsis of 25.5.1.2 [reverse.iterator] change as indicated: 

    namespace std {
template <class Iterator>
class reverse_iterator : public
             iterator<typename iterator_traits<Iterator>::iterator_category,
             typename iterator_traits<Iterator>::value_type,
             typename iterator_traits<Iterator>::difference_type,
             typename iterator_traits<Iterator&>::pointer,
             typename iterator_traits<Iterator>::reference> {
public:
  [..]
  typedef typename iterator_traits<Iterator&>::pointer pointer;
  [..]
protected:
  Iterator current;
private:
  mutable Iterator deref_tmp; // exposition only
};
  2. Change [reverse.iter.opref]/1 as indicated: 
    pointer operator->() const;

    1 Returns Effects: &(operator*()). 

    deref_tmp = current;
--deref_tmp;
return deref_tmp;

[Alternate Proposed Resolution from 2775, which was closed as a dup of this issue]

This wording is relative to N4606.

  1. Modify [reverse.iter.opref] as indicated:

    constexpr pointer operator->() const;

    -1- Returns: addressof(operator*()).Effects: If Iterator is a pointer type, as if by: 

    Iterator tmp = current;
return --tmp;

    Otherwise, as if by: 

    Iterator tmp = current;
--tmp;
return tmp.operator->();

[2018-08-06; Daniel rebases to current working draft and reduces the proposed wording to that preferred by LEWG]

[2018-11-13; Casey Carter comments]

The acceptance of P0896R4 during the San Diego meeting resolves this issue: The wording in [reverse.iter.elem] for operator-> is equivalent to the PR for LWG 1052.

Proposed resolution:

This wording is relative to N4762.

  1. Modify 25.5.1.6 [reverse.iter.elem] as indicated:

    constexpr pointer operator->() const;

    -2- Returns: addressof(operator*()).Effects: If Iterator is a pointer type, as if by: 

    Iterator tmp = current;
return --tmp;

    Otherwise, as if by: 

    Iterator tmp = current;
--tmp;
return tmp.operator->();