Date: 2004-01-21

# Iterator concept and adapter issues

Date: | 2004-01-21
Copyright: | Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.

Index

- Issues from Matt's TR issues list
  - 9.1 iterator_access overspecified?
  - 9.2 operators of iterator_facade overspecified
  - 9.3 enable_if_interoperable needs standardese
  - 9.4 enable_if_convertible unspecified, conflicts with requires
  - 9.5 iterator_adaptor has an extraneous 'bool' at the start of the template definition
  - 9.6 Name of private member shouldn't be normative
  - 9.7 iterator_adaptor operations specifications are a bit inconsistent
  - 9.8 Specialized adaptors text should be normative
  - 9.9 Reverse_iterator text is too informal
  - 9.10 'prior' is undefined
  - 9.11 "In other words" is bad wording
  - 9.12 Transform_iterator shouldn't mandate private member
  - 9.13 Unclear description of counting iterator
  - 9.14 Counting_iterator's difference type
  - 9.15 How to detect lvalueness?
  - 9.16 is_writable_iterator returns false positives
  - 9.17 is_swappable_iterator returns false positives
  - 9.18 Are is_readable, is_writable, and is_swappable useful?
  - 9.19 Non-Uniformity of the "lvalue_iterator Bit"
  - 9.20 Traversal Concepts and Tags
  - 9.21 iterator_facade Derived template argument underspecified
  - 9.22 return type of Iterator difference for iterator facade
  - 9.23 Iterator_facade: minor wording Issue
  - 9.24 Use of undefined name in iterator_facade table
  - 9.25 Iterator_facade: wrong return type
  - 9.26 Iterator_facade: unclear returns clause for operator[]
  - 9.27 Iterator_facade: redundant clause
  - 9.28 indirect_iterator: incorrect specification of default constructor
  - 9.29 indirect_iterator: unclear specification of template constructor
  - 9.30 transform_iterator argument irregularity
  - 9.31 function_output_iterator overconstrained
  - 9.32 Should output_proxy really be a named type?
  - 9.33 istreambuf_iterator isn't a Readable Iterator
  - 9.34 iterator_facade free functions unspecified
  - 9.35 iterator_facade: too many equals?
  - 9.36 iterator_facade function requirements
- More Issues (not from Matt's list)
  - 9.37x Inheritance in iterator_adaptor and other adaptors is an overspecification
  - 9.38x Problem with specification of a->m in Readable Iterator
  - 9.39x counting_iterator Traversal argument unspecified
  - 9.40x indirect_iterator requirements muddled
  - 9.41x Problem with transform_iterator requirements
  - 9.42x filter_iterator details unspecified
  - 9.43x transform_iterator interoperability too restrictive
  - 9.44y `indirect_iterator` and smart pointers
  - 9.45y N1530: Typos and editorial changes in proposal text (not standardese)
  - 9.46y N1530: `base()` return-by-value is costly
  - 9.47x Forgot default constructible in Forward Traversal Iterator
  - 9.48x Editorial changes (non-normative text)
  - 9.49x Clarification of iterator_facade requirements and type members

# Issues from Matt's TR issues list

## 9.1 iterator_access overspecified?

Submitter: | Pete Becker
Status: | New

The proposal includes:

    enum iterator_access {
       readable_iterator = 1, writable_iterator = 2,
       swappable_iterator = 4, lvalue_iterator = 8
    };

In general, the standard specifies thing like this as a bitmask type with a list of defined names, and specifies neither the exact type nor the specific values. Is there a reason for iterator_access to be more specific?

Proposed resolution:
| The `iterator_access` enum will be removed, so this is no longer an issue. See the resolution to 9.15.

## 9.2 operators of iterator_facade overspecified

Submitter: | Pete Becker
Status: | New

In general, we've provided operational semantics for things like operator++. That is, we've said that ++iter must work, without requiring either a member function or a non-member function. iterator_facade specifies most operators as member functions. There's no inherent reason for these to be members, so we should remove this requirement. Similarly, some operations are specified as non-member functions but could be implemented as members. Again, the standard doesn't make either of these choices, and TR1 shouldn't, either. So: `operator*()`, `operator++()`, `operator++(int)`, `operator--()`, `operator--(int)`, `operator+=`, `operator-=`, `operator-(difference_type)`, `operator-(iterator_facade instance)`, and `operator+` should be specified with operational semantics and not explicitly required to be members or non-members.

Proposed resolution:
| Not a defect.
Rationale: | The standard uses valid expressions such as `++iter` in requirements tables, such as for input iterator. However, for classes, such as `reverse_iterator`, the standard uses function prototypes, as we have done here for `iterator_facade`. Further, the prototype specification does not prevent the implementor from using members or non-members, since nothing the user can do in a conforming program can detect how the function is implemented.

## 9.3 enable_if_interoperable needs standardese

Submitter: | Pete Becker
Status: | New

The only discussion of what this means is in a note, so is non-normative. Further, the note seems to be incorrect. It says that enable_if_interoperable only works for types that "are interoperable, by which we mean they are convertible to each other." This requirement is too strong: it should be that one of the types is convertible to the other. N1541 48

Proposed resolution:
> | Add normative text. Relax requirements in the proposed way. Change: [Note: The `enable_if_interoperable` template used above is for exposition purposes. The member operators should be only be in an overload set provided the derived types `Dr1` and `Dr2` are interoperable, by which we mean they are convertible to each other. The `enable_if_interoperable` approach uses SFINAE to take the operators out of the overload set when the types are not interoperable.] To: The `enable_if_interoperable` template used above is for exposition purposes. The member operators should only be in an overload set provided the derived types `Dr1` and `Dr2` are interoperable, meaning that at least one of the types is convertible to the other. The `enable_if_interoperable` approach uses SFINAE to take the operators out of the overload set when the types are not interoperable. The operators should behave as-if `enable_if_interoperable` were defined to be:
>     template <bool, typename> enable_if_interoperable_impl
>     {};

>     template <typename T> enable_if_interoperable_impl<true,T>
>     { typedef T type; };

>     template<typename Dr1, typename Dr2, typename T>
>     struct enable_if_interoperable
>       : enable_if_interoperable_impl<
>             is_convertible<Dr1,Dr2>::value || is_convertible<Dr2,Dr1>::value
>           , T
>         >
>     {};

## 9.4 enable_if_convertible unspecified, conflicts with requires

Submitter: | Pete Becker
Status: | New

In every place where enable_if_convertible is used it's used like this (simplified):

    template<class T>
    struct C
    {
      template<class T1>
      C(T1, enable_if_convertible<T1, T>::type* = 0);
    };

The idea being that this constructor won't compile if T1 isn't convertible to T. As a result, the constructor won't be considered as a possible overload when constructing from an object x where the type of x isn't convertible to T. In addition, however, each of these constructors has a requires clause that requires convertibility, so the behavior of a program that attempts such a construction is undefined. Seems like the enable_if_convertible part is irrelevant, and should be removed. There are two problems. First, enable_if_convertible is never specified, so we don't know what this is supposed to do. Second: we could reasonably say that this overload should be disabled in certain cases or we could reasonably say that behavior is undefined, but we can't say both.

Thomas Witt writes that the goal of putting in enable_if_convertible here is to make sure that a specific overload doesn't interfere with the generic case except when that overload makes sense. He agrees that what we currently have is deficient. Dave Abrahams writes that there is no conflict with the requires cause because the requires clause only takes effect when the function is actually called. The presence of the constructor signature can/will be detected by is_convertible without violating the requires clause, and thus it makes a difference to disable those constructor instantiations that would be disabled by enable_if_convertible even if calling them invokes undefined behavior. There was more discussion on the reflector: c++std-lib-12312, c++std-lib-12325, c++std-lib- 12330, c++std-lib-12334, c++std-lib-12335, c++std-lib-12336, c++std-lib-12338, c++std-lib- 12362.

Proposed resolution:
> | Change: [Note: The `enable_if_convertible<X,Y>::type` expression used in this section is for exposition purposes. The converting constructors for specialized adaptors should be only be in an overload set provided that an object of type `X` is implicitly convertible to an object of type `Y`. The `enable_if_convertible` approach uses SFINAE to take the constructor out of the overload set when the types are not implicitly convertible.] To: The `enable_if_convertible<X,Y>::type` expression used in this section is for exposition purposes. The converting constructors for specialized adaptors should be only be in an overload set provided that an object of type `X` is implicitly convertible to an object of type `Y`. The signatures involving `enable_if_convertible` should behave as-if `enable_if_convertible` were defined to be:
>     template <bool> enable_if_convertible_impl
>     {};

>     template <> enable_if_convertible_impl<true>
>     { struct type; };

>     template<typename From, typename To>
>     struct enable_if_convertible
>       : enable_if_convertible_impl<is_convertible<From,To>::value>
>     {};
If an expression other than the default argument is used to supply the value of a function parameter whose type is written in terms of `enable_if_convertible`, the program is ill-formed, no diagnostic required. [Note: The `enable_if_convertible` approach uses SFINAE to take the constructor out of the overload set when the types are not implicitly convertible. ]

## 9.5 iterator_adaptor has an extraneous 'bool' at the start of the template definition

Submitter: | Pete Becker
Status: | New

The title says it all; this is probably just a typo.

Proposed resolution:
| Remove the 'bool'.

## 9.6 Name of private member shouldn't be normative

Submitter: | Pete Becker
Status: | New

iterator_adaptor has a private member named m_iterator. Presumably this is for exposition only, since it's an implementation detail. It needs to be marked as such.

Proposed resolution:
| Mark the member `m_iterator` as exposition only. Note/DWA: I think this is NAD because the user can't detect it, though I'm happy to mark it exposition only. In [lib.iterator.adaptor] Change:
    Base m_iterator;
to:
    Base m_iterator; // exposition only

## 9.7 iterator_adaptor operations specifications are a bit inconsistent

Submitter: | Pete Becker
Status: | New

iterator_adpator() has a Requires clause, that Base must be default constructible. iterator_adaptor(Base) has no Requires clause, although the Returns clause says that the Base member is copy construced from the argument (this may actually be an oversight in N1550, which doesn't require iterators to be copy constructible or assignable).

Proposed resolution:
| Add a requirements section for the template parameters of iterator_adaptor, and state that Base must be Copy Constructible and Assignable. N1550 does in fact include requirements for copy constructible and assignable in the requirements tables. To clarify, we've also added the requirements to the text.

## 9.8 Specialized adaptors text should be normative

Submitter: | Pete Becker
Status: | New

similar to 9.3, "Specialized Adaptors" has a note describing enable_if_convertible. This should be normative text.

Proposed resolution:
| Changed it to normative text. See the resolution of 9.4

## 9.9 Reverse_iterator text is too informal

Submitter: | Pete Becker
Status: | New

reverse iterator "flips the direction of the base iterator's motion". This needs to be more formal, as in the current standard. Something like: "iterates through the controlled sequence in the opposite direction"

Proposed resolution:
| Change: The reverse iterator adaptor flips the direction of a base iterator's motion. Invoking `operator++()` moves the base iterator backward and invoking `operator--()` moves the base iterator forward. to: The reverse iterator adaptor iterates through the adapted iterator range in the opposite direction.

## 9.10 'prior' is undefined

Submitter: | Pete Becker
Status: | New

reverse_iterator::dereference is specified as calling a function named 'prior' which has no specification.

Proposed resolution:
| Change the specification to avoid using `prior` as follows. Remove:
    typename reverse_iterator::reference dereference() const { return *prior(this->base()); }
> And at the end of the operations section add: `reference operator*() const;`
> Effects: |
>     Iterator tmp = m_iterator;
>     return *--tmp;
Rationale: | The style of specification has changed because of issue 9.37x.

## 9.11 "In other words" is bad wording

Submitter: | Pete Becker
Status: | New

Transform iterator has a two-part specification: it does this, in other words, it does that. "In other words" always means "I didn't say it right, so I'll try again." We need to say it once.

Proposed resolution:
| Change: The transform iterator adapts an iterator by applying some function object to the result of dereferencing the iterator. In other words, the `operator*` of the transform iterator first dereferences the base iterator, passes the result of this to the function object, and then returns the result. to: The transform iterator adapts an iterator by modifying the `operator*` to apply a function object to the result of dereferencing the iterator and returning the result.

## 9.12 Transform_iterator shouldn't mandate private member

Submitter: | Pete Becker
Status: | New

transform_iterator has a private member named 'm_f' which should be marked "exposition only."

Proposed resolution:
| Mark the member `m_f` as exposition only. Note/DWA: I think this is NAD because the user can't detect it, though I'm happy to mark it exposition only. Change:
    UnaryFunction m_f;
to:
    UnaryFunction m_f;   // exposition only

## 9.13 Unclear description of counting iterator

Submitter: | Pete Becker
Status: | New

The description of Counting iterator is unclear. "The counting iterator adaptor implements dereference by returning a reference to the base object. The other operations are implemented by the base m_iterator, as per the inheritance from iterator_adaptor."

Proposed resolution:
| Change: The counting iterator adaptor implements dereference by returning a reference to the base object. The other operations are implemented by the base `m_iterator`, as per the inheritance from `iterator_adaptor`. to: `counting_iterator` adapts an object by adding an `operator*` that returns the current value of the object. All other iterator operations are forwarded to the adapted object.

## 9.14 Counting_iterator's difference type

Submitter: | Pete Becker
Status: | New

Counting iterator has the following note:

> [Note: implementers are encouraged to provide an implementation of distance_to and a difference_type that avoids overflows in the cases when the Incrementable type is a numeric type.]

I'm not sure what this means. The user provides a template argument named Difference, but there's no difference_type. I assume this is just a glitch in the wording. But if implementors are encouraged to ignore this argument if it won't work right, why is it there?

Proposed resolution:
| The `difference_type` was inherited from `iterator_adaptor`. However, we've removed the explicit inheritance, so explicit typedefs have been added. See the resolution of 9.37x.

## 9.15 How to detect lvalueness?

Submitter: | Dave Abrahams
Status: | New

Shortly after N1550 was accepted, we discovered that an iterator's lvalueness can be determined knowing only its value_type. This predicate can be calculated even for old-style iterators (on whose reference type the standard places few requirements). A trait in the Boost iterator library does it by relying on the compiler's unwillingness to bind an rvalue to a T& function template parameter. Similarly, it is possible to detect an iterator's readability knowing only its value_type. Thus, any interface which asks the user to explicitly describe an iterator's lvalue-ness or readability seems to introduce needless complexity.

Proposed resolution:
> | Remove the `is_writable` and `is_swappable` traits, and remove the requirements in the Writable Iterator and Swappable Iterator concepts that require their models to support these traits. Change the `is_readable` specification. Remove the requirement for support of the `is_readable` trait from the Readable Iterator concept. Remove the `iterator_tag` class and transplant the logic for choosing an iterator category into `iterator_facade`. Change the specification of `traversal_category`. Remove Access parameters from N1530 In N1550: Remove: Since the access concepts are not related via refinement, but instead cover orthogonal issues, we do not use tags for the access concepts, but instead use the equivalent of a bit field. We provide an access mechanism for mapping iterator types to the new traversal tags and access bit field. Our design reuses `iterator_traits<Iter>::iterator_category` as the access mechanism. To that end, the access and traversal information is bundled into a single type using the following iterator_tag class.
>     enum iterator_access { readable_iterator = 1, writable_iterator = 2,
>         swappable_iterator = 4, lvalue_iterator = 8 };

>     template <unsigned int access_bits, class TraversalTag>
>     struct iterator_tag : /* appropriate old category or categories */ {
>       static const iterator_access access =
>         (iterator_access)access_bits &
>           (readable_iterator | writable_iterator | swappable_iterator);
>       typedef TraversalTag traversal;
>     };
> The `access_bits` argument is declared to be `unsigned int` instead of the enum `iterator_access` for convenience of use. For example, the expression `(readable_iterator | writable_iterator)` produces an unsigned int, not an `iterator_access`. The purpose of the `lvalue_iterator` part of the `iterator_access` enum is to communicate to `iterator_tag` whether the reference type is an lvalue so that the appropriate old category can be chosen for the base class. The `lvalue_iterator` bit is not recorded in the `iterator_tag::access` data member. The `iterator_tag` class template is derived from the appropriate iterator tag or tags from the old requirements based on the access bits and traversal tag passed as template parameters. The algorithm for determining the old tag or tags picks the least refined old concepts that include all of the requirements of the access and traversal concepts (that is, the closest fit), if any such category exists. For example, the category tag for a Readable Single Pass Iterator will always be derived from `input_iterator_tag`, while the category tag for a Single Pass Iterator that is both Readable and Writable will be derived from both `input_iterator_tag` and `output_iterator_tag`. We also provide several helper classes that make it convenient to obtain the access and traversal characteristics of an iterator. These helper classes work both for iterators whose `iterator_category` is `iterator_tag` and also for iterators using the original iterator categories.
>     template <class Iterator> struct is_readable  { typedef ... type; };
>     template <class Iterator> struct is_writable { typedef ... type; };
>     template <class Iterator> struct is_swappable { typedef ... type; };
>     template <class Iterator> struct traversal_category { typedef ... type; };
> After: Like the old iterator requirements, we provide tags for purposes of dispatching based on the traversal concepts. The tags are related via inheritance so that a tag is convertible to another tag if the concept associated with the first tag is a refinement of the second tag. Add: Our design reuses `iterator_traits<Iter>::iterator_category` to indicate an iterator's traversal capability. To specify capabilities not captured by any old-style iterator category, an iterator designer can use an `iterator_category` type that is convertible to both the the most-derived old iterator category tag which fits, and the appropriate new iterator traversal tag. We do not provide tags for the purposes of dispatching based on the access concepts, in part because we could not find a way to automatically infer the right access tags for old-style iterators. An iterator's writability may be dependent on the assignability of its `value_type` and there's no known way to detect whether an arbitrary type is assignable. Fortunately, the need for dispatching based on access capability is not as great as the need for dispatching based on traversal capability. From the Readable Iterator Requirements table, remove:
> `is_readable<X>::type` | `true_type` |
> From the Writable Iterator Requirements table, remove:
> `is_writable<X>::type` | `true_type` |
> From the Swappable Iterator Requirements table, remove:
> `is_swappable<X>::type` | `true_type` |
From [lib.iterator.synopsis] replace:
    template <class Iterator> struct is_readable;
    template <class Iterator> struct is_writable;
    template <class Iterator> struct is_swappable;
    template <class Iterator> struct traversal_category;

    enum iterator_access { readable_iterator = 1, writable_iterator = 2,
        swappable_iterator = 4, lvalue_iterator = 8 };

    template <unsigned int access_bits, class TraversalTag>
    struct iterator_tag : /* appropriate old category or categories */ {
      static const iterator_access access =
        (iterator_access)access_bits &
          (readable_iterator | writable_iterator | swappable_iterator);
      typedef TraversalTag traversal;
    };
with:
    template <class Iterator> struct is_readable_iterator;
    template <class Iterator> struct iterator_traversal;
> In [lib.iterator.traits], remove: The `iterator_tag` class template is an iterator category tag that encodes the access enum and traversal tag in addition to being compatible with the original iterator tags. The `iterator_tag` class inherits from one of the original iterator tags according to the following pseudo-code.
>     inherit-category(access, traversal-tag) =
>          if ((access & readable_iterator) && (access & lvalue_iterator)) {
>              if (traversal-tag is convertible to random_access_traversal_tag)
>                  return random_access_iterator_tag;
>              else if (traversal-tag is convertible to bidirectional_traversal_tag)
>                  return bidirectional_iterator_tag;
>              else if (traversal-tag is convertible to forward_traversal_tag)
>                  return forward_iterator_tag;
>              else if (traversal-tag is convertible to single_pass_traversal_tag)
>                  if (access-tag is convertible to writable_iterator_tag)
>                      return input_output_iterator_tag;
>                  else
>                      return input_iterator_tag;
>              else
>                  return null_category_tag;
>          } else if ((access & readable_iterator) and (access & writable_iterator)
>                     and traversal-tag is convertible to single_pass_iterator_tag)
>              return input_output_iterator_tag;
>          else if (access & readable_iterator
>                   and traversal-tag is convertible to single_pass_iterator_tag)
>              return input_iterator_tag;
>          else if (access & writable_iterator
>                   and traversal-tag is convertible to incrementable_iterator_tag)
>              return output_iterator_tag;
>          else
>              return null_category_tag;
> If the argument for `TraversalTag` is not convertible to `incrementable_iterator_tag` then the program is ill-formed. Change: The `is_readable`, `is_writable`, `is_swappable`, and `traversal_category` class templates are traits classes. For iterators whose `iterator_traits<Iter>::iterator_category` type is `iterator_tag`, these traits obtain the `access` enum and `traversal` member type from within `iterator_tag`. For iterators whose `iterator_traits<Iter>::iterator_category` type is not `iterator_tag` and instead is a tag convertible to one of the original tags, the appropriate traversal tag and access bits are deduced. The following pseudo-code describes the algorithm.
>     is-readable(Iterator) =
>         cat = iterator_traits<Iterator>::iterator_category;
>         if (cat == iterator_tag<Access,Traversal>)
>             return Access & readable_iterator;
>         else if (cat is convertible to input_iterator_tag)
>             return true;
>         else
>             return false;

>     is-writable(Iterator) =
>         cat = iterator_traits<Iterator>::iterator_category;
>         if (cat == iterator_tag<Access,Traversal>)
>             return Access & writable_iterator;
>         else if (cat is convertible to output_iterator_tag)
>              return true;
>         else if (
>              cat is convertible to forward_iterator_tag
>              and iterator_traits<Iterator>::reference is a
>                  mutable reference)
>             return true;
>         else
>             return false;

>     is-swappable(Iterator) =
>         cat = iterator_traits<Iterator>::iterator_category;
>         if (cat == iterator_tag<Access,Traversal>)
>             return Access & swappable_iterator;
>         else if (cat is convertible to forward_iterator_tag) {
>             if (iterator_traits<Iterator>::reference is a const reference)
>                 return false;
>             else
>                 return true;
>         } else
>             return false;

>     traversal-category(Iterator) =
>         cat = iterator_traits<Iterator>::iterator_category;
>         if (cat == iterator_tag<Access,Traversal>)
>             return Traversal;
>         else if (cat is convertible to random_access_iterator_tag)
>             return random_access_traversal_tag;
>         else if (cat is convertible to bidirectional_iterator_tag)
>             return bidirectional_traversal_tag;
>         else if (cat is convertible to forward_iterator_tag)
>             return forward_traversal_tag;
>         else if (cat is convertible to input_iterator_tag)
>             return single_pass_iterator_tag;
>         else if (cat is convertible to output_iterator_tag)
>             return incrementable_iterator_tag;
>         else
>             return null_category_tag;
> The following specializations provide the access and traversal category tags for pointer types.
>     template <typename T>
>     struct is_readable<const T*> { typedef true_type type; };
>     template <typename T>
>     struct is_writable<const T*> { typedef false_type type; };
>     template <typename T>
>     struct is_swappable<const T*> { typedef false_type type; };

>     template <typename T>
>     struct is_readable<T*> { typedef true_type type; };
>     template <typename T>
>     struct is_writable<T*> { typedef true_type type; };
>     template <typename T>
>     struct is_swappable<T*> { typedef true_type type; };

>     template <typename T>
>     struct traversal_category<T*>
>     {
>       typedef random_access_traversal_tag type;
>     };
> to: The `is_readable_iterator` class template satisfies the UnaryTypeTrait requirements. Given an iterator type `X`, `is_readable_iterator<X>::value` yields `true` if, for an object `a` of type `X`, `*a` is convertible to `iterator_traits<X>::value_type`, and `false` otherwise. `iterator_traversal<X>::type` is
>     category-to-traversal(iterator_traits<X>::iterator_category)
> where category-to-traversal is defined as follows
>     category-to-traversal(C) =
>         if (C is convertible to incrementable_traversal_tag)
>             return C;
>         else if (C is convertible to random_access_iterator_tag)
>             return random_access_traversal_tag;
>         else if (C is convertible to bidirectional_iterator_tag)
>             return bidirectional_traversal_tag;
>         else if (C is convertible to forward_iterator_tag)
>             return forward_traversal_tag;
>         else if (C is convertible to input_iterator_tag)
>             return single_pass_traversal_tag;
>         else if (C is convertible to output_iterator_tag)
>             return incrementable_traversal_tag;
>         else
>             the program is ill-formed
In N1530: In [lib.iterator.helper.synopsis]: Change:
    const unsigned use_default_access = -1;

    struct iterator_core_access { /* implementation detail */ };

    template <
        class Derived
      , class Value
      , unsigned AccessCategory
      , class TraversalCategory
      , class Reference  = Value&
      , class Difference = ptrdiff_t
    >
    class iterator_facade;

    template <
        class Derived
      , class Base
      , class Value      = use_default
      , unsigned Access  = use_default_access
      , class Traversal  = use_default
      , class Reference  = use_default
      , class Difference = use_default
    >
    class iterator_adaptor;

    template <
        class Iterator
      , class Value = use_default
      , unsigned Access  = use_default_access
      , class Traversal  = use_default
      , class Reference = use_default
      , class Difference = use_default
    >
    class indirect_iterator;
To:
    struct iterator_core_access { /* implementation detail */ };

    template <
        class Derived
      , class Value
      , class CategoryOrTraversal
      , class Reference  = Value&
      , class Difference = ptrdiff_t
    >
    class iterator_facade;

    template <
        class Derived
      , class Base
      , class Value      = use_default
      , class CategoryOrTraversal  = use_default
      , class Reference  = use_default
      , class Difference = use_default
    >
    class iterator_adaptor;

    template <
        class Iterator
      , class Value = use_default
      , class CategoryOrTraversal = use_default
      , class Reference = use_default
      , class Difference = use_default
    >
    class indirect_iterator;
Change:
    template <
        class Incrementable
      , unsigned Access  = use_default_access
      , class Traversal  = use_default
      , class Difference = use_default
    >
    class counting_iterator
To:
    template <
        class Incrementable
      , class CategoryOrTraversal  = use_default
      , class Difference = use_default
    >
    class counting_iterator;
In [lib.iterator.facade]: Change:
    template <
        class Derived
      , class Value
      , unsigned AccessCategory
      , class TraversalCategory
      , class Reference  = /* see below */
      , class Difference = ptrdiff_t
    >
    class iterator_facade {
to:
    template <
        class Derived
      , class Value
      , class CategoryOrTraversal
      , class Reference  = Value&
      , class Difference = ptrdiff_t
    >
    class iterator_facade {
Change:
    typedef iterator_tag<AccessCategory, TraversalCategory> iterator_category;
to:
    typedef /* see below */ iterator_category;
Change:
    // Comparison operators
    template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
              class Dr2, class V2, class AC2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1, Dr2, bool>::type // exposition
    operator ==(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
                iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);

    template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
              class Dr2, class V2, class AC2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1, Dr2, bool>::type
    operator !=(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
                iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);

    template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
              class Dr2, class V2, class AC2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1, Dr2, bool>::type
    operator <(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
               iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);

    template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
              class Dr2, class V2, class AC2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1, Dr2, bool>::type
    operator <=(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
                iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);

    template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
              class Dr2, class V2, class AC2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1, Dr2, bool>::type
    operator >(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
               iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);

    template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
              class Dr2, class V2, class AC2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1, Dr2, bool>::type
    operator >=(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
                iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);

    template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
              class Dr2, class V2, class AC2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1, Dr2, bool>::type
    operator >=(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
                iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);

    // Iterator difference
    template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
              class Dr2, class V2, class AC2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1, Dr2, bool>::type
    operator -(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
               iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);

    // Iterator addition
    template <class Derived, class V, class AC, class TC, class R, class D>
    Derived operator+ (iterator_facade<Derived, V, AC, TC, R, D> const&,
                       typename Derived::difference_type n)
to:
    // Comparison operators
    template <class Dr1, class V1, class TC1, class R1, class D1,
              class Dr2, class V2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1,Dr2,bool>::type // exposition
    operator ==(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
                iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);

    template <class Dr1, class V1, class TC1, class R1, class D1,
              class Dr2, class V2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1,Dr2,bool>::type
    operator !=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
                iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);

    template <class Dr1, class V1, class TC1, class R1, class D1,
              class Dr2, class V2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1,Dr2,bool>::type
    operator <(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
               iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);

    template <class Dr1, class V1, class TC1, class R1, class D1,
              class Dr2, class V2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1,Dr2,bool>::type
    operator <=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
                iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);

    template <class Dr1, class V1, class TC1, class R1, class D1,
              class Dr2, class V2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1,Dr2,bool>::type
    operator >(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
               iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);

    template <class Dr1, class V1, class TC1, class R1, class D1,
              class Dr2, class V2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1,Dr2,bool>::type
    operator >=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
                iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);

    // Iterator difference
    template <class Dr1, class V1, class TC1, class R1, class D1,
              class Dr2, class V2, class TC2, class R2, class D2>
    /* see below */
    operator-(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
              iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);

    // Iterator addition
    template <class Dr, class V, class TC, class R, class D>
    Derived operator+ (iterator_facade<Dr,V,TC,R,D> const&,
                       typename Derived::difference_type n);

    template <class Dr, class V, class TC, class R, class D>
    Derived operator+ (typename Derived::difference_type n,
                       iterator_facade<Dr,V,TC,R,D> const&);
After the `iterator_facade` synopsis, add: The `iterator_category` member of `iterator_facade` is
    iterator-category(CategoryOrTraversal, value_type, reference)
where iterator-category is defined as follows:
    iterator-category(C,R,V) :=
       if (C is convertible to std::input_iterator_tag
           || C is convertible to std::output_iterator_tag
       )
           return C

       else if (C is not convertible to incrementable_traversal_tag)
           the program is ill-formed

       else return a type X satisfying the following two constraints:

          1. X is convertible to X1, and not to any more-derived
             type, where X1 is defined by:

               if (R is a reference type
                   && C is convertible to forward_traversal_tag)
               {
                   if (C is convertible to random_access_traversal_tag)
                       X1 = random_access_iterator_tag
                   else if (C is convertible to bidirectional_traversal_tag)
                       X1 = bidirectional_iterator_tag
                   else
                       X1 = forward_iterator_tag
               }
               else
               {
                   if (C is convertible to single_pass_traversal_tag
                       && R is convertible to V)
                       X1 = input_iterator_tag
                   else
                       X1 = C
               }

          2. category-to-traversal(X) is convertible to the most
             derived traversal tag type to which X is also
             convertible, and not to any more-derived traversal tag
             type.

> In [lib.iterator.facade] `iterator_facade` requirements:

> Remove:

> > `AccessCategory` must be an unsigned value which uses no more bits than the greatest value of `iterator_access`.

> In the Iterator Adaptor section, change:

> > Several of the template parameters of `iterator_adaptor` default to `use_default` (or `use_default_access`).

> to:

> > Several of the template parameters of `iterator_adaptor` default to `use_default`.

> In [lib.iterator.special.adaptors]:

> Change:

>     template <
>         class Iterator
>       , class Value = use_default
>       , unsigned Access  = use_default_access
>       , class Traversal  = use_default
>       , class Reference = use_default
>       , class Difference = use_default
>     >
>     class indirect_iterator

> to:

>     template <
>         class Iterator
>       , class Value = use_default
>       , class CategoryOrTraversal = use_default
>       , class Reference = use_default
>       , class Difference = use_default
>     >
>     class indirect_iterator

> Change:

>     template <
>         class Iterator2, class Value2, unsigned Access2, class Traversal2
>       , class Reference2, class Difference2
>     >
>     indirect_iterator(

> to:

>     template <
>         class Iterator2, class Value2, class Category2
>       , class Reference2, class Difference2
>     >
>     indirect_iterator(

> Change:

>     template <
>         class Incrementable
>       , unsigned Access = use_default_access
>       , class Traversal = use_default
>       , class Difference = use_default
>     >
>     class counting_iterator

> to:

>     template <
>         class Incrementable
>       , class CategoryOrTraversal = use_default
>       , class Difference = use_default
>     >
>     class counting_iterator

> Change:

>     typedef iterator_tag<
>           writable_iterator
>         , incrementable_traversal_tag
>     > iterator_category;

> to:

> > typedef std::output_iterator_tag iterator_category;

> In [lib.iterator.adaptor]

> Change:

>     template <
>         class Derived
>       , class Base
>       , class Value      = use_default
>       , unsigned Access  = use_default_access
>       , class Traversal  = use_default
>       , class Reference  = use_default
>       , class Difference = use_default
>     >
>     class iterator_adaptor

> To:

>     template <
>         class Derived
>       , class Base
>       , class Value               = use_default
>       , class CategoryOrTraversal = use_default
>       , class Reference           = use_default
>       , class Difference = use_default
>     >
>     class iterator_adaptor

Rationale: |

1. There are two reasons for removing `is_writable` and `is_swappable`. The first is that we do not know of a way to fix the specification so that it gives the correct answer for all iterators. Second, there was only a weak motivation for having `is_writable` and `is_swappable` there in the first place. The main motivation was simply uniformity: we have tags for the old iterator categories so we should have tags for the new iterator categories. While having tags and the capability to dispatch based on the traversal categories is often used, we see less of a need for dispatching based on writability and swappability, since typically algorithms that need these capabilities have no alternative if they are not provided.
2. We discovered that the `is_readable` trait can be implemented using only the iterator type itself and its `value_type`. Therefore we remove the requirement for `is_readable` from the Readable Iterator concept, and change the definition of `is_readable` so that it works for any iterator type.
3. The purpose of the `iterator_tag` class was to bundle the traversal and access category tags into the `iterator_category` typedef. With `is_writable` and `is_swappable` gone, and `is_readable` no longer in need of special hints, there is no reason for iterators to provide information about the access capabilities of an iterator. Thus there is no need for the `iterator_tag`. The traversal tag can be directly used for the `iterator_category`. If a new iterator is intended to be backward compatible with old iterator concepts, a tag type that is convertible to both one of the new traversal tags and also to an old iterator tag can be created and use for the `iterator_category`.
4. The changes to the specification of `traversal_category` are a direct result of the removal of `iterator_tag`.

## 9.16 is_writable_iterator returns false positives

Submitter: | Dave Abrahams
Status: | New

is_writable_iterator returns false positives for forward iterators whose value_type has a private assignment operator, or whose reference type is not a reference (currently legal).

Proposed Resolution:
| See the resolution to 9.15.

## 9.17 is_swappable_iterator returns false positives

Submitter: | Dave Abrahams
Status: | New

is_swappable_iterator has the same problems as is_writable_iterator. In addition, if we allow users to write their own iter_swap functions it's easy to imagine old-style iterators for which is_swappable returns false negatives.

Proposed Resolution:
| See the resolution to 9.15.

## 9.18 Are is_readable, is_writable, and is_swappable useful?

Submitter: | Dave Abrahams
Status: | New

I am concerned that there is little use for any of is_readable, is_writable, or is_swappable, and that not only do they unduly constrain iterator implementors but they add overhead to iterator_facade and iterator_adaptor in the form of a template parameter which would otherwise be unneeded. Since we can't implement two of them accurately for old-style iterators, I am having a hard time justifying their impact on the rest of the proposal(s).

Proposed Resolution:
| See the resolution to 9.15.

## 9.19 Non-Uniformity of the "lvalue_iterator Bit"

Submitter: | Dave Abrahams
Status: | New

The proposed iterator_tag class template accepts an "access bits" parameter which includes a bit to indicate the iterator's lvalueness (whether its dereference operator returns a reference to its value_type. The relevant part of N1550 says:

> The purpose of the lvalue_iterator part of the iterator_access enum is to communicate to iterator_tagwhether the reference type is an lvalue so that the appropriate old category can be chosen for the base class. The lvalue_iterator bit is not recorded in the iterator_tag::access data member.

The lvalue_iterator bit is not recorded because N1550 aims to improve orthogonality of the iterator concepts, and a new-style iterator's lvalueness is detectable by examining its reference type. This inside/outside difference is awkward and confusing.

Proposed Resolution:
| The iterator_tag class will be removed, so this is no longer an issue. See the resolution to 9.15.

## 9.20 Traversal Concepts and Tags

Submitter: | Dave Abrahams
Status: | New

Howard Hinnant pointed out some inconsistencies with the naming of these tag types:

    incrementable_iterator_tag // ++r, r++
    single_pass_iterator_tag // adds a == b, a != b
    forward_traversal_iterator_tag // adds multi-pass
    bidirectional_traversal_iterator_tag // adds --r, r--
    random_access_traversal_iterator_tag // adds r+n,n+r,etc.

Howard thought that it might be better if all tag names contained the word "traversal". It's not clear that would result in the best possible names, though. For example, incrementable iterators can only make a single pass over their input. What really distinguishes single pass iterators from incrementable iterators is not that they can make a single pass, but that they are equality comparable. Forward traversal iterators really distinguish themselves by introducing multi-pass capability. Without entering a "Parkinson's Bicycle Shed" type of discussion, it might be worth giving the names of these tags (and the associated concepts) some extra attention.

Proposed resolution:
| Change the names of the traversal tags to the following names:
    incrementable_traversal_tag
    single_pass_traversal_tag
    forward_traversal_tag
    bidirectional_traversal_tag
    random_access_traversal_tag
> In [lib.iterator.traversal]: Change:
> `traversal_category<X>::type` | Convertible to `incrementable_iterator_tag` |
> to:
> `iterator_traversal<X>::type` | Convertible to `incrementable_traversal_tag` |
> Change:
> `traversal_category<X>::type` | Convertible to `single_pass_iterator_tag` |
> to:
> `iterator_traversal<X>::type` | Convertible to `single_pass_traversal_tag` |
> Change:
> `traversal_category<X>::type` | Convertible to `forward_traversal_iterator_tag` |
> to:
> `iterator_traversal<X>::type` | Convertible to `forward_traversal_tag` |
> Change:
> `traversal_category<X>::type` | Convertible to `bidirectional_traversal_iterator_tag` |
> to:
> `iterator_traversal<X>::type` | Convertible to `bidirectional_traversal_tag` |
> Change:
> `traversal_category<X>::type` | Convertible to `random_access_traversal_iterator_tag` |  |
> to:
> `iterator_traversal<X>::type` | Convertible to `random_access_traversal_tag` |  |
In [lib.iterator.synopsis], change:
    struct incrementable_iterator_tag { };
    struct single_pass_iterator_tag : incrementable_iterator_tag { };
    struct forward_traversal_tag : single_pass_iterator_tag { };
to:
    struct incrementable_traversal_tag { };
    struct single_pass_traversal_tag : incrementable_traversal_tag { };
    struct forward_traversal_tag : single_pass_traversal_tag { };
Remove:
    struct null_category_tag { };
    struct input_output_iterator_tag : input_iterator_tag, output_iterator_tag {};

## 9.21 iterator_facade Derived template argument underspecified

Submitter: | Pete Becker
Status: | New

The first template argument to iterator_facade is named Derived, and the proposal says:

> The Derived template parameter must be a class derived from iterator_facade.

First, iterator_facade is a template, so cannot be derived from. Rather, the class must be derived from a specialization of iterator_facade. More important, isn't Derived required to be the class that is being defined? That is, if I understand it right, the definition of D here this is not valid:

    class C : public iterator_facade<C, ... > { ... };
    class D : public iterator_facade<C, ...> { ... };

In the definition of D, the Derived argument to iterator_facade is a class derived from a specialization of iterator_facade, so the requirement is met. Shouldn't the requirement be more like "when using iterator_facade to define an iterator class Iter, the class Iter must be derived from a specialization of iterator_facade whose first template argument is Iter." That's a bit awkward, but at the moment I don't see a better way of phrasing it.

Proposed resolution:
| In [lib.iterator.facade] Remove: The `Derived` template parameter must be a class derived from `iterator_facade`. Change: The following table describes the other requirements on the `Derived` parameter. Depending on the resulting iterator's `iterator_category`, a subset of the expressions listed in the table are required to be valid. The operations in the first column must be accessible to member functions of class `iterator_core_access`. to: The following table describes the typical valid expressions on `iterator_facade`'s `Derived` parameter, depending on the iterator concept(s) it will model. The operations in the first column must be made accessible to member functions of class `iterator_core_access`. In addition, `static_cast<Derived*>(iterator_facade*)` shall be well-formed. In [lib.iterator.adaptor] Change: The `iterator_adaptor` is a base class template derived from an instantiation of `iterator_facade`. to: Each specialization of the `iterator_adaptor` class template is derived from a specialization of `iterator_facade`. Change: The `Derived` template parameter must be a derived class of `iterator_adaptor`. To: `static_cast<Derived*>(iterator_adaptor*)` shall be well-formed.

[Note: The proposed resolution to Issue 9.37 contains related changes]

## 9.22 return type of Iterator difference for iterator facade

Submitter: | Pete Becker
Status: | New

The proposal says:

    template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
    class Dr2, class V2, class AC2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1, Dr2, bool>::type
    operator -(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
    iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);

Shouldn't the return type be one of the two iterator types? Which one? The idea is that if one of the iterator types can be converted to the other type, then the subtraction is okay. Seems like the return type should then be the type that was converted to. Is that right?

Proposed resolution:
| See resolution to 9.34.

## 9.23 Iterator_facade: minor wording Issue

Submitter: | Pete Becker
Status: | New

In the table that lists the required (sort of) member functions of iterator types that are based on iterator_facade, the entry for c.equal(y) says:

> true iff c and y refer to the same position. Implements c == y and c != y. The second sentence is inside out. c.equal(y) does not implement either of these operations. It is used to implement them. Same thing in the description of c.distance_to(z).

Proposed resolution:
| remove "implements" descriptions from table. See resolution to 9.34

## 9.24 Use of undefined name in iterator_facade table

Submitter: | Pete Becker
Status: | New

Several of the descriptions use the name X without defining it. This seems to be a carryover from the table immediately above this section, but the text preceding that table says "In the table below, X is the derived iterator type." Looks like the X:: qualifiers aren't really needed; X::reference can simply be reference, since that's defined by the iterator_facade specialization itself.

Proposed resolution:
> > | Remove references to X. In [lib.iterator.facade] operations `operator->() const;`: Change:
> > Returns: | If `X::reference` is a reference type, an object of type `X::pointer` equal to:
> >     &static_cast<Derived const*>(this)->dereference()
> > Otherwise returns an object of unspecified type such that, given an object `a` of type `X`, `a->m` is equivalent to `(w = *a, w.m)` for some temporary object `w` of type `X::value_type`. The type `X::pointer` is `Value*` if `is_writable_iterator<X>::value` is `true`, and `Value const*` otherwise.
> > to:
> > Returns: | If `reference` is a reference type, an object of type `pointer` equal to:
> >     &static_cast<Derived const*>(this)->dereference()
> > Otherwise returns an object of unspecified type such that, `(*static_cast<Derived const*>(this))->m` is equivalent to `(w = **static_cast<Derived const*>(this), w.m)` for some temporary object `w` of type `value_type`.
Further changes are covered by issue 9.26.

## 9.25 Iterator_facade: wrong return type

Submitter: | Pete Becker
Status: | New

Several of the member functions return a Derived object or a Derived&. Their Effects clauses end with:

    return *this;

This should be

    return *static_cast<Derived*>(this);

Proposed resolution:
| In [lib.iterator.facade], in the effects clause of the following operations:
    Derived& operator++()
    Derived& operator--()
    Derived& operator+=(difference_type n)
    Derived& operator-=(difference_type n)
Change: `return *this` to: `return *static_cast<Derived*>(this);`

## 9.26 Iterator_facade: unclear returns clause for operator[]

Submitter: | Pete Becker
Status: | New

The returns clause for `operator[](difference_type n)` const says:

> Returns: an object convertible to X::reference and holding a copy p of a+n such that, for a constant object v of type X::value_type, X::reference(a[n] = v) is equivalent to p = v. This needs to define 'a', but assuming it's supposed to be `*this` (or maybe `*(Derived*)this`), it still isn't clear what this says. Presumably, the idea is that you can index off of an iterator and assign to the result. But why the requirement that it hold a copy of a+n? Granted, that's probably how it's implemented, but it seems over-constrained. And the last phrase seems wrong. p is an iterator; there's no requirement that you can assign a value_type object to it. Should that be `*p = v`? But why the cast in reference(a[n] = v)?

Proposed resolution:
> > | In section operator[]: Change: Writable iterators built with `iterator_facade` implement the semantics required by the preferred resolution to issue 299 and adopted by proposal n1477: the result of `p[n]` is a proxy object containing a copy of `p+n`, and `p[n] = x` is equivalent to `*(p + n) = x`. This approach will work properly for any random-access iterator regardless of the other details of its implementation. A user who knows more about the implementation of her iterator is free to implement an `operator[]` which returns an lvalue in the derived iterator class; it will hide the one supplied by `iterator_facade` from clients of her iterator. to: Writable iterators built with `iterator_facade` implement the semantics required by the preferred resolution to issue 299 and adopted by proposal n1550: the result of `p[n]` is an object convertible to the iterator's `value_type`, and `p[n] = x` is equivalent to `*(p + n) = x` (Note: This result object may be implemented as a proxy containing a copy of `p+n`). This approach will work properly for any random-access iterator regardless of the other details of its implementation. A user who knows more about the implementation of her iterator is free to implement an `operator[]` that returns an lvalue in the derived iterator class; it will hide the one supplied by `iterator_facade` from clients of her iterator. In [lib.iterator.facade] operations: Change:
> > Returns: | an object convertible to `X::reference` and holding a copy p of `a+n` such that, for a constant object `v` of type `X::value_type`, `X::reference(a[n] = v)` is equivalent to `p = v`.
> > to:
> > Returns: | an object convertible to `value_type`. For constant objects `v` of type `value_type`, and `n` of type `difference_type`, `(*this)[n] = v` is equivalent to `*(*this + n) = v`, and `static_cast<value_type const&>((*this)[n])` is equivalent to `static_cast<value_type const&>(*(*this + n))`

## 9.27 Iterator_facade: redundant clause

Submitter: | Pete Becker
Status: | New

`operator-` has both an effects clause and a returns clause. Looks like the returns clause should be removed.

Proposed resolution:
  | Remove the returns clause. In [lib.iterator.facade] operations: Remove:
  Returns: | `static_cast<Derived const*>(this)->advance(-n);`

## 9.28 indirect_iterator: incorrect specification of default constructor

Submitter: | Pete Becker
Status: | New

The default constructor returns "An instance of indirect_iterator with a default constructed base object", but the constructor that takes an Iterator object returns "An instance of indirect_iterator with the iterator_adaptor subobject copy constructed from x." The latter is the correct form, since it does not reach inside the base class for its semantics. So the default constructor shoudl return "An instance of indirect_iterator with a default-constructed iterator_adaptor subobject."

Proposed resolution:
  | Change:
  Returns: | An instance of `indirect_iterator` with a default constructed base object.
  to:
  Returns: | An instance of `indirect_iterator` with a default-constructed `m_iterator`.
Rationale: | Inheritance from iterator_adaptor has been removed, so we instead give the semantics in terms of the (exposition only) member `m_iterator`.

## 9.29 indirect_iterator: unclear specification of template constructor

Submitter: | Pete Becker
Status: | New

The templated constructor that takes an indirect_iterator with a different set of template arguments says that it returns "An instance of indirect_iterator that is a copy of [the argument]". But the type of the argument is different from the type of the object being constructed, and there is no description of what a "copy" means. The Iterator template parameter for the argument must be convertible to the Iterator template parameter for the type being constructed, which suggests that the argument's contained Iterator object should be converted to the target type's Iterator type. Is that what's meant here? (Pete later writes: In fact, this problem is present in all of the specialized adaptors that have a constructor like this: the constructor returns "a copy" of the argument without saying what a copy is.)

Proposed resolution:
  | Change:
  Returns: | An instance of `indirect_iterator` that is a copy of `y`.
  to:
  Returns: | An instance of `indirect_iterator` whose `m_iterator` subobject is constructed from `y.base()`.
Rationale: | Inheritance from iterator_adaptor has been removed, so we instead give the semantics in terms of the member `m_iterator`.

## 9.30 transform_iterator argument irregularity

Submitter: | Pete Becker
Status: | New

The specialized adaptors that take both a Value and a Reference template argument all take them in that order, i.e. Value precedes Reference in the template argument list, with the exception of transform_iterator, where Reference precedes Value. This seems like a possible source of confusion. Is there a reason why this order is preferable?

Proposed resolution:
| NAD
Rationale: | defaults for Value depend on Reference. A sensible Value can almost always be computed from Reference. The first parameter is UnaryFunction, so the argument order is already different from the other adapters.

## 9.31 function_output_iterator overconstrained

Submitter: | Pete Becker
Status: | New

function_output_iterator requirements says: "The UnaryFunction must be Assignable, Copy Constructible, and the expression f(x) must be valid, where f is an object of type UnaryFunction and x is an object of a type accepted by f."

Everything starting with "and," somewhat reworded, is actually a constraint on output_proxy::operator=. All that's needed to create a function_output_iterator object is that the UnaryFunction type be Assignable and CopyConstructible. That's also sufficient to dereference and to increment such an object. It's only when you try to assign through a dereferenced iterator that f(x) has to work, and then only for the particular function object that the iterator holds and for the particular value that is being assigned.

Addition from Jeremy:
  The constructor for `function_output_iterator` is also slightly overconstrained because it requires the `UnaryFunction` to have a default constructor even when the default constructor of `function_output_iterator` is not used.

Proposed resolution:
| Change: `output_proxy operator*();` to: `/* see below */ operator*();` After `function_output_iterator& operator++(int);` add:
    private:
      UnaryFunction m_f;     // exposition only
Change: The `UnaryFunction` must be Assignable, Copy Constructible, and the expression `f(x)` must be valid, where `f` is an object of type `UnaryFunction` and `x` is an object of a type accepted by `f`. The resulting `function_output_iterator` is a model of the Writable and Incrementable Iterator concepts. to: `UnaryFunction` must be Assignable and Copy Constructible. After the requirements section, add:

`function_output_iterator` models

> `function_output_iterator` is a model of the Writable and Incrementable Iterator concepts.

Change:

  Returns: | An instance of `function_output_iterator` with `f` stored as a data member.

to:

  Effects: | Constructs an instance of `function_output_iterator` with `m_f` constructed from `f`.

Change:

  `output_proxy operator*();`

  Returns: | An instance of `output_proxy` constructed with a copy of the unary function `f`.

to:

  `operator*();`

  Returns: | An object `r` of unspecified type such that `r = t` is equivalent to `m_f(t)` for all `t`.

Remove:

  `function_output_iterator::output_proxy` operations

  `output_proxy(UnaryFunction& f);`

  Returns: | An instance of `output_proxy` with `f` stored as a data member.

  `template <class T> output_proxy& operator=(const T& value);`

  Effects: |
      m_f(value);
      return *this;

Change:

    explicit function_output_iterator(const UnaryFunction& f = UnaryFunction());

to:

    explicit function_output_iterator();

    explicit function_output_iterator(const UnaryFunction& f);

## 9.32 Should output_proxy really be a named type?

Submitter: | Pete Becker
Status: | New

This means someone can store an output_proxy object for later use, whatever that means. It also constrains output_proxy to hold a copy of the function object, rather than a pointer to the iterator object. Is all this mechanism really necessary?

Proposed resolution:
| See issue 9.31.

## 9.33 istreambuf_iterator isn't a Readable Iterator

Submitter: | Pete Becker
Status: | New

c++std-lib-12333:

> N1550 requires that for a Readable Iterator a of type X, `*a` returns an object of type `iterator_traits<X>::reference`. `istreambuf_iterator::operator*` returns `charT`, but `istreambuf_iterator::reference` is `charT&`. So am I overlooking something, or is `istreambuf_iterator` not Readable.

Proposed resolution:
> | Remove all constraints on `iterator_traits<X>::reference` in Readable Iterator and Lvalue Iterator. Change Lvalue Iterator to refer to `T&` instead of `iterator_traits<X>::reference`. Change: A class or built-in type `X` models the Readable Iterator concept for the value type `T` if the following expressions are valid and respect the stated semantics. `U` is the type of any specified member of type `T`. to: A class or built-in type `X` models the Readable Iterator concept for value type `T` if, in addition to `X` being Assignable and Copy Constructible, the following expressions are valid and respect the stated semantics. `U` is the type of any specified member of type `T`. From the Input Iterator Requirements table, remove:
> `iterator_traits<X>::reference` | Convertible to `iterator_traits<X>::value_type` |
> Change:
> `*a` | `iterator_traits<X>::reference` | pre: `a` is dereferenceable. If `a == b` then `*a` is equivalent to `*b`
> to:
> `*a` | Convertible to `T` | pre: `a` is dereferenceable. If `a == b` then `*a` is equivalent to `*b`.
> Change: The Lvalue Iterator concept adds the requirement that the `reference` type be a reference to the value type of the iterator. to: The Lvalue Iterator concept adds the requirement that the return type of `operator*` type be a reference to the value type of the iterator. Change:
> Lvalue Iterator Requirements
> Expression | Return Type | Assertion
> --- | --- | ---
> `iterator_traits<X>::reference` | `T&` | `T` is cv `iterator_traits<X>::value_type` where cv is an optional cv-qualification
> to:
> Lvalue Iterator Requirements
> Expression | Return Type | Note/Assertion
> --- | --- | ---
> `*a` | `T&` | `T` is cv `iterator_traits<X>::value_type` where cv is an optional cv-qualification. pre: `a` is dereferenceable. If `a == b` then `*a` is equivalent to `*b`.
At the end of the section reverse_iterator models, add: The type `iterator_traits<Iterator>::reference` must be the type of `*i`, where `i` is an object of type `Iterator`.
Rationale: | Ideally there should be requirements on the reference type, however, since Readable Iterator is suppose to correspond to the current standard iterator requirements (which do not place requirements on the reference type) we will leave them off for now. There is a DR in process with respect to the reference type in the stadard iterator requirements. Once that is resolved we will revisit this issue for Readable Iterator and Lvalue Iterator. We added Assignable to the requirements for Readable Iterator. This is needed to have Readable Iterator coincide with the capabilities of Input Iterator.

## 9.34 iterator_facade free functions unspecified

Submitter: | Pete Becker
Status: | New

c++std-lib-12562:

> The template functions `operator==`, `operator!=`, `operator<`, `operator<=`, `operator>`, `operator>=`, and `operator-` that take two arguments that are specializations of iterator_facade have no specification. The template function operator+ that takes an argument that is a specialization of iterator_facade and an argument of type difference_type has no specification.

Proposed resolution:
| Add the missing specifications.
    template <class Dr, class V, class TC, class R, class D>
    Derived operator+ (iterator_facade<Dr,V,TC,R,D> const&,
                       typename Derived::difference_type n);

    template <class Dr, class V, class TC, class R, class D>
    Derived operator+ (typename Derived::difference_type n,
                       iterator_facade<Dr,V,TC,R,D> const&);
Effects: |
    Derived tmp(static_cast<Derived const*>(this));
    return tmp += n;
    template <class Dr1, class V1, class TC1, class R1, class D1,
              class Dr2, class V2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1,Dr2,bool>::type
    operator ==(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
                iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
Returns: | if `is_convertible<Dr2,Dr1>::value`, then `lhs.equal(rhs)`. Otherwise, `rhs.equal(lhs)`.
    template <class Dr1, class V1, class TC1, class R1, class D1,
              class Dr2, class V2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1,Dr2,bool>::type
    operator !=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
                iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
Returns: | if `is_convertible<Dr2,Dr1>::value`, then `!lhs.equal(rhs)`. Otherwise, `!rhs.equal(lhs)`.
    template <class Dr1, class V1, class TC1, class R1, class D1,
              class Dr2, class V2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1,Dr2,bool>::type
    operator <(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
               iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
Returns: | if `is_convertible<Dr2,Dr1>::value`, then `lhs.distance_to(rhs) < 0`. Otherwise, `rhs.distance_to(lhs) > 0`.
    template <class Dr1, class V1, class TC1, class R1, class D1,
              class Dr2, class V2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1,Dr2,bool>::type
    operator <=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
                iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
Returns: | if `is_convertible<Dr2,Dr1>::value`, then `lhs.distance_to(rhs) <= 0`. Otherwise, `rhs.distance_to(lhs) >= 0`.
    template <class Dr1, class V1, class TC1, class R1, class D1,
              class Dr2, class V2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1,Dr2,bool>::type
    operator >(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
               iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
Returns: | if `is_convertible<Dr2,Dr1>::value`, then `lhs.distance_to(rhs) > 0`. Otherwise, `rhs.distance_to(lhs) < 0`.
    template <class Dr1, class V1, class TC1, class R1, class D1,
              class Dr2, class V2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1,Dr2,bool>::type
    operator >=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
                iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
Returns: | if `is_convertible<Dr2,Dr1>::value`, then `lhs.distance_to(rhs) >= 0`. Otherwise, `rhs.distance_to(lhs) <= 0`.
    template <class Dr1, class V1, class TC1, class R1, class D1,
              class Dr2, class V2, class TC2, class R2, class D2>
    typename enable_if_interoperable<Dr1,Dr2,difference>::type
    operator -(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
               iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
Return Type: | if `is_convertible<Dr2,Dr1>::value`, then `difference` shall be `iterator_traits<Dr1>::difference_type`. Otherwise, `difference` shall be `iterator_traits<Dr2>::difference_type`.
Returns: | if `is_convertible<Dr2,Dr1>::value`, then `-lhs.distance_to(rhs)`. Otherwise, `rhs.distance_to(lhs)`.

## 9.35 iterator_facade: too many equals?

Submitter: | Pete Becker
Status: | New

c++std-lib-12563:

> The table listing the functions required for types derived from iterator_facade has two functions named equal and two named distance_to:

>     c.equal(b)
>     c.equal(y)
>     c.distance_to(b)
>     c.distance_to(z)

> where b and c are const objects of the derived type, y and z are constant objects of certain iterator types that are interoperable with the derived type. Seems like the 'b' versions are redundant: in both cases, the other version will take a 'b'. In fact, iterator_adaptor is specified to use iterator_facade, but does not provide the 'b' versions of these functions.

> Are the 'b' versions needed?

Proposed resolution:
> | Remove the 'b' versions. In `iterator_facade` requirements, remove:
> `c.equal(b)` | convertible to bool | true iff `b` and `c` are equivalent. | Single Pass Iterator
> and remove:
> `c.distance_to(b)` | convertible to X::difference_type | equivalent to `distance(c, b)` | Random Access Traversal Iterator

## 9.36 iterator_facade function requirements

Submitter: | Pete Becker
Status: | New

c++std-lib-12636:

> The table that lists required functions for the derived type X passed to iterator_facade lists, among others:

> for a single pass iterator:

>     c.equal(b)
>     c.equal(y)

> where b and c are const X objects, and y is a const object of a single pass iterator that is interoperable with X. Since X is interoperable with itself, c.equal(b) is redundant. There is a difference in their descriptions, but its meaning isn't clear. The first is "true iff b and c are equivalent", and the second is "true iff c and y refer to the same position." Is there a difference between the undefined term "equivalent" and "refer to the same position"?

> Similarly, for a random access traversal iterator:

>     c.distance_to(b)
>     c.distance_to(z)

> where z is a constant object of a random access traversal iterator that is interoperable with X. Again, X is interoperable with itself, so c.distance_to(b) is redundant. Also, the specification for c.distance_to(z) isn't valid. It's written as "equivalent to distance(c, z)". The template function distance takes two arguments of the same type, so distance(c, z) isn't valid if c and z are different types. Should it be distance(c, (X)z)?

Proposed resolution:
> | Removed the 'b' versions (see 9.35) and added the cast. Change:
> `c.distance_to(z)` | convertible to X::difference_type | equivalent to `distance(c, z)`. Implements `c - z`, `c < z`, `c <= z`, `c > z`, and `c >= c`. | Random Access Traversal Iterator
> to:
> `c.distance_to(z)` | convertible to `F::difference_type` | equivalent to `distance(c, X(z))`. | Random Access Traversal Iterator

# More Issues (not from Matt's list)

## 9.37x Inheritance in iterator_adaptor and other adaptors is an overspecification

Submitter: | Pete Becker
Status: | New

c++std-lib-12696: The paper requires that iterator_adaptor be derived from an appropriate instance of iterator_facade, and that most of the specific forms of adaptors be derived from appropriate instances of iterator_adaptor. That seems like overspecification, and we ought to look at specifying these things in terms of what the various templates provide rather than how they're implemented.

Proposed resolution:
> | Remove the specfication of inheritance, and add explicit specification of all the functionality that was inherited from the specialized iterators. In iterator_adaptor, inheritance is retained, sorry NAD. Also, the Interoperable Iterators concept is added to the new iterator concepts, and this concept is used in the specification of the iterator adaptors. In n1550, after [lib.random.access.traversal.iterators], add: Interoperable Iterators [lib.interoperable.iterators] A class or built-in type `X` that models Single Pass Iterator is interoperable with a class or built-in type `Y` that also models Single Pass Iterator if the following expressions are valid and respect the stated semantics. In the tables below, `x` is an object of type `X`, `y` is an object of type `Y`, `Distance` is `iterator_traits<Y>::difference_type`, and `n` represents a constant object of type `Distance`.
> Expression | Return Type | Assertion/Precondition/Postcondition
> --- | --- | ---
> `y = x` | `Y` | post: `y == x`
> `Y(x)` | `Y` | post: `Y(x) == x`
> `x == y` | convertible to `bool` | `==` is an equivalence relation over its domain.
> `y == x` | convertible to `bool` | `==` is an equivalence relation over its domain.
> `x != y` | convertible to `bool` | `bool(a==b) != bool(a!=b)` over its domain.
> `y != x` | convertible to `bool` | `bool(a==b) != bool(a!=b)` over its domain.
> If `X` and `Y` both model Random Access Traversal Iterator then the following additional requirements must be met.
> Expression | Return Type | Operational Semantics | Assertion/ Precondition
> --- | --- | --- | ---
> `x < y` | convertible to `bool` | `y - x > 0` | `<` is a total ordering relation
> `y < x` | convertible to `bool` | `x - y > 0` | `<` is a total ordering relation
> `x > y` | convertible to `bool` | `y < x` | `>` is a total ordering relation
> `y > x` | convertible to `bool` | `x < y` | `>` is a total ordering relation
> `x >= y` | convertible to `bool` | `!(x < y)` |
> `y >= x` | convertible to `bool` | `!(y < x)` |
> `x <= y` | convertible to `bool` | `!(x > y)` |
> `y <= x` | convertible to `bool` | `!(y > x)` |
> `y - x` | `Distance` | `distance(Y(x),y)` | pre: there exists a value `n` of `Distance` such that `x + n == y`. `y == x + (y - x)`.
> `x - y` | `Distance` | `distance(y,Y(x))` | pre: there exists a value `n` of `Distance` such that `y + n == x`. `x == y + (x - y)`.
> In N1530: In [lib.iterator.adaptor] Change:
>     class iterator_adaptor
>       : public iterator_facade<Derived, /* see details ...*/>
> To:
>     class iterator_adaptor
>       : public iterator_facade<Derived, *V'*, *C'*, *R'*, *D'*> // see details
Change the text from: The `Base` type must implement the expressions involving `m_iterator` in the specifications... until the end of the iterator_adaptor requirements section, to: The `Base` argument shall be Assignable and Copy Constructible. Add:

`iterator_adaptor` base class parameters

> The V', C', R', and D' parameters of the `iterator_facade` used as a base class in the summary of `iterator_adaptor` above are defined as follows:

>     V' = if (Value is use_default)
>               return iterator_traits<Base>::value_type
>           else
>               return Value

>     C' = if (CategoryOrTraversal is use_default)
>               return iterator_traversal<Base>::type
>           else
>               return CategoryOrTraversal

>     R' = if (Reference is use_default)
>               if (Value is use_default)
>                   return iterator_traits<Base>::reference
>               else
>                   return Value&
>           else
>               return Reference

>     D' = if (Difference is use_default)
>               return iterator_traits<Base>::difference_type
>           else
>               return Difference

In [lib.iterator.special.adaptors]

Change:

    class indirect_iterator
      : public iterator_adaptor</* see discussion */>
    {
        friend class iterator_core_access;

to:

    class indirect_iterator
    {
     public:
        typedef /* see below */ value_type;
        typedef /* see below */ reference;
        typedef /* see below */ pointer;
        typedef /* see below */ difference_type;
        typedef /* see below */ iterator_category;

Change:

    private: // as-if specification
        typename indirect_iterator::reference dereference() const
        {
            return **this->base();
        }

to:

        Iterator const& base() const;
        reference operator*() const;
        indirect_iterator& operator++();
        indirect_iterator& operator--();
    private:
       Iterator m_iterator; // exposition

After the synopsis add:

> The member types of `indirect_iterator` are defined according to the following pseudo-code, where `V` is `iterator_traits<Iterator>::value_type`

>     if (Value is use_default) then
>         typedef remove_const<pointee<V>::type>::type value_type;
>     else
>         typedef remove_const<Value>::type value_type;

>     if (Reference is use_default) then
>         if (Value is use_default) then
>             typedef indirect_reference<V>::type reference;
>         else
>             typedef Value& reference;
>     else
>         typedef Reference reference;

>     if (Value is use_default) then
>         typedef pointee<V>::type* pointer;
>     else
>         typedef Value* pointer;

>     if (Difference is use_default)
>         typedef iterator_traits<Iterator>::difference_type difference_type;
>     else
>         typedef Difference difference_type;

>     if (CategoryOrTraversal is use_default)
>         typedef iterator-category(
>             iterator_traversal<Iterator>::type,``reference``,``value_type``
>         ) iterator_category;
>     else
>         typedef iterator-category(
>             CategoryOrTraversal,``reference``,``value_type``
>         ) iterator_category;

[Note: See resolution to 9.44y for a description of `pointee` and `indirect_reference`]

After the requirements section, add:

`indirect_iterator` models

> In addition to the concepts indicated by `iterator_category` and by `iterator_traversal<indirect_iterator>::type`, a specialization of `indirect_iterator` models the following concepts, Where `v` is an object of `iterator_traits<Iterator>::value_type`:

> > - Readable Iterator if `reference(*v)` is convertible to `value_type`.
> > - Writable Iterator if `reference(*v) = t` is a valid expression (where `t` is an object of type `indirect_iterator::value_type`)
> > - Lvalue Iterator if `reference` is a reference type.

> `indirect_iterator<X,V1,C1,R1,D1>` is interoperable with `indirect_iterator<Y,V2,C2,R2,D2>` if and only if `X` is interoperable with `Y`.

Before `indirect_iterator();` add:

> In addition to the operations required by the concepts described above, specializations of `indirect_iterator` provide the following operations.

Change:

  Returns: | An instance of `indirect_iterator` with the `iterator_adaptor` subobject copy constructed from `x`.

to:

  Returns: | An instance of `indirect_iterator` with `m_iterator` copy constructed from `x`.

At the end of the indirect_iterator operations add:

> `Iterator const& base() const;`

> Returns: | `m_iterator`

> `reference operator*() const;`

> Returns: | `**m_iterator`

> `indirect_iterator& operator++();`

> Effects: | `++m_iterator`
> Returns: | `*this`

> `indirect_iterator& operator--();`

> Effects: | `--m_iterator`
> Returns: | `*this`

Change:

    template <class Iterator>
    class reverse_iterator :
      public iterator_adaptor< reverse_iterator<Iterator>, Iterator >
    {
      friend class iterator_core_access;

to:

    template <class Iterator>
    class reverse_iterator
    {
    public:
      typedef iterator_traits<Iterator>::value_type value_type;
      typedef iterator_traits<Iterator>::reference reference;
      typedef iterator_traits<Iterator>::pointer pointer;
      typedef iterator_traits<Iterator>::difference_type difference_type;
      typedef /* see below */ iterator_category;

Change:

    private: // as-if specification
      typename reverse_iterator::reference dereference() const { return *prior(this->base()); }

      void increment() { --this->base_reference(); }
      void decrement() { ++this->base_reference(); }

      void advance(typename reverse_iterator::difference_type n)
      {
          this->base_reference() += -n;
      }

      template <class OtherIterator>
      typename reverse_iterator::difference_type
      distance_to(reverse_iterator<OtherIterator> const& y) const
      {
          return this->base_reference() - y.base();
      }

to:

      Iterator const& base() const;
      reference operator*() const;
      reverse_iterator& operator++();
      reverse_iterator& operator--();
    private:
      Iterator m_iterator; // exposition

After the synopsis for `reverse_iterator`, add:
  If `Iterator` models Random Access Traversal Iterator and Readable Lvalue Iterator, then `iterator_category` is convertible to `random_access_iterator_tag`. Otherwise, if `Iterator` models Bidirectional Traversal Iterator and Readable Lvalue Iterator, then `iterator_category` is convertible to `bidirectional_iterator_tag`. Otherwise, `iterator_category` is convertible to `input_iterator_tag`.
Change:

  reverse_iterator requirements

  The base `Iterator` must be a model of Bidirectional Traversal Iterator. The resulting `reverse_iterator` will be a model of the most refined standard traversal and access concepts that are modeled by `Iterator`.

to:

  reverse_iterator requirements

  `Iterator` must be a model of Bidirectional Traversal Iterator.

`reverse_iterator` models

> A specialization of `reverse_iterator` models the same iterator traversal and iterator access concepts modeled by its `Iterator` argument. In addition, it may model old iterator concepts specified in the following table:

> If `I` models | then `reverse_iterator<I>` models
> --- | ---
> Readable Lvalue Iterator, Bidirectional Traversal Iterator | Bidirectional Iterator
> Writable Lvalue Iterator, Bidirectional Traversal Iterator | Mutable Bidirectional Iterator
> Readable Lvalue Iterator, Random Access Traversal Iterator | Random Access Iterator
> Writable Lvalue Iterator, Random Access Traversal Iterator | Mutable Random Access Iterator

> `reverse_iterator<X>` is interoperable with `reverse_iterator<Y>` if and only if `X` is interoperable with `Y`.

Change:

  Returns: | An instance of `reverse_iterator` with a default constructed base object.

to:

  Effects: | Constructs an instance of `reverse_iterator` with `m_iterator` default constructed.

Change:

  Effects: | Constructs an instance of `reverse_iterator` with a base object copy constructed from `x`.

to:

  Effects: | Constructs an instance of `reverse_iterator` with a `m_iterator` constructed from `x`.

Change:

  Returns: | An instance of `reverse_iterator` that is a copy of `r`.

to:

  Effects: | Constructs instance of `reverse_iterator` whose `m_iterator` subobject is constructed from `y.base()`.

At the end of the operations for `reverse_iterator`, add:

  `Iterator const& base() const;`

  Returns: | `m_iterator`

  `reference operator*() const;`

  Effects: |

      Iterator tmp = m_iterator;
      return *--tmp;

  `reverse_iterator& operator++();`

  Effects: | `--m_iterator`
  Returns: | `*this`

  `reverse_iterator& operator--();`

  Effects: | `++m_iterator`
  Returns: | `*this`

Change:

    class transform_iterator
      : public iterator_adaptor</* see discussion */>
    {
      friend class iterator_core_access;

to:

    class transform_iterator
    {
    public:
      typedef /* see below */ value_type;
      typedef /* see below */ reference;
      typedef /* see below */ pointer;
      typedef iterator_traits<Iterator>::difference_type difference_type;
      typedef /* see below */ iterator_category;

After `UnaryFunction functor() const;` add:

    Iterator const& base() const;
    reference operator*() const;
    transform_iterator& operator++();
    transform_iterator& operator--();

Change:

    private:
      typename transform_iterator::value_type dereference() const;
      UnaryFunction m_f;
    };

to:

    private:
      Iterator m_iterator; // exposition only
      UnaryFunction m_f;   // exposition only
    };

After the synopsis, add:
  If `Iterator` models Readable Lvalue Iterator and if `Iterator` models Random Access Traversal Iterator, then `iterator_category` is convertible to `random_access_iterator_tag`. Otherwise, if `Iterator` models Bidirectional Traversal Iterator, then `iterator_category` is convertible to `bidirectional_iterator_tag`. Otherwise `iterator_category` is convertible to `forward_iterator_tag`. If `Iterator` does not model Readable Lvalue Iterator then `iterator_category` is convertible to `input_iterator_tag`.
In the requirements section, change:

  The type `Iterator` must at least model Readable Iterator. The resulting `transform_iterator` models the most refined of the following that is also modeled by `Iterator`.

  > - Writable Lvalue Iterator if `result_of<UnaryFunction(iterator_traits<Iterator>::reference)>::type` is a non-const reference.
  > - Readable Lvalue Iterator if `result_of<UnaryFunction(iterator_traits<Iterator>::reference)>::type` is a const reference.
  > - Readable Iterator otherwise.

  The `transform_iterator` models the most refined standard traversal concept that is modeled by `Iterator`.

  The `reference` type of `transform_iterator` is `result_of<UnaryFunction(iterator_traits<Iterator>::reference)>::type`. The `value_type` is `remove_cv<remove_reference<reference> >::type`.

to:
  The argument `Iterator` shall model Readable Iterator.

After the requirements section, add:

`transform_iterator` models

> The resulting `transform_iterator` models the most refined of the following options that is also modeled by `Iterator`.

> > - Writable Lvalue Iterator if `transform_iterator::reference` is a non-const reference.
> > - Readable Lvalue Iterator if `transform_iterator::reference` is a const reference.
> > - Readable Iterator otherwise.

> The `transform_iterator` models the most refined standard traversal concept that is modeled by the `Iterator` argument.

> If `transform_iterator` is a model of Readable Lvalue Iterator then it models the following original iterator concepts depending on what the `Iterator` argument models.

> If `Iterator` models | then `transform_iterator` models
> --- | ---
> Single Pass Iterator | Input Iterator
> Forward Traversal Iterator | Forward Iterator
> Bidirectional Traversal Iterator | Bidirectional Iterator
> Random Access Traversal Iterator | Random Access Iterator

> If `transform_iterator` models Writable Lvalue Iterator then it is a mutable iterator (as defined in the old iterator requirements).

> `transform_iterator<F1, X, R1, V1>` is interoperable with `transform_iterator<F2, Y, R2, V2>` if and only if `X` is interoperable with `Y`.

Remove the private operations section heading and remove:

    ``typename transform_iterator::value_type dereference() const;``

    :Returns: ``m_f(transform_iterator::dereference());``

After the entry for `functor()`, add:

    ``Iterator const& base() const;``

    :Returns: ``m_iterator``

    ``reference operator*() const;``

    :Returns: ``m_f(*m_iterator)``

    ``transform_iterator& operator++();``

    :Effects: ``++m_iterator``
    :Returns: ``*this``

    ``transform_iterator& operator--();``

    :Effects: ``--m_iterator``
    :Returns: ``*this``

Change:

    template <class Predicate, class Iterator>
    class filter_iterator
       : public iterator_adaptor<
             filter_iterator<Predicate, Iterator>, Iterator
           , use_default
           , /* see details */
         >
    {
     public:

to:

    template <class Predicate, class Iterator>
    class filter_iterator
    {
     public:
       typedef iterator_traits<Iterator>::value_type value_type;
       typedef iterator_traits<Iterator>::reference reference;
       typedef iterator_traits<Iterator>::pointer pointer;
       typedef iterator_traits<Iterator>::difference_type difference_type;
       typedef /* see below */ iterator_category;

Change:

    private: // as-if specification
       void increment()
       {
           ++(this->base_reference());
           satisfy_predicate();
       }

       void satisfy_predicate()
       {
           while (this->base() != this->m_end && !this->m_predicate(*this->base()))
               ++(this->base_reference());
       }

       Predicate m_predicate;
       Iterator m_end;

to:

        Iterator const& base() const;
        reference operator*() const;
        filter_iterator& operator++();
    private:
        Predicate m_pred; // exposition only
        Iterator m_iter;  // exposition only
        Iterator m_end;   // exposition only

Change:
  The base `Iterator` parameter must be a model of Readable Iterator and Single Pass Iterator. The resulting `filter_iterator` will be a model of Forward Traversal Iterator if `Iterator` is, otherwise the `filter_iterator` will be a model of Single Pass Iterator. The access category of the `filter_iterator` will be the same as the access category of `Iterator`.
to:
  The `Iterator` argument shall meet the requirements of Readable Iterator and Single Pass Iterator or it shall meet the requirements of Input Iterator.

After the requirements section, add:

`filter_iterator` models

> The concepts that `filter_iterator` models are dependent on which concepts the `Iterator` argument models, as specified in the following tables.

> If `Iterator` models | then `filter_iterator` models
> --- | ---
> Single Pass Iterator | Single Pass Iterator
> Forward Traversal Iterator | Forward Traversal Iterator

> If `Iterator` models | then `filter_iterator` models
> --- | ---
> Readable Iterator | Readable Iterator
> Writable Iterator | Writable Iterator
> Lvalue Iterator | Lvalue Iterator

> If `Iterator` models | then `filter_iterator` models
> --- | ---
> Readable Iterator, Single Pass Iterator | Input Iterator
> Readable Lvalue Iterator, Forward Traversal Iterator | Forward Iterator
> Writable Lvalue Iterator, Forward Traversal Iterator | Mutable Forward Iterator

> `filter_iterator<P1, X>` is interoperable with `filter_iterator<P2, Y>` if and only if `X` is interoperable with `Y`.

Change:

  Returns: | a `filter_iterator` whose predicate is a default constructed `Predicate` and whose `end` is a default constructed `Iterator`.

to:

  Effects: | Constructs a `filter_iterator` whose``m_pred``, `m_iter`, and `m_end` members are a default constructed.

Change:

  Returns: | A `filter_iterator` at position `x` that filters according to predicate `f` and that will not increment past `end`.

to:

  Effects: | Constructs a `filter_iterator` where `m_iter` is either the first position in the range `[x,end)` such that `f(*m_iter) == true` or else``m_iter == end``. The member `m_pred` is constructed from `f` and `m_end` from `end`.

Change:

  Returns: | A `filter_iterator` at position `x` that filters according to a default constructed `Predicate` and that will not increment past `end`.

to:

  Effects: | Constructs a `filter_iterator` where `m_iter` is either the first position in the range `[x,end)` such that `m_pred(*m_iter) == true` or else``m_iter == end``. The member `m_pred` is default constructed.

Change:

  Returns: | A copy of iterator `t`.

to:

  Effects: | Constructs a filter iterator whose members are copied from `t`.

Change:

  Returns: | A copy of the predicate object used to construct `*this`.

to:

  Returns: | `m_pred`

Change:

  Returns: | The object `end` used to construct `*this`.

to:

  Returns: | `m_end`

At the end of the operations section, add:

> `reference operator*() const;`

> Returns: | `*m_iter`

> `filter_iterator& operator++();`

> Effects: | Increments `m_iter` and then continues to increment `m_iter` until either `m_iter == m_end` or `m_pred(*m_iter) == true`.
> Returns: | `*this`

Change:

    class counting_iterator
      : public iterator_adaptor<
            counting_iterator<Incrementable, Access, Traversal, Difference>
          , Incrementable
          , Incrementable
          , Access
          , /* see details for traversal category */
          , Incrementable const&
          , Incrementable const*
          , /* distance = Difference or a signed integral type */>
    {
        friend class iterator_core_access;
     public:

to:

    class counting_iterator
    {
     public:
        typedef Incrementable value_type;
        typedef const Incrementable& reference;
        typedef const Incrementable* pointer;
        typedef /* see below */ difference_type;
        typedef /* see below */ iterator_category;

Change:

    private:
        typename counting_iterator::reference dereference() const
        {
            return this->base_reference();
        }

to:

        Incrementable const& base() const;
        reference operator*() const;
        counting_iterator& operator++();
        counting_iterator& operator--();
    private:
        Incrementable m_inc; // exposition

After the synopsis, add:

> If the `Difference` argument is `use_default` then `difference_type` is an unspecified signed integral type. Otherwise `difference_type` is `Difference`.

> `iterator_category` is determined according to the following algorithm:

>     if (CategoryOrTraversal is not use_default)
>         return CategoryOrTraversal
>     else if (numeric_limits<Incrementable>::is_specialized)
>         return iterator-category(
>             random_access_traversal_tag, Incrementable, const Incrementable&)
>     else
>         return iterator-category(
>              iterator_traversal<Incrementable>::type,
>              Incrementable, const Incrementable&)

Change:
  [Note: implementers are encouraged to provide an implementation of
    `distance_to` and a `difference_type` that avoids overflows in the cases when the `Incrementable` type is a numeric type.]
to:
  [Note: implementers are encouraged to provide an implementation of
    `operator-` and a `difference_type` that avoid overflows in the cases where `std::numeric_limits<Incrementable>::is_specialized` is true.]
Change:

  The `Incrementable` type must be Default Constructible, Copy Constructible, and Assignable. The default distance is an implementation defined signed integegral type.

  The resulting `counting_iterator` models Readable Lvalue Iterator.

to:
  The `Incrementable` argument shall be Copy Constructible and Assignable.
Change:
  Furthermore, if you wish to create a counting iterator that is a Forward Traversal Iterator, then the following expressions must be valid:
to:
  If `iterator_category` is convertible to `forward_iterator_tag` or `forward_traversal_tag`, the following must be well-formed:
Change:
  If you wish to create a counting iterator that is a Bidirectional Traversal Iterator, then pre-decrement is also required:
to:
  If `iterator_category` is convertible to `bidirectional_iterator_tag` or `bidirectional_traversal_tag`, the following expression must also be well-formed:
Change:
  If you wish to create a counting iterator that is a Random Access Traversal Iterator, then these additional expressions are also required:
to:
  If `iterator_category` is convertible to `random_access_iterator_tag` or `random_access_traversal_tag`, the following must must also be valid:

After the requirements section, add:

`counting_iterator` models

> Specializations of `counting_iterator` model Readable Lvalue Iterator. In addition, they model the concepts corresponding to the iterator tags to which their `iterator_category` is convertible. Also, if `CategoryOrTraversal` is not `use_default` then `counting_iterator` models the concept corresponding to the iterator tag `CategoryOrTraversal`. Otherwise, if `numeric_limits<Incrementable>::is_specialized`, then `counting_iterator` models Random Access Traversal Iterator. Otherwise, `counting_iterator` models the same iterator traversal concepts modeled by `Incrementable`.

> `counting_iterator<X,C1,D1>` is interoperable with `counting_iterator<Y,C2,D2>` if and only if `X` is interoperable with `Y`.

At the begining of the operations section, add:

> In addition to the operations required by the concepts modeled by `counting_iterator`, `counting_iterator` provides the following operations.

Change:

  Returns: | A default constructed instance of `counting_iterator`.

to:

  Requires: | `Incrementable` is Default Constructible.
  Effects: | Default construct the member `m_inc`.

Change:

  Returns: | An instance of `counting_iterator` that is a copy of `rhs`.

to:

  Effects: | Construct member `m_inc` from `rhs.m_inc`.

Change:

  Returns: | An instance of `counting_iterator` with its base object copy constructed from `x`.

to:

  Effects: | Construct member `m_inc` from `x`.

At the end of the operations section, add:

> `reference operator*() const;`

> Returns: | `m_inc`

> `counting_iterator& operator++();`

> Effects: | `++m_inc`
> Returns: | `*this`

> `counting_iterator& operator--();`

> Effects: | `--m_inc`
> Returns: | `*this`

> `Incrementable const& base() const;`

> Returns: | `m_inc`

## 9.38x Problem with specification of a->m in Readable Iterator

Submitter: | Howard Hinnant
Status: | New

c++std-lib-12585:

Readable Iterator Requirements says:

> `a->m` | `U&` | pre: `(*a).m` is well-defined. Equivalent to `(*a).m`

Do we mean to outlaw iterators with proxy references from meeting the readable requirements?

Would it be better for the requirements to read `static_cast<T>(*a).m` instead of `(*a).m` ?

Proposed resolution:
| NAD.
Rationale: | We think you're misreading "pre:". If `(*a).m` is not well defined, then the iterator is not required to provide `a->m`. So a proxy iterator is not required to provide `a->m`. As an aside, it is possible for proxy iterators to support `->`, so changing the requirements to read `static_cast<T>(*a).m` is interesting. However, such a change to Readable Iterator would mean that it no longer corresponds to the input iterator requirements. So old iterators would not necessarily conform to new iterator requirements.

## 9.39x counting_iterator Traversal argument unspecified

Submitter: | Pete Becker

c++std-lib-12635:

counting_iterator takes an argument for its Traversal type, with a default value of use_default. It is derived from an instance of iterator_adaptor, where the argument passed for the Traversal type is described as "`/* see details for traversal category */`". The details for counting_iterator describe constraints on the Incrementable type imposed by various traversal categories. There is no description of what the argument to iterator_adaptor should be.

Proposed resolution:
| We no longer inherit from iterator_adaptor. So instead, we specify the iterator_category in terms of the Traversal type (which is now called CategoryOrTraversal). Also the requirements and models section was reorganized to match these changes and to make more sense.

## 9.40x indirect_iterator requirements muddled

Submitter: | Pete Becker

c++std-lib-12640:

> > The value_type of the Iterator template parameter should itself be dereferenceable. The return type of the `operator*` for the value_type must be the same type as the Reference template parameter.

> I'd say this a bit differently, to emphasize what's required: iterator_traits<Iterator>::value_type must be dereferenceable. The Reference template parameter must be the same type as `*iterator_traits<Iterator>::value_type()`.

> > The Value template parameter will be the value_type for the indirect_iterator, unless Value is const. If Value is const X, then value_type will be non- const X.

> Also non-volatile, right? In other words, if Value isn't use_default, it just gets passed as the Value argument for iterator_adaptor.

> > The default for Value is:

> >     iterator_traits< iterator_traits<Iterator>::value_type >::value_type

> > If the default is used for Value, then there must be a valid specialization of iterator_traits for the value type of the base iterator.

> The earlier requirement is that `iterator_traits<Iterator>::value_type` must be dereferenceable. Now it's being treated as an iterator. Is this just a pun, or is `iterator_traits<Iterator>::value_type` required to be some form of iterator? If it's the former we need to find a different way to say it. If it's the latter we need to say so.

Proposed resolution:
> | Change: The `value_type` of the `Iterator` template parameter should itself be dereferenceable. The return type of the `operator*` for the `value_type` must be the same type as the `Reference` template parameter. The `Value` template parameter will be the `value_type` for the `indirect_iterator`, unless `Value` is const. If `Value` is `const X`, then `value_type` will be non- `const X`. The default for `Value` is:
>     iterator_traits< iterator_traits<Iterator>::value_type >::value_type
If the default is used for `Value`, then there must be a valid specialization of `iterator_traits` for the value type of the base iterator. The `Reference` parameter will be the `reference` type of the `indirect_iterator`. The default is `Value&`. The `Access` and `Traversal` parameters are passed unchanged to the corresponding parameters of the `iterator_adaptor` base class, and the `Iterator` parameter is passed unchanged as the `Base` parameter to the `iterator_adaptor` base class. to: The expression `*v`, where `v` is an object of `iterator_traits<Iterator>::value_type`, shall be valid expression and convertible to `reference`. `Iterator` shall model the traversal concept indicated by `iterator_category`. `Value`, `Reference`, and `Difference` shall be chosen so that `value_type`, `reference`, and `difference_type` meet the requirements indicated by `iterator_category`. [Note: there are further requirements on the `iterator_traits<Iterator>::value_type` if the `Value` parameter is not `use_default`, as implied by the algorithm for deducing the default for the `value_type` member.]
Rationale: | Not included above is the specification of the `value_type`, `reference`, etc., members, which is handled by the changes in 9.37x.

## 9.41x Problem with transform_iterator requirements

Submitter: | Pete Becker

c++std-lib-12641:

> The reference type of transform_iterator is `result_of< UnaryFunction(iterator_traits<Iterator>::reference) >::type`. The `value_type` is `remove_cv<remove_reference<reference> >::type`.

These are the defaults, right? If the user supplies their own types that's what gets passed to iterator_adaptor. And again, the specification should be in terms of the specialization of iterator_adaptor, and not in terms of the result:

Reference argument to iterator_adaptor:

    if (Reference != use_default)
        Reference
    else
        result_of<
            UnaryFunction(iterator_traits<Iterator>::reference)
        >::type

Value argument to iterator_adaptor:

    if (Value != use_default)
        Value
    else if (Reference != use_default)
        remove_reference<reference>::type
    else
        remove_reference<
            result_of<
                UnaryFunction(iterator_traits<Iterator>::reference)
            >::type
        >::type

There's probably a better way to specify that last alternative, but I've been at this too long, and it's all turning into a maze of twisty passages, all alike.

Proposed resolution:
| Replace: The reference type of transform_iterator is `result_of< UnaryFunction(iterator_traits<Iterator>::reference) >::type`. The `value_type` is `remove_cv<remove_reference<reference> >::type`. with: If `Reference` is `use_default` then the `reference` member of `transform_iterator` is `result_of< UnaryFunction(iterator_traits<Iterator>::reference) >::type`. Otherwise, `reference` is `Reference`. If `Value` is `use_default` then the `value_type` member is `remove_cv<remove_reference<reference> >::type`. Otherwise, `value_type` is `Value`.

## 9.42x filter_iterator details unspecified

Submitter: | Pete Becker

c++std-lib-12642:

The paper says:

    template<class Predicate, class Iterator>
    class filter_iterator
         : public iterator_adaptor<
             filter_iterator<Predicate, Iterator>,
             Iterator,
             use_default,
             /* see details */ >

That comment covers the Access, Traversal, Reference, and Difference arguments. The only specification for any of these in the details is:

> The access category of the filter_iterator will be the same as the access category of Iterator.

Needs more.

Proposed resolution:
| Add to the synopsis:
    typedef iterator_traits<Iterator>::value_type value_type;
    typedef iterator_traits<Iterator>::reference reference;
    typedef iterator_traits<Iterator>::pointer pointer;
    typedef iterator_traits<Iterator>::difference_type difference_type;
    typedef /* see below */ iterator_category;
and add just after the synopsis: If `Iterator` models Readable Lvalue Iterator and Forward Traversal Iterator then `iterator_category` is convertible to `std::forward_iterator_tag`. Otherwise `iterator_category` is convertible to `std::input_iterator_tag`.

## 9.43x transform_iterator interoperability too restrictive

Submitter: | Jeremy Siek

We do not need to require that the function objects have the same type, just that they be convertible.

Proposed resolution:
| Change:
    template<class OtherIterator, class R2, class V2>
    transform_iterator(
          transform_iterator<UnaryFunction, OtherIterator, R2, V2> const& t
        , typename enable_if_convertible<OtherIterator, Iterator>::type* = 0 // exposition
    );
to:
    template<class F2, class I2, class R2, class V2>
    transform_iterator(
          transform_iterator<F2, I2, R2, V2> const& t
        , typename enable_if_convertible<I2, Iterator>::type* = 0      // exposition only
        , typename enable_if_convertible<F2, UnaryFunction>::type* = 0 // exposition only
    );

## 9.44y `indirect_iterator` and smart pointers

Submitter: | Dave Abrahams

`indirect_iterator` should be able to iterate over containers of smart pointers, but the mechanism that allows it was left out of the specification, even though it's present in the Boost specification

Proposed resolution:
| Add `pointee` and `indirect_reference` to deal with this capability. In [lib.iterator.helper.synopsis], add:
    template <class Dereferenceable>
    struct pointee;

    template <class Dereferenceable>
    struct indirect_reference;
After `indirect_iterator`'s abstract, add:

Class template `pointee`

    template <class Dereferenceable>
    struct pointee
    {
        typedef /* see below */ type;
    };

Requires: | For an object `x` of type `Dereferenceable`, `*x` is well-formed. If `++x` is ill-formed it shall neither be ambiguous nor shall it violate access control, and `Dereferenceable::element_type` shall be an accessible type. Otherwise `iterator_traits<Dereferenceable>::value_type` shall be well formed. [Note: These requirements need not apply to explicit or partial specializations of `pointee`]

`type` is determined according to the following algorithm, where `x` is an object of type `Dereferenceable`:

    if ( ++x is ill-formed )
    {
        return ``Dereferenceable::element_type``
    }
    else if (``*x`` is a mutable reference to
             std::iterator_traits<Dereferenceable>::value_type)
    {
        return iterator_traits<Dereferenceable>::value_type
    }
    else
    {
        return iterator_traits<Dereferenceable>::value_type const
    }

Class template `indirect_reference`

    template <class Dereferenceable>
    struct indirect_reference
    {
        typedef /* see below */ type;
    };

Requires: | For an object `x` of type `Dereferenceable`, `*x` is well-formed. If `++x` is ill-formed it shall neither be ambiguous nor shall it violate access control, and `pointee<Dereferenceable>::type&` shall be well-formed. Otherwise `iterator_traits<Dereferenceable>::reference` shall be well formed. [Note: These requirements need not apply to explicit or partial specializations of `indirect_reference`]

`type` is determined according to the following algorithm, where `x` is an object of type `Dereferenceable`:

    if ( ++x is ill-formed )
        return ``pointee<Dereferenceable>::type&``
    else
        std::iterator_traits<Dereferenceable>::reference

See proposed resolution to Issue 9.37x for more changes related to this issue.

## 9.45y N1530: Typos and editorial changes in proposal text (not standardese)

Submitter: | Dave Abrahams

1. "because specification helps to highlight that the `Reference` template parameter may not always be identical to the iterator's `reference` type, and will keep users making mistakes based on that assumption."

   Proposed resolution:
   | add "from" before "making"

2. mention of obsolete projection_iterator

> Proposed Resolution:
> | From n1530, in the Specialized Adaptors section, remove: `projection_iterator`, which is similar to `transform_iterator` except that when dereferenced it returns a reference instead of a value.
> Rationale: | This iterator was in the original boost library, but the new iterator concepts allowed this iterator to be folded into `transform_iterator`.

## 9.46y N1530: `base()` return-by-value is costly

Submitter: | Dave Abrahams

We've had some real-life reports that iterators that use `iterator_adaptor`'s `base()` function can be inefficient when the `Base` iterator is expensive to copy. Iterators, of all things, should be efficient.

Proposed resolution:
| In [lib.iterator.adaptor] Change:
    Base base() const;
to:
    Base const& base() const;
twice (once in the synopsis and once in the public operations section).

## 9.47x Forgot default constructible in Forward Traversal Iterator

Submitter: | Jeremy Siek

We want Forward Traversal Iterator plus Readable Lvalue Iterator to match the old Foward Iterator requirements, so we need Forward Traversal Iterator to include Default Constructible.

Proposed resolution:
> | Change: A class or built-in type `X` models the Forward Traversal Iterator concept if the following expressions are valid and respect the stated semantics.
> Forward Traversal Iterator Requirements (in addition to Single Pass Iterator)
> to: A class or built-in type `X` models the Forward Traversal Iterator concept if, in addition to `X` meeting the requirements of Default Constructible and Single Pass Iterator, the following expressions are valid and respect the stated semantics.
> Forward Traversal Iterator Requirements (in addition to Default Constructible and Single Pass Iterator)

## 9.48x Editorial changes (non-normative text)

Change:
  Iterator facade uses the Curiously Recurring Template Pattern (CRTP) [Cop95] so that the user can specify the behavior of `iterator_facade` in a derived class. Former designs used policy objects to specify the behavior. `iterator_facade` does not use policy objects for several reasons:
to:
  Iterator facade uses the Curiously Recurring Template Pattern (CRTP) [Cop95] so that the user can specify the behavior of `iterator_facade` in a derived class. Former designs used policy objects to specify the behavior, but that approach was discarded for several reasons:
Change:
  iterator's `operator++` returns the iterator type itself means that all iterators generated by `iterator_facade` would be instantiations of `iterator_facade`. Cumbersome type generator
to:
  iterator's `operator++` returns the iterator type itself would mean that all iterators built with the library would have to be specializations of `iterator_facade<...>`, rather than something more descriptive like `indirect_iterator<T*>`. Cumbersome type generator
Change:
  The return type for `operator->` and `operator[]` is not explicitly specified. Instead it requires each `iterator_facade` instantiation to meet the requirements of its `iterator_category`.
To:
  The return types for `iterator_facade`'s `operator->` and `operator[]` are not explicitly specified. Instead, those types are described in terms of a set of requirements, which must be satisfied by the `iterator_facade` implementation.

## 9.49x Clarification of iterator_facade requirements and type members

A general cleanup and simplification of the requirements and description of type members for `iterator_facade`.

The user is only allowed to add `const` as a qualifier.

Change:
  `typedef remove_cv<Value>::type value_type;`
to:
  `typedef remove_const<Value>::type value_type;`

We use to have an unspecified type for `pointer`, to match the return type of `operator->`, but there's no real reason to make them match, so we just use the simpler `Value*` for `pointer`.

Change:

> `typedef /* see description of operator-> */ pointer;`

To:
  `typedef Value* pointer;`
Remove:
  Some of the constraints on template parameters to `iterator_facade` are expressed in terms of resulting nested types and should be viewed in the context of their impact on `iterator_traits<Derived>`.
Change:
  The `Derived` template parameter must be a class derived from `iterator_facade`.
and:
  The following table describes the other requirements on the `Derived` parameter. Depending on the resulting iterator's `iterator_category`, a subset of the expressions listed in the table are required to be valid. The operations in the first column must be accessible to member functions of class `iterator_core_access`.
to:
  The following table describes the typical valid expressions on `iterator_facade`'s `Derived` parameter, depending on the iterator concept(s) it will model. The operations in the first column must be made accessible to member functions of class `iterator_core_access`. In addition, `static_cast<Derived*>(iterator_facade*)` shall be well-formed.
Remove:

  The nested `::value_type` type will be the same as `remove_cv<Value>::type`, so the `Value` parameter must be an (optionally `const`-qualified) non-reference type.

  The nested `::reference` will be the same as the `Reference` parameter; it must be a suitable reference type for the resulting iterator. The default for the `Reference` parameter is `Value&`.

Change:

> In the table below, `X` is the derived iterator type, `a` is an object of type `X`, `b` and `c` are objects of type `const X`, `n` is an object of `X::difference_type`, `y` is a constant object of a single pass iterator type interoperable with X, and `z` is a constant object of a random access traversal iterator type interoperable with `X`.

> Expression | Return Type | Assertion/Note | Required to implement Iterator Concept(s)
> --- | --- | --- | ---
> `c.dereference()` | `X::reference` |  | Readable Iterator, Writable Iterator
> `c.equal(b)` | convertible to bool | true iff `b` and `c` are equivalent. | Single Pass Iterator
> `c.equal(y)` | convertible to bool | true iff `c` and `y` refer to the same position. Implements `c == y` and `c != y`. | Single Pass Iterator
> `a.advance(n)` | unused |  | Random Access Traversal Iterator
> `a.increment()` | unused |  | Incrementable Iterator
> `a.decrement()` | unused |  | Bidirectional Traversal Iterator
> `c.distance_to(b)` | convertible to X::difference_type | equivalent to `distance(c, b)` | Random Access Traversal Iterator
> `c.distance_to(z)` | convertible to X::difference_type | equivalent to `distance(c, z)`. Implements `c - z`, `c < z`, `c <= z`, `c > z`, and `c >= c`. | Random Access Traversal Iterator

to:

> In the table below, `F` is `iterator_facade<X,V,C,R,D>`, `a` is an object of type `X`, `b` and `c` are objects of type `const X`, `n` is an object of `F::difference_type`, `y` is a constant object of a single pass iterator type interoperable with `X`, and `z` is a constant object of a random access traversal iterator type interoperable with `X`.

> iterator_facade Core Operations

> Expression | Return Type | Assertion/Note | Used to implement Iterator Concept(s)
> --- | --- | --- | ---
> `c.dereference()` | `F::reference` |  | Readable Iterator, Writable Iterator
> `c.equal(y)` | convertible to bool | true iff `c` and `y` refer to the same position. | Single Pass Iterator
> `a.increment()` | unused |  | Incrementable Iterator
> `a.decrement()` | unused |  | Bidirectional Traversal Iterator
> `a.advance(n)` | unused |  | Random Access Traversal Iterator
> `c.distance_to(z)` | convertible to `F::difference_type` | equivalent to `distance(c, X(z))`. | Random Access Traversal Iterator