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
    >
{};

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. ]

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

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.

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.

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;

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.

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

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.

1. 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.
2. Change the is_readable specification. Remove the requirement for support of the is_readable trait from the Readable Iterator concept.
3. Remove the iterator_tag class and transplant the logic for choosing an iterator category into iterator_facade.
4. Change the specification of traversal_category.
5. 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.

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 {};

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.

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.

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);

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))

Remove the returns clause.

In [lib.iterator.facade] operations:

Remove:

Returns:	static_cast<Derived const*>(this)->advance(-n);

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.

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().

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:

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.

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&);

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);

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);

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);

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

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

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:

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.]

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.

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.

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
);

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:

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).

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)