Date: | 2004-01-27 |
---|
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. |
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. |
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:
|
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:
|
Submitter: | Pete Becker |
---|---|
Status: | New |
The title says it all; this is probably just a typo.
Proposed resolution: | |
---|---|
Remove the 'bool'. |
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: | |
---|---|
In [lib.iterator.adaptor] Change: Base m_iterator; to: Base m_iterator; // exposition only |
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. |
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 |
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. |
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:
|
|||
Rationale: | The style of specification has changed because of issue 9.37x. |
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. |
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 |
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. |
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. |
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: | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
In N1550: Remove:
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:
From the Readable Iterator Requirements table, remove:
From the Writable Iterator Requirements table, remove:
From the Swappable Iterator Requirements table, remove:
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:
Change:
to:
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_iteratorto:
template < class Iterator , class Value = use_default , class CategoryOrTraversal = use_default , class Reference = use_default , class Difference = use_default > class indirect_iteratorChange:
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_iteratorto:
template < class Incrementable , class CategoryOrTraversal = use_default , class Difference = use_default > class counting_iteratorChange:
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_adaptorTo:
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: |
---|
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. |
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. |
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. |
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. |
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:
to:
Change:
to:
Change:
to:
Change:
to:
Change:
to:
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 {}; |
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]
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. |
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 |
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;:
|
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)
|
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[]:
In [lib.iterator.facade] operations:
|
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:
|
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: | |||||
---|---|---|---|---|---|
|
|||||
Rationale: | Inheritance from iterator_adaptor has been removed, so we instead give the semantics in terms of the (exposition only) member m_iterator. |
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: | |||||
---|---|---|---|---|---|
|
|||||
Rationale: | Inheritance from iterator_adaptor has been removed, so we instead give the semantics in terms of the member m_iterator. |
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. |
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.
Proposed resolution: | |
---|---|
After function_output_iterator& operator++(int); add: private: UnaryFunction m_f; // exposition only
After the requirements section, add: |
function_output_iterator models
function_output_iterator is a model of the Writable and Incrementable Iterator concepts.
Returns: | An instance of function_output_iterator with f stored as a data member. |
---|
Effects: | Constructs an instance of function_output_iterator with m_f constructed from f. |
---|
output_proxy operator*();
Returns: | An instance of output_proxy constructed with a copy of the unary function f. |
---|
operator*();
Returns: | An object r of unspecified type such that r = t is equivalent to m_f(t) for all t. |
---|
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);
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. |
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.
From the Input Iterator Requirements table, remove:
Change:
to:
Change:
to:
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. |
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&);
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);
|
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:
and remove:
|
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:
to:
|
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:
In N1530:
|
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.
Returns: | An instance of indirect_iterator with the iterator_adaptor subobject copy constructed from x. |
---|
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
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.
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.
Returns: | An instance of reverse_iterator with a default constructed base object. |
---|
Effects: | Constructs an instance of reverse_iterator with m_iterator default constructed. |
---|
Effects: | Constructs an instance of reverse_iterator with a base object copy constructed from x. |
---|
Effects: | Constructs an instance of reverse_iterator with a m_iterator constructed from x. |
---|
Returns: | An instance of reverse_iterator that is a copy of r. |
---|
Effects: | Constructs instance of reverse_iterator whose m_iterator subobject is constructed from y.base(). |
---|
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 };
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.
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
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.
Returns: | a filter_iterator whose predicate is a default constructed Predicate and whose end is a default constructed Iterator. |
---|
Effects: | Constructs a filter_iterator whose``m_pred``, m_iter, and m_end members are a default constructed. |
---|
Returns: | A filter_iterator at position x that filters according to predicate f and that will not increment past end. |
---|
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. |
---|
Returns: | A filter_iterator at position x that filters according to a default constructed Predicate and that will not increment past end. |
---|
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. |
---|
Returns: | A copy of iterator t. |
---|
Effects: | Constructs a filter iterator whose members are copied from t. |
---|
Returns: | A copy of the predicate object used to construct *this. |
---|
Returns: | m_pred |
---|
Returns: | The object end used to construct *this. |
---|
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&)
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.
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.
Returns: | A default constructed instance of counting_iterator. |
---|
Requires: | Incrementable is Default Constructible. |
---|---|
Effects: | Default construct the member m_inc. |
Returns: | An instance of counting_iterator that is a copy of rhs. |
---|
Effects: | Construct member m_inc from rhs.m_inc. |
---|
Returns: | An instance of counting_iterator with its base object copy constructed from x. |
---|
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
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. |
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. |
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_typeIf 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:
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. |
|
Rationale: | Not included above is the specification of the value_type, reference, etc., members, which is handled by the changes in 9.37x. |
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:
|
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. |
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 ); |
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.
Submitter: | Dave Abrahams |
---|
"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" |
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.
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). |
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:
to:
|
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.
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;
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