| Author: | David Abrahams, Jeremy Siek, Thomas Witt |
|---|---|
| Contact: | dave@boost-consulting.com, jsiek@osl.iu.edu, witt@acm.org |
| Organization: | Boost Consulting, Indiana University Open Systems Lab, University of Hanover Institute for Transport Railway Operation and Construction |
| Date: | 2003-11-24 |
| Number: | This is a revised version of N1530=03-0113, which was accepted for Technical Report 1 by the C++ standard committee's library working group. |
| copyright: | Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. All rights reserved |
|---|
| abstract: | We propose a set of class templates that help programmers build standard-conforming iterators, both from scratch and by adapting other iterators. |
|---|
Iterators play an important role in modern C++ programming. The iterator is the central abstraction of the algorithms of the Standard Library, allowing algorithms to be re-used in in a wide variety of contexts. The C++ Standard Library contains a wide variety of useful iterators. Every one of the standard containers comes with constant and mutable iterators 2, and also reverse versions of those same iterators which traverse the container in the opposite direction. The Standard also supplies istream_iterator and ostream_iterator for reading from and writing to streams, insert_iterator, front_insert_iterator and back_insert_iterator for inserting elements into containers, and raw_storage_iterator for initializing raw memory [7].
Despite the many iterators supplied by the Standard Library, obvious and useful iterators are missing, and creating new iterator types is still a common task for C++ programmers. The literature documents several of these, for example line_iterator [3] and Constant_iterator [9]. The iterator abstraction is so powerful that we expect programmers will always need to invent new iterator types.
Although it is easy to create iterators that almost conform to the standard, the iterator requirements contain subtleties which can make creating an iterator which actually conforms quite difficult. Further, the iterator interface is rich, containing many operators that are technically redundant and tedious to implement. To automate the repetitive work of constructing iterators, we propose iterator_facade, an iterator base class template which provides the rich interface of standard iterators and delegates its implementation to member functions of the derived class. In addition to reducing the amount of code necessary to create an iterator, the iterator_facade also provides compile-time error detection. Iterator implementation mistakes that often go unnoticed are turned into compile-time errors because the derived class implementation must match the expectations of the iterator_facade.
A common pattern of iterator construction is the adaptation of one iterator to form a new one. The functionality of an iterator is composed of four orthogonal aspects: traversal, indirection, equality comparison and distance measurement. Adapting an old iterator to create a new one often saves work because one can reuse one aspect of functionality while redefining the other. For example, the Standard provides reverse_iterator, which adapts any Bidirectional Iterator by inverting its direction of traversal. As with plain iterators, iterator adaptors defined outside the Standard have become commonplace in the literature:
| [1] | We use the term concept to mean a set of requirements that a type must satisfy to be used with a particular template parameter. |
| [2] | The term mutable iterator refers to iterators over objects that can be changed by assigning to the dereferenced iterator, while constant iterator refers to iterators over objects that cannot be modified. |
To fulfill the need for constructing adaptors, we propose the iterator_adaptor class template. Instantiations of iterator_adaptor serve as a base classes for new iterators, providing the default behavior of forwarding all operations to the underlying iterator. The user can selectively replace these features in the derived iterator class. This proposal also includes a number of more specialized adaptors, such as the transform_iterator that applies some user-specified function during the dereference of the iterator.
This proposal is purely an addition to the C++ standard library. However, note that this proposal relies on the proposal for New Iterator Concepts.
This proposal is formulated in terms of the new iterator concepts as proposed in n1550, since user-defined and especially adapted iterators suffer from the well known categorization problems that are inherent to the current iterator categories.
This proposal does not strictly depend on proposal n1550, as there is a direct mapping between new and old categories. This proposal could be reformulated using this mapping if n1550 was not accepted.
The question of iterator interoperability is poorly addressed in the current standard. There are currently two defect reports that are concerned with interoperability issues.
Issue 179 concerns the fact that mutable container iterator types are only required to be convertible to the corresponding constant iterator types, but objects of these types are not required to interoperate in comparison or subtraction expressions. This situation is tedious in practice and out of line with the way built in types work. This proposal implements the proposed resolution to issue 179, as most standard library implementations do nowadays. In other words, if an iterator type A has an implicit or user defined conversion to an iterator type B, the iterator types are interoperable and the usual set of operators are available.
Issue 280 concerns the current lack of interoperability between reverse iterator types. The proposed new reverse_iterator template fixes the issues raised in 280. It provides the desired interoperability without introducing unwanted overloads.
While the iterator interface is rich, there is a core subset of the interface that is necessary for all the functionality. We have identified the following core behaviors for iterators:
In addition to the behaviors listed above, the core interface elements include the associated types exposed through iterator traits: value_type, reference, difference_type, and iterator_category.
Iterator facade uses the Curiously Recurring Template Pattern (CRTP) [Cop95] so that the user can specify the behavior of iterator_facade in a derived class. Former designs used policy objects to specify the behavior. iterator_facade does not use policy objects for several reasons:
- the creation and eventual copying of the policy object may create overhead that can be avoided with the current approach.
- The policy object approach does not allow for custom constructors on the created iterator types, an essential feature if iterator_facade should be used in other library implementations.
- Without the use of CRTP, the standard requirement that an iterator's operator++ returns the iterator type itself means that all iterators generated by iterator_facade would be specializations of iterator_facade. Cumbersome type generator metafunctions would be needed to build new parameterized iterators, and a separate iterator_adaptor layer would be impossible.
The user of iterator_facade derives his iterator class from a specialization of iterator_facade and passes the derived iterator class as iterator_facade's first template parameter. The order of the other template parameters have been carefully chosen to take advantage of useful defaults. For example, when defining a constant lvalue iterator, the user can pass a const-qualified version of the iterator's value_type as iterator_facade's Value parameter and omit the Reference parameter which follows.
The derived iterator class must define member functions implementing the iterator's core behaviors. The following table describes expressions which are required to be valid depending on the category of the derived iterator type. These member functions are described briefly below and in more detail in the iterator facade requirements.
Expression Effects i.dereference() Access the value referred to i.equal(j) Compare for equality with j i.increment() Advance by one position i.decrement() Retreat by one position i.advance(n) Advance by n positions i.distance_to(j) Measure the distance to j
In addition to implementing the core interface functions, an iterator derived from iterator_facade typically defines several constructors. To model any of the standard iterator concepts, the iterator must at least have a copy constructor. Also, if the iterator type X is meant to be automatically interoperate with another iterator type Y (as with constant and mutable iterators) then there must be an implicit conversion from X to Y or from Y to X (but not both), typically implemented as a conversion constructor. Finally, if the iterator is to model Forward Traversal Iterator or a more-refined iterator concept, a default constructor is required.
iterator_facade and the operator implementations need to be able to access the core member functions in the derived class. Making the core member functions public would expose an implementation detail to the user. The design used here ensures that implementation details do not appear in the public interface of the derived iterator type.
Preventing direct access to the core member functions has two advantages. First, there is no possibility for the user to accidently use a member function of the iterator when a member of the value_type was intended. This has been an issue with smart pointer implementations in the past. The second and main advantage is that library implementers can freely exchange a hand-rolled iterator implementation for one based on iterator_facade without fear of breaking code that was accessing the public core member functions directly.
In a naive implementation, keeping the derived class' core member functions private would require it to grant friendship to iterator_facade and each of the seven operators. In order to reduce the burden of limiting access, iterator_core_access is provided, a class that acts as a gateway to the core member functions in the derived iterator class. The author of the derived class only needs to grant friendship to iterator_core_access to make his core member functions available to the library.
iterator_core_access will be typically implemented as an empty class containing only private static member functions which invoke the iterator core member functions. There is, however, no need to standardize the gateway protocol. Note that even if iterator_core_access used public member functions it would not open a safety loophole, as every core member function preserves the invariants of the iterator.
The indexing operator for a generalized iterator presents special challenges. A random access iterator's operator[] is only required to return something convertible to its value_type. Requiring that it return an lvalue would rule out currently-legal random-access iterators which hold the referenced value in a data member (e.g. counting_iterator), because *(p+n) is a reference into the temporary iterator p+n, which is destroyed when operator[] returns.
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 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.
The reference type of a readable iterator (and today's input iterator) need not in fact be a reference, so long as it is convertible to the iterator's value_type. When the value_type is a class, however, it must still be possible to access members through operator->. Therefore, an iterator whose reference type is not in fact a reference must return a proxy containing a copy of the referenced value from its operator->.
The return type for operator-> and operator[] is not explicitly specified. Instead it requires each iterator_facade specialization to meet the requirements of its iterator_category.
| [Cop95] | [Coplien, 1995] Coplien, J., Curiously Recurring Template Patterns, C++ Report, February 1995, pp. 24-27. |
The iterator_adaptor class template adapts some Base 3 type to create a new iterator. Instantiations of iterator_adaptor are derived from a corresponding instantiation of iterator_facade and implement the core behaviors in terms of the Base type. In essence, iterator_adaptor merely forwards all operations to an instance of the Base type, which it stores as a member.
| [3] | The term "Base" here does not refer to a base class and is not meant to imply the use of derivation. We have followed the lead of the standard library, which provides a base() function to access the underlying iterator object of a reverse_iterator adaptor. |
The user of iterator_adaptor creates a class derived from an instantiation of iterator_adaptor and then selectively redefines some of the core member functions described in the table above. The Base type need not meet the full requirements for an iterator. It need only support the operations used by the core interface functions of iterator_adaptor that have not been redefined in the user's derived class.
Several of the template parameters of iterator_adaptor default to use_default. This allows the user to make use of a default parameter even when she wants to specify a parameter later in the parameter list. Also, the defaults for the corresponding associated types are somewhat complicated, so metaprogramming is required to compute them, and use_default can help to simplify the implementation. Finally, the identity of the use_default type is not left unspecified 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 from making mistakes based on that assumption.
This proposal also contains several examples of specialized adaptors which were easily implemented using iterator_adaptor:
Based on examples in the Boost library, users have generated many new adaptors, among them a permutation adaptor which applies some permutation to a random access iterator, and a strided adaptor, which adapts a random access iterator by multiplying its unit of motion by a constant factor. In addition, the Boost Graph Library (BGL) uses iterator adaptors to adapt other graph libraries, such as LEDA [10] and Stanford GraphBase [8], to the BGL interface (which requires C++ Standard compliant iterators).
struct use_default;
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;
template <class Iterator>
class reverse_iterator;
template <
class UnaryFunction
, class Iterator
, class Reference = use_default
, class Value = use_default
>
class transform_iterator;
template <class Predicate, class Iterator>
class filter_iterator;
template <
class Incrementable
, class CategoryOrTraversal = use_default
, class Difference = use_default
>
class counting_iterator
template <class UnaryFunction>
class function_output_iterator;
iterator_facade is a base class template that implements the interface of standard iterators in terms of a few core functions and associated types, to be supplied by a derived iterator class.
template <
class Derived
, class Value
, class CategoryOrTraversal
, class Reference = Value&
, class Difference = ptrdiff_t
>
class iterator_facade {
public:
typedef remove_const<Value>::type value_type;
typedef Reference reference;
typedef Value* pointer;
typedef Difference difference_type;
typedef /* see below */ iterator_category;
reference operator*() const;
/* see below */ operator->() const;
/* see below */ operator[](difference_type n) const;
Derived& operator++();
Derived operator++(int);
Derived& operator--();
Derived operator--(int);
Derived& operator+=(difference_type n);
Derived& operator-=(difference_type n);
Derived operator-(difference_type n) const;
};
// 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);
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>
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 addition
template <class Derived, class V, class TC, class R, class D>
Derived operator+ (iterator_facade<Derived, V, TC, R, D> const&,
typename Derived::difference_type n)
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, 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
>
{};
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 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.
| Expression | Return Type | Assertion/Note | Used to implement Iterator Concept(s) |
|---|---|---|---|
| c.dereference() | F::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 F::difference_type | equivalent to distance(c, b) | Random Access Traversal Iterator |
| c.distance_to(z) | convertible to F::difference_type | equivalent to distance(c, z). Implements c - z, c < z, c <= z, c > z, and c >= c. | Random Access Traversal Iterator |
The iterator_category member of iterator_facade<X,V,R,C,D> is a type which satisfies the following conditions:
if C is convertible to std::input_iterator_tag or C is convertible to std::output_iterator_tag, iterator_category is the same as C.
Otherwise, if C is not convertible to incrementable_traversal_tag, the program is ill-formed
Otherwise:
iterator_category is convertible to the iterator category tag or tags given by the following algorithm, and not to any more-derived iterator category tag or tags:
if (R is a reference type && C is convertible to forward_traversal_tag) { if (C is convertible to random_access_traversal_tag) return random_access_iterator_tag else if (C is convertible to bidirectional_traversal_tag) return bidirectional_iterator_tag else return forward_traversal_tag } else { if (C is convertible to single_pass_traversal_tag && R is convertible to V) { if (V is const) return input_iterator_tag else return input_iterator_tag and output_iterator_tag } else return output_iterator_tag }iterator_traversal<X>::type is convertible to the most derived traversal tag type to which C is also convertible, and not to any more-derived traversal tag type.
The operations in this section are described in terms of operations on the core interface of Derived which may be inaccessible (i.e. private). The implementation should access these operations through member functions of class iterator_core_access.
reference operator*() const;
| Returns: | static_cast<Derived const*>(this)->dereference() |
|---|
operator->() const; (see below)
| 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. |
|---|
unspecified operator[](difference_type n) const;
| Returns: | an object convertible to reference and holding a copy p of *static_cast<Derived const*>(this) + n such that, for a constant object v of type value_type, (*static_cast<Derived const*>(this))[n] = v is equivalent to p = v. |
|---|
Derived& operator++();
| Effects: | static_cast<Derived*>(this)->increment(); return *static_cast<Derived*>(this); |
|---|
Derived operator++(int);
| Effects: | Derived tmp(static_cast<Derived const*>(this)); ++*this; return tmp; |
|---|
Derived& operator--();
| Effects: | static_cast<Derived*>(this)->decrement(); return static_cast<Derived*>(this); |
|---|
Derived operator--(int);
| Effects: | Derived tmp(static_cast<Derived const*>(this)); --*this; return tmp; |
|---|
Derived& operator+=(difference_type n);
| Effects: | static_cast<Derived*>(this)->advance(n); return static_cast<Derived*>(this); |
|---|
Derived& operator-=(difference_type n);
| Effects: | static_cast<Derived*>(this)->advance(-n); return static_cast<Derived*>(this); |
|---|
Derived operator-(difference_type n) const;
| Effects: | Derived tmp(static_cast<Derived const*>(this)); return tmp -= n; |
|---|
Each specialization of the iterator_adaptor class template is derived from a specialization of iterator_facade. The core interface functions expected by iterator_facade are implemented in terms of the iterator_adaptor's Base template parameter. A class derived from iterator_adaptor typically redefines some of the core interface functions to adapt the behavior of the Base type. Whether the derived class models any of the standard iterator concepts depends on the operations supported by the Base type and which core interface functions of iterator_facade are redefined in the Derived class.
template <
class Derived
, class Base
, class Value = use_default
, class CategoryOrTraversal = use_default
, class Reference = use_default
, class Difference = use_default
>
class iterator_adaptor
: public iterator_facade<Derived, V, C, R, D> // see details
{
friend class iterator_core_access;
public:
iterator_adaptor();
explicit iterator_adaptor(Base iter);
Base base() const;
protected:
Base const& base_reference() const;
Base& base_reference();
private: // Core iterator interface for iterator_facade.
typename iterator_adaptor::reference dereference() const;
template <
class OtherDerived, class OtherIterator, class V, class C, class R, class D
>
bool equal(iterator_adaptor<OtherDerived, OtherIterator, V, C, R, D> const& x) const;
void advance(typename iterator_adaptor::difference_type n);
void increment();
void decrement();
template <
class OtherDerived, class OtherIterator, class V, class C, class R, class D
>
typename iterator_adaptor::difference_type distance_to(
iterator_adaptor<OtherDerived, OtherIterator, V, C, R, D> const& y) const;
private:
Base m_iterator; // exposition only
};
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
The Derived template parameter must be a publicly derived from iterator_adaptor. In order for Derived to model the iterator concepts corresponding to iterator_traits<Derived>::iterator_category, the expressions involving m_iterator in the specifications of those private member functions of iterator_adaptor that may be called by iterator_facade<Derived, ``\ *V*\, \ *C*\, \ *R*\, \ *D*\>`` in evaluating any valid expression involving Derived in those concepts' requirements.
iterator_adaptor();
| Requires: | The Base type must be Default Constructible. |
|---|---|
| Returns: | An instance of iterator_adaptor with m_iterator default constructed. |
explicit iterator_adaptor(Base iter);
| Returns: | An instance of iterator_adaptor with m_iterator copy constructed from iter. |
|---|
Base base() const;
| Returns: | m_iterator |
|---|
Base const& base_reference() const;
| Returns: | A const reference to m_iterator. |
|---|
Base& base_reference();
| Returns: | A non-const reference to m_iterator. |
|---|
typename iterator_adaptor::reference dereference() const;
| Returns: | *m_iterator |
|---|
template < class OtherDerived, class OtherIterator, class V, class C, class R, class D > bool equal(iterator_adaptor<OtherDerived, OtherIterator, V, C, R, D> const& x) const;
| Returns: | m_iterator == x.base() |
|---|
void advance(typename iterator_adaptor::difference_type n);
| Effects: | m_iterator += n; |
|---|
void increment();
| Effects: | ++m_iterator; |
|---|
void decrement();
| Effects: | --m_iterator; |
|---|
template <
class OtherDerived, class OtherIterator, class V, class C, class R, class D
>
typename iterator_adaptor::difference_type distance_to(
iterator_adaptor<OtherDerived, OtherIterator, V, C, R, D> const& y) const;
| Returns: | y.base() - m_iterator |
|---|
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. ]
The indirect iterator adapts an iterator by applying an extra dereference inside of operator*(). For example, this iterator adaptor makes it possible to view a container of pointers (e.g. list<foo*>) as if it were a container of the pointed-to type (e.g. list<foo>) .
template <
class Iterator
, class Value = use_default
, class CategoryOrTraversal = use_default
, class Reference = use_default
, class Difference = use_default
>
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;
indirect_iterator();
indirect_iterator(Iterator x);
template <
class Iterator2, class Value2, class Category2
, class Reference2, class Difference2
>
indirect_iterator(
indirect_iterator<
Iterator2, Value2, Category2, Reference2, Difference2
> const& y
, typename enable_if_convertible<Iterator2, Iterator>::type* = 0 // exposition
);
};
The member types of indirect_iterator are defined according to the following pseudo-code. We use the abbreviation V=iterator_traits<Iterator>::value_type.:
if (Value is use_default) then
typedef iterator_traits<V>::value_type value_type;
else
typedef remove_const<Value>::type value_type;
if (Reference is use_default) then
if (Value is use_default) then
typedef iterator_traits<V>::reference reference;
else
typedef Value& reference;
else
typedef Reference reference;
if (Value is use_default) then
typedef ?? pointer;
else
typedef Value* pointer;
if (Difference is use_default)
typedef iterator_traits<Iterator>::difference_type difference_type;
else
typedef Difference difference_type;
The member indirect_iterator::iterator_category is a type that satisfies the requirements of the concepts modeled by the indirect iterator as specified in the models section.
The Iterator argument shall meet the requirements of Readable Iterator. The CategoryOrTraversal argument shall be one of the standard iterator tags or use_default. If CategoryOrTraversal is an iterator tag, the template parameter Iterator argument shall meet the traversal requirements corresponding to the iterator tag.
The expression *v, where v is an object of type iterator_traits<Iterator>::value_type, must be a valid expression and must be convertible to indirect_iterator::reference. Also indirect_iterator::reference must be convertible to indirect_iterator::value. 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.
If CategoryOrTraversal is a standard iterator tag, indirect_iterator is a model of the iterator concept corresponding to the tag, otherwise indirect_iterator satisfies the requirements of the most refined standard traversal concept that is satisfied by the Iterator argument.
indirect_iterator models Readable Iterator. If indirect_iterator::reference(*v) = t is a valid expression (where t is an object of type indirect_iterator::value_type) then indirect_iterator models Writable Iterator. If indirect_iterator::reference is a reference then indirect_iterator models Lvalue Iterator.
indirect_iterator();
| Requires: | Iterator must be Default Constructible. |
|---|---|
| Returns: | An instance of indirect_iterator with a default-constructed iterator_adaptor subobject. |
indirect_iterator(Iterator x);
| Returns: | An instance of indirect_iterator with the iterator_adaptor subobject copy constructed from x. |
|---|
template <
class Iterator2, class Value2, unsigned Access, class Traversal
, class Reference2, class Difference2
>
indirect_iterator(
indirect_iterator<
Iterator2, Value2, Access, Traversal, Reference2, Difference2
> const& y
, typename enable_if_convertible<Iterator2, Iterator>::type* = 0 // exposition
);
| Requires: | Iterator2 is implicitly convertible to Iterator. |
|---|---|
| Returns: | An instance of indirect_iterator whose iterator_adaptor subobject is constructed from y.base(). |
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.
template <class Iterator>
class reverse_iterator :
public iterator_adaptor< reverse_iterator<Iterator>, Iterator >
{
friend class iterator_core_access;
public:
reverse_iterator() {}
explicit reverse_iterator(Iterator x) ;
template<class OtherIterator>
reverse_iterator(
reverse_iterator<OtherIterator> const& r
, typename enable_if_convertible<OtherIterator, Iterator>::type* = 0 // exposition
);
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();
}
};
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();
| Requires: | Iterator must be Default Constructible. |
|---|---|
| Returns: | An instance of reverse_iterator with a default constructed base object. |
explicit reverse_iterator(Iterator x);
| Returns: | An instance of reverse_iterator with a base object copy constructed from x. |
|---|
template<class OtherIterator>
reverse_iterator(
reverse_iterator<OtherIterator> const& r
, typename enable_if_convertible<OtherIterator, Iterator>::type* = 0 // exposition
);
| Requires: | OtherIterator is implicitly convertible to Iterator. |
|---|---|
| Returns: | An instance of reverse_iterator that is a copy of r. |
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.
template <class UnaryFunction,
class Iterator,
class Reference = use_default,
class Value = use_default>
class transform_iterator
: public iterator_adaptor</* see discussion */>
{
friend class iterator_core_access;
public:
transform_iterator();
transform_iterator(Iterator const& x, UnaryFunction f);
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
, typename enable_if_convertible<F2, UnaryFunction>::type* = 0 // exposition
);
UnaryFunction functor() const;
private:
typename transform_iterator::value_type dereference() const;
UnaryFunction m_f;
};
The type UnaryFunction must be Assignable, Copy Constructible, and the expression f(*i) must be valid where f is an object of type UnaryFunction, i is an object of type Iterator, and where the type of f(*i) must be result_of<UnaryFunction(iterator_traits<Iterator>::reference)>::type.
The type Iterator must at least model Readable Iterator. The resulting transform_iterator models the most refined of the following options 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.
transform_iterator();
| Returns: | An instance of transform_iterator with m_f and m_iterator default constructed. |
|---|
transform_iterator(Iterator const& x, UnaryFunction f);
| Returns: | An instance of transform_iterator with m_f initialized to f and m_iterator initialized to x. |
|---|
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
);
| Returns: | An instance of transform_iterator that is a copy of t. |
|---|---|
| Requires: | OtherIterator is implicitly convertible to Iterator. |
UnaryFunction functor() const;
| Returns: | m_f |
|---|
typename transform_iterator::value_type dereference() const;
| Returns: | m_f(transform_iterator::dereference()); |
|---|
The filter iterator adaptor creates a view of an iterator range in which some elements of the range are skipped over. A predicate function object controls which elements are skipped. When the predicate is applied to an element, if it returns true then the element is retained and if it returns false then the element is skipped over. When skipping over elements, it is necessary for the filter adaptor to know when to stop so as to avoid going past the end of the underlying range. Therefore the constructor of the filter iterator takes two iterator parameters: the position for the filtered iterator and the end of the range.
template <class Predicate, class Iterator>
class filter_iterator
: public iterator_adaptor<
filter_iterator<Predicate, Iterator>, Iterator
, use_default
, /* see details */
>
{
public:
filter_iterator();
filter_iterator(Predicate f, Iterator x, Iterator end = Iterator());
filter_iterator(Iterator x, Iterator end = Iterator());
template<class OtherIterator>
filter_iterator(
filter_iterator<Predicate, OtherIterator> const& t
, typename enable_if_convertible<OtherIterator, Iterator>::type* = 0 // exposition
);
Predicate predicate() const;
Iterator end() const;
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;
};
The base Iterator parameter must be a model of Readable Iterator and Single Pass Iterator. The resulting filter_iterator will be a model of Forward Traversal Iterator if Iterator is, otherwise the filter_iterator will be a model of Single Pass Iterator. The access category of the filter_iterator will be the same as the access category of Iterator.
The Predicate must be Assignable, Copy Constructible, and the expression p(x) must be valid where p is an object of type Predicate, x is an object of type iterator_traits<Iterator>::value_type, and where the type of p(x) must be convertible to bool.
filter_iterator();
| Requires: | Predicate and Iterator must be Default Constructible. |
|---|---|
| Returns: | a filter_iterator whose predicate is a default constructed Predicate and whose end is a default constructed Iterator. |
filter_iterator(Predicate f, Iterator x, Iterator end = Iterator());
| Returns: | A filter_iterator at position x that filters according to predicate f and that will not increment past end. |
|---|
filter_iterator(Iterator x, Iterator end = Iterator());
| Requires: | Predicate must be Default Constructible. |
|---|---|
| Returns: | A filter_iterator at position x that filters according to a default constructed Predicate and that will not increment past end. |
template <class OtherIterator>
filter_iterator(
filter_iterator<Predicate, OtherIterator> const& t
, typename enable_if_convertible<OtherIterator, Iterator>::type* = 0 // exposition
);``
| Requires: | OtherIterator is implicitly convertible to Iterator. |
|---|---|
| Returns: | A copy of iterator t. |
Predicate predicate() const;
| Returns: | A copy of the predicate object used to construct *this. |
|---|
Iterator end() const;
| Returns: | The object end used to construct *this. |
|---|
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.
template <
class Incrementable
, unsigned Access = use_default_access
, class Traversal = use_default
, class Difference = use_default
>
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:
counting_iterator();
counting_iterator(counting_iterator const& rhs);
counting_iterator(Incrementable x);
private:
typename counting_iterator::reference dereference() const
{
return this->base_reference();
}
};
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.
Furthermore, if you wish to create a counting iterator that is a Forward Traversal Iterator, then the following expressions must be valid:
Incrementable i, j; ++i // pre-increment i == j // operator equal
If you wish to create a counting iterator that is a Bidirectional Traversal Iterator, then pre-decrement is also required:
--i
If you wish to create a counting iterator that is a Random Access Traversal Iterator, then these additional expressions are also required:
counting_iterator::difference_type n; i += n n = i - j i < j
counting_iterator();
| Returns: | A default constructed instance of counting_iterator. |
|---|
counting_iterator(counting_iterator const& rhs);
| Returns: | An instance of counting_iterator that is a copy of rhs. |
|---|
counting_iterator(Incrementable x);
| Returns: | An instance of counting_iterator with its base object copy constructed from x. |
|---|
The function output iterator adaptor makes it easier to create custom output iterators. The adaptor takes a unary function and creates a model of Output Iterator. Each item assigned to the output iterator is passed as an argument to the unary function. The motivation for this iterator is that creating a conforming output iterator is non-trivial, particularly because the proper implementation usually requires a proxy object.
template <class UnaryFunction>
class function_output_iterator {
public:
typedef iterator_tag<
writable_iterator
, incrementable_traversal_tag
> iterator_category;
typedef void value_type;
typedef void difference_type;
typedef void pointer;
typedef void reference;
explicit function_output_iterator(const UnaryFunction& f = UnaryFunction());
struct output_proxy {
output_proxy(UnaryFunction& f);
template <class T> output_proxy& operator=(const T& value);
};
output_proxy operator*();
function_output_iterator& operator++();
function_output_iterator& operator++(int);
};
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.
explicit function_output_iterator(const UnaryFunction& f = UnaryFunction());
| Returns: | An instance of function_output_iterator with f stored as a data member. |
|---|
output_proxy operator*();
| Returns: | An instance of output_proxy constructed with a copy of the unary function f. |
|---|
function_output_iterator& operator++();
| Returns: | *this |
|---|
function_output_iterator& operator++(int);
| Returns: | *this |
|---|
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; |
|---|