Project: ArangoDB (App Definition and Development / Database)
File: facade-and-adaptor.pdf

Iterator Facade and Adaptor
Author : David Abrahams, Jeremy Siek, Thomas Witt
Contact : dave@boost-consulting.com ,jsiek@osl.iu.edu ,witt@styleadvisor.com
Organization :Boost Consulting , Indiana University Open Systems Lab ,Zephyr Asso-
ciates, Inc.
Date : 2004-11-01
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.
abstract: We propose a set of class templates that help programmers build standard-
conforming iterators, both from scratch and by adapting other iterators.
Table of Contents
Motivation
Impact on the Standard
Design
Iterator Concepts
Interoperability
Iterator Facade
Usage
Iterator Core Access
operator[]
operator->
Iterator Adaptor
Specialized Adaptors
Proposed Text
Header <iterator_helper> synopsis [lib.iterator.helper.synopsis]
Iterator facade [lib.iterator.facade]
Class template iterator_facade
iterator_facade Requirements
iterator_facade operations
Iterator adaptor [lib.iterator.adaptor]
Class template iterator_adaptor
iterator_adaptor requirements
1iterator_adaptor base class parameters
iterator_adaptor public operations
iterator_adaptor protected member functions
iterator_adaptor private member functions
Specialized adaptors [lib.iterator.special.adaptors]
Indirect iterator
Class template pointee
Class template indirect_reference
Class template indirect_iterator
indirect_iterator requirements
indirect_iterator models
indirect_iterator operations
Reverse iterator
Class template reverse_iterator
reverse_iterator requirements
reverse_iterator models
reverse_iterator operations
Transform iterator
Class template transform_iterator
transform_iterator requirements
transform_iterator models
transform_iterator operations
Filter iterator
Class template filter_iterator
filter_iterator requirements
filter_iterator models
filter_iterator operations
Counting iterator
Class template counting_iterator
counting_iterator requirements
counting_iterator models
counting_iterator operations
Function output iterator
Class template function_output_iterator
Header
function_output_iterator requirements
function_output_iterator models
function_output_iterator operations
Motivation
Iterators play an important role in modern C++ programming. The iterator is the central abstrac-
tion of the algorithms of the Standard Library, allowing algorithms to be re-used in in a wide va-
riety of contexts. The C++ Standard Library contains a wide variety of useful iterators. Every
one of the standard containers comes with constant and mutable iterators2, and also reverse ver-
sions of those same iterators which traverse the container in the opposite direction. The Standard
2also 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:
•Checked iter[13] adds bounds-checking to an existing iterator.
•The iterators of the View Template Library[14], which adapts containers, are themselves adaptors
over the underlying iterators.
•Smart iterators [5] adapt an iterator’s dereferencing behavior by applying a function object to the
object being referenced and returning the result.
•Custom iterators [4], in which a variety of adaptor types are enumerated.
•Compound iterators [1], which access a slice out of a container of containers.
•Several iterator adaptors from the MTL [12]. The MTL contains a strided iterator, where each
call to operator++() moves the iterator ahead by some constant factor, and a scaled iterator,
which multiplies the dereferenced value by some constant.
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.
1We use the term concept to mean a set of requirements that a type must satisfy to be used with a
particular template parameter.
2The 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.
3Impact on the Standard
This proposal is purely an addition to the C++ standard library. However, note that this proposal
relies on the proposal for New Iterator Concepts.
Design
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.
Interoperability
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 179concerns 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 280concerns 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.
Iterator Facade
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:
•dereferencing
•incrementing
•decrementing
•equality comparison
•random-access motion
•distance measurement
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, but that approach was discarded for several reasons:
1.the creation and eventual copying of the policy object may create overhead that
can be avoided with the current approach.
42.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.
3.Without the use of CRTP, the standard requirement that an iterator’s opera-
tor++ returns the iterator type itself would mean that all iterators built with the
library would have to be specializations of iterator_facade<...> , rather than
something more descriptive like indirect_iterator<T*> . Cumbersome type gen-
erator metafunctions would be needed to build new parameterized iterators, and
a separate iterator_adaptor layer would be impossible.
Usage
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 asiterator_facade ’sValue 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 npositions
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 Xis meant to be automatically interoperate
with another iterator type Y(as with constant and mutable iterators) then there must be an implicit
conversion from XtoYor from YtoX(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 Core Access
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 possi-
bility 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.
5In 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.
operator[]
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 an object convertible
to the iterator’s value_type , and p[n] = x is equivalent to *(p + n) = x (Note: This result object
may be implemented as a proxy containing a copy of p+n). This approach will work properly for any
random-access iterator regardless of the other details of its implementation. A user who knows more
about the implementation of her iterator is free to implement an operator[] that returns an lvalue in
the derived iterator class; it will hide the one supplied by iterator_facade from clients of her iterator.
operator->
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 types for iterator_facade ’soperator-> and operator[] are not explicitly speciﬁed.
Instead, those types are described in terms of a set of requirements, which must be satisﬁed by the
iterator_facade implementation.
Iterator Adaptor
Theiterator_adaptor class template adapts some Base3type 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.
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 iterator_facade
core requirements table. The Base type need not meet the full requirements for an iterator; it need
[Cop95] [Coplien, 1995] Coplien, J., Curiously Recurring Template Patterns, C++ Report, February
1995, pp. 24-27.
3The 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.
6only 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 imple-
mentation. 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.
Specialized Adaptors
This proposal also contains several examples of specialized adaptors which were easily implemented
using iterator_adaptor :
•indirect_iterator , which iterates over iterators, pointers, or smart pointers and applies an extra
level of dereferencing.
•A new reverse_iterator , which inverts the direction of a Base iterator’s motion, while allowing
adapted constant and mutable iterators to interact in the expected ways (unlike those in most
implementations of C++98).
•transform_iterator , which applies a user-deﬁned function object to the underlying values when
dereferenced.
•filter_iterator , which provides a view of an iterator range in which some elements of the
underlying range are skipped.
•counting_iterator , which adapts any incrementable type (e.g. integers, iterators) so that incre-
menting/decrementing the adapted iterator and dereferencing it produces successive values of the
Base type.
•function_output_iterator , which makes it easier to create custom output iterators.
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).
Proposed Text
Header <iterator_helper> synopsis [lib.iterator.helper.synopsis]
struct use_default;
struct iterator_core_access { /* implementation detail */ };
template <
class Derived
, class Value
, class CategoryOrTraversal
, class Reference = Value&
, class Difference = ptrdiff_t
7>
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 Dereferenceable>
struct pointee;
template <class Dereferenceable>
struct indirect_reference;
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;
8Iterator facade [lib.iterator.facade]
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.
Class template iterator_facade
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;
protected:
typedef iterator_facade iterator_facade_;
};
// 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,
9iterator_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&);
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
10type, 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.
[Note: the intention is to allow iterator_category to be one of the ﬁve original category tags when
convertibility to one of the traversal tags would add no information]
The enable_if_interoperable template used above is for exposition purposes. The member op-
erators should only be in an overload set provided the derived types Dr1and Dr2are interoperable,
meaning that at least one of the types is convertible to the other. The enable_if_interoperable ap-
proach 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
>
{};
iterator_facade Requirements
The following table describes the typical valid expressions on iterator_facade ’sDerived parameter,
depending on the iterator concept(s) it will model. The operations in the ﬁrst column must be made ac-
cessible to member functions of class iterator_core_access . In addition, static_cast<Derived*>(iterator_facade*)
shall be well-formed.
11In the table below, Fisiterator_facade<X,V,C,R,D> ,ais an object of type X,bandcare objects of
type const X ,nis an object of F::difference_type ,yis a constant object of a single pass iterator type
interoperable with X, and zis 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 It-
erator Concept(s)
c.dereference() F::reference Readable Iterator, Writable
Iterator
c.equal(y) convertible to bool true iﬀ candyrefer to the
same position.Single Pass Iterator
a.increment() unused Incrementable Iterator
a.decrement() unused Bidirectional Traversal Iter-
ator
a.advance(n) unused Random Access Traversal
Iterator
c.distance_to(z) convertible to
F::difference_typeequivalent to dis-
tance(c, X(z)) .Random Access Traversal
Iterator
iterator_facade operations
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; (seebelow )
Returns: Ifreference 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 wof type value_type .
unspeciﬁed operator[](difference_type n) const;
Returns: an object convertible to value_type . For constant objects vof type value_type ,
andnof type difference_type ,(*this)[n] = v is equivalent to *(*this + n) = v ,
andstatic_cast<value_type const&>((*this)[n]) is equivalent to static_cast<value_type
const&>(*(*this + n))
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;
12Derived& 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;
template <class Dr, class V, class TC, class R, class D>
Derived operator+ (iterator_facade<Dr,V,TC,R,D> const&,
typename Derived::difference_type n);
template <class Dr, class V, class TC, class R, class D>
Derived operator+ (typename Derived::difference_type n,
iterator_facade<Dr,V,TC,R,D> const&);
Eﬀects: Derived tmp(static_cast<Derived const*>(this));
return tmp += n;
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator ==(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
Returns: ifis_convertible<Dr2,Dr1>::value
then ((Dr1 const&)lhs).equal((Dr2 const&)rhs) .
Otherwise, ((Dr2 const&)rhs).equal((Dr1 const&)lhs) .
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator !=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
Returns: ifis_convertible<Dr2,Dr1>::value
then !((Dr1 const&)lhs).equal((Dr2 const&)rhs) .
Otherwise, !((Dr2 const&)rhs).equal((Dr1 const&)lhs) .
13template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator <(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
Returns: ifis_convertible<Dr2,Dr1>::value
then ((Dr1 const&)lhs).distance_to((Dr2 const&)rhs) < 0 .
Otherwise, ((Dr2 const&)rhs).distance_to((Dr1 const&)lhs) > 0 .
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator <=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
Returns: ifis_convertible<Dr2,Dr1>::value
then ((Dr1 const&)lhs).distance_to((Dr2 const&)rhs) <= 0 .
Otherwise, ((Dr2 const&)rhs).distance_to((Dr1 const&)lhs) >= 0 .
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator >(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
Returns: ifis_convertible<Dr2,Dr1>::value
then ((Dr1 const&)lhs).distance_to((Dr2 const&)rhs) > 0 .
Otherwise, ((Dr2 const&)rhs).distance_to((Dr1 const&)lhs) < 0 .
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator >=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
Returns: ifis_convertible<Dr2,Dr1>::value
then ((Dr1 const&)lhs).distance_to((Dr2 const&)rhs) >= 0 .
Otherwise, ((Dr2 const&)rhs).distance_to((Dr1 const&)lhs) <= 0 .
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,difference>::type
operator -(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
Return Type: ifis_convertible<Dr2,Dr1>::value
then difference shall be iterator_traits<Dr1>::difference_type .
Otherwise difference shall be iterator_traits<Dr2>::difference_type
Returns: ifis_convertible<Dr2,Dr1>::value
then -((Dr1 const&)lhs).distance_to((Dr2 const&)rhs) .
Otherwise, ((Dr2 const&)rhs).distance_to((Dr1 const&)lhs) .
14Iterator adaptor [lib.iterator.adaptor]
Each specialization of the iterator_adaptor class template is derived from a specialization of itera-
tor_facade . The core interface functions expected by iterator_facade are implemented in terms of
theiterator_adaptor ’sBase 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.
Class template iterator_adaptor
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 const& iter);
typedef Base base_type;
Base const& base() const;
protected:
typedef iterator_adaptor iterator_adaptor_;
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 OtherItera-
tor, class V, class C, class R, class D
>
bool equal(iterator_adaptor<OtherDerived, OtherItera-
tor, V, C, R, D> const& x) const;
void advance(typename iterator_adaptor::difference_type n);
void increment();
void decrement();
template <
class OtherDerived, class OtherItera-
tor, class V, class C, class R, class D
>
typename iterator_adaptor::difference_type distance_to(
iterator_adaptor<OtherDerived, OtherItera-
tor, V, C, R, D> const& y) const;
15private:
Base m_iterator; // exposition only
};
iterator_adaptor requirements
static_cast<Derived*>(iterator_adaptor*) shall be well-formed. The Base argument shall be
Assignable and Copy Constructible.
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
iterator_adaptor public operations
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 const& iter);
Returns: An instance of iterator_adaptor with m_iterator copy constructed from iter .
Base const& base() const;
Returns: m_iterator
16iterator_adaptor protected member functions
Base const& base_reference() const;
Returns: A const reference to m_iterator .
Base& base_reference();
Returns: A non-const reference to m_iterator .
iterator_adaptor private member functions
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 OtherItera-
tor, 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
Specialized adaptors [lib.iterator.special.adaptors]
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 Xis 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>
{};
17If 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. ]
Indirect iterator
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> ).indirect_iterator depends on two
auxiliary traits, pointee andindirect_reference , to provide support for underlying iterators whose
value_type is not an iterator.
Class template pointee
template <class Dereferenceable>
struct pointee
{
typedef /* see below */ type;
};
Requires: For an object xof type Dereferenceable ,*xis well-formed. If ++xis ill-formed
it shall neither be ambiguous nor shall it violate access control, and Dereference-
able::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 xis an object of type Dereference-
able :
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;
};
18Requires: For an object xof type Dereferenceable ,*xis well-formed. If ++xis 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 xis an object of type Dereference-
able :
if ( ++x is ill-formed )
return ‘‘pointee<Dereferenceable>::type&‘‘
else
std::iterator_traits<Dereferenceable>::reference
Class template indirect_iterator
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, Itera-
tor>::type* = 0 // exposition
);
Iterator const& base() const;
reference operator*() const;
indirect_iterator& operator++();
indirect_iterator& operator--();
private:
Iterator m_iterator; // exposition
};
19The member types of indirect_iterator are deﬁned according to the following pseudo-code, where
Visiterator_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;
indirect_iterator requirements
The expression *v, where vis an object of iterator_traits<Iterator>::value_type , shall be valid
expression and convertible to reference .Iterator shall model the traversal concept indicated by it-
erator_category .Value ,Reference , and Difference shall be chosen so that value_type ,reference ,
anddifference_type meet the requirements indicated by iterator_category .
[Note: there are further requirements on the iterator_traits<Iterator>::value_type if the
Value parameter is not use_default , as implied by the algorithm for deducing the default for the
value_type member.]
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 vis an object of itera-
tor_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 tis an object of
type indirect_iterator::value_type )
20•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 Xis interoperable with Y.
indirect_iterator operations
In addition to the operations required by the concepts described above, specializations of indirect_iterator
provide the following operations.
indirect_iterator();
Requires: Iterator must be Default Constructible.
Eﬀects: Constructs an instance of indirect_iterator with a default-constructed m_iterator .
indirect_iterator(Iterator x);
Eﬀects: Constructs an instance of indirect_iterator with m_iterator 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 // expo-
sition
);
Requires: Iterator2 is implicitly convertible to Iterator .
Eﬀects: Constructs an instance of indirect_iterator whose m_iterator subobject is
constructed from y.base() .
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
Reverse iterator
The reverse iterator adaptor iterates through the adapted iterator range in the opposite direction.
21Class template reverse_iterator
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;
reverse_iterator() {}
explicit reverse_iterator(Iterator x) ;
template<class OtherIterator>
reverse_iterator(
reverse_iterator<OtherIterator> const& r
, typename enable_if_convertible<OtherIterator, Itera-
tor>::type* = 0 // exposition
);
Iterator const& base() const;
reference operator*() const;
reverse_iterator& operator++();
reverse_iterator& operator--();
private:
Iterator m_iterator; // exposition
};
IfIterator models Random Access Traversal Iterator and Readable Lvalue Iterator, then itera-
tor_category is convertible to random_access_iterator_tag . Otherwise, if Iterator models Bidirec-
tional Traversal Iterator and Readable Lvalue Iterator, then iterator_category is convertible to bidi-
rectional_iterator_tag . Otherwise, iterator_category is convertible to input_iterator_tag .
reverse_iterator requirements
Iterator must be a model of Bidirectional Traversal Iterator. The type iterator_traits<Iterator>::reference
must be the type of *i, where iis an object of type 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:
IfImodels then reverse_iterator<I> models
Readable Lvalue Iterator, Bidirectional Traversal
IteratorBidirectional Iterator
Writable Lvalue Iterator, Bidirectional Traversal
IteratorMutable Bidirectional Iterator
Readable Lvalue Iterator, Random Access
Traversal IteratorRandom Access Iterator
Writable Lvalue Iterator, Random Access
Traversal IteratorMutable Random Access Iterator
22reverse_iterator<X> is interoperable with reverse_iterator<Y> if and only if Xis interoperable
with Y.
reverse_iterator operations
In addition to the operations required by the concepts modeled by reverse_iterator ,reverse_iterator
provides the following operations.
reverse_iterator();
Requires: Iterator must be Default Constructible.
Eﬀects: Constructs an instance of reverse_iterator with m_iterator default constructed.
explicit reverse_iterator(Iterator x);
Eﬀects: Constructs an instance of reverse_iterator with m_iterator copy constructed
from x.
template<class OtherIterator>
reverse_iterator(
reverse_iterator<OtherIterator> const& r
, typename enable_if_convertible<OtherIterator, Itera-
tor>::type* = 0 // exposition
);
Requires: OtherIterator is implicitly convertible to Iterator .
Eﬀects: Constructs instance of reverse_iterator whose m_iterator subobject is con-
structed 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
Transform iterator
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.
23Class template transform_iterator
template <class UnaryFunction,
class Iterator,
class Reference = use_default,
class Value = use_default>
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;
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 // ex-
position only
, typename enable_if_convertible<F2, UnaryFunction>::type* = 0 // ex-
position only
);
UnaryFunction functor() const;
Iterator const& base() const;
reference operator*() const;
transform_iterator& operator++();
transform_iterator& operator--();
private:
Iterator m_iterator; // exposition only
UnaryFunction m_f; // exposition only
};
IfReference isuse_default then the reference member of transform_iterator isresult_of<UnaryFunction(iterator_traits<Iterator>::reference)>::type .
Otherwise, reference isReference .
IfValue isuse_default then the value_type member is remove_cv<remove_reference<reference>
>::type . Otherwise, value_type isValue .
IfIterator models Readable Lvalue Iterator and if Iterator models Random Access Traver-
sal Iterator, then iterator_category is convertible to random_access_iterator_tag . Otherwise, if
Iterator models Bidirectional Traversal Iterator, then iterator_category is convertible to bidi-
rectional_iterator_tag . Otherwise iterator_category is convertible to forward_iterator_tag .
IfIterator does not model Readable Lvalue Iterator then iterator_category is convertible to in-
put_iterator_tag .
transform_iterator requirements
The type UnaryFunction must be Assignable, Copy Constructible, and the expression f(*i) must be
valid where fis an object of type UnaryFunction ,iis an object of type Iterator , and where the type
off(*i) must be result_of<UnaryFunction(iterator_traits<Iterator>::reference)>::type .
The argument Iterator shall model Readable Iterator.
24transform_iterator models
The resulting transform_iterator models the most reﬁned of the following 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
theIterator argument.
Iftransform_iterator is a model of Readable Lvalue Iterator then it models the following original
iterator concepts depending on what the Iterator argument models.
IfIterator 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
Iftransform_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 Xis interoperable with Y.
transform_iterator operations
In addition to the operations required by the concepts modeled by transform_iterator ,trans-
form_iterator provides the following operations.
transform_iterator();
Returns: An instance of transform_iterator with m_f and m_iterator default con-
structed.
transform_iterator(Iterator const& x, UnaryFunction f);
Returns: An instance of transform_iterator with m_finitialized to fand m_iterator
initialized to x.
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 // expo-
sition only
, typename enable_if_convertible<F2, UnaryFunction>::type* = 0 // expo-
sition only
);
Returns: An instance of transform_iterator with m_finitialized to t.functor() and
m_iterator initialized to t.base() .
Requires: OtherIterator is implicitly convertible to Iterator .
UnaryFunction functor() const;
Returns: m_f
25Iterator 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
Filter iterator
The ﬁlter iterator adaptor creates a view of an iterator range in which some elements of the range are
skipped. 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. A ﬁlter iterator is therefore constructed
with pair of iterators indicating the range of elements in the unﬁltered sequence to be traversed.
Class template filter_iterator
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;
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, Itera-
tor>::type* = 0 // exposition
);
Predicate predicate() const;
Iterator end() const;
Iterator const& base() const;
reference operator*() const;
filter_iterator& operator++();
private:
Predicate m_pred; // exposition only
26Iterator m_iter; // exposition only
Iterator m_end; // exposition only
};
IfIterator models Readable Lvalue Iterator and Bidirectional Traversal Iterator then itera-
tor_category is convertible to std::bidirectional_iterator_tag . Otherwise, 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 .
filter_iterator requirements
The Iterator argument shall meet the requirements of Readable Iterator and Single Pass Iterator or
it shall meet the requirements of Input Iterator.
The Predicate argument must be Assignable, Copy Constructible, and the expression p(x) must be
valid where pis an object of type Predicate ,xis an object of type iterator_traits<Iterator>::value_type ,
and where the type of p(x) must be convertible to bool .
filter_iterator models
The concepts that filter_iterator models are dependent on which concepts the Iterator argument
models, as speciﬁed in the following tables.
IfIterator models then filter_iterator models
Single Pass Iterator Single Pass Iterator
Forward Traversal Iterator Forward Traversal Iterator
Bidirectional Traversal Iterator Bidirectional Traversal Iterator
IfIterator models then filter_iterator models
Readable Iterator Readable Iterator
Writable Iterator Writable Iterator
Lvalue Iterator Lvalue Iterator
IfIterator 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
Writable Lvalue Iterator, Bidirectional Iterator Mutable Bidirectional Iterator
filter_iterator<P1, X> is interoperable with filter_iterator<P2, Y> if and only if Xis inter-
operable with Y.
filter_iterator operations
In addition to those operations required by the concepts that filter_iterator models, filter_iterator
provides the following operations.
filter_iterator();
Requires: Predicate andIterator must be Default Constructible.
Eﬀects: Constructs a filter_iterator whose“m pred“, m_iter , and m_end members are
a default constructed.
filter_iterator(Predicate f, Iterator x, Iterator end = Iterator());
27Eﬀects: 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 fandm_end from end.
filter_iterator(Iterator x, Iterator end = Iterator());
Requires: Predicate must be Default Constructible and Predicate is a class type (not a
function pointer).
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.
template <class OtherIterator>
filter_iterator(
filter_iterator<Predicate, OtherIterator> const& t
, typename enable_if_convertible<OtherIterator, Itera-
tor>::type* = 0 // exposition
);‘‘
Requires: OtherIterator is implicitly convertible to Iterator .
Eﬀects: Constructs a ﬁlter iterator whose members are copied from t.
Predicate predicate() const;
Returns: m_pred
Iterator end() const;
Returns: m_end
Iterator const& base() const;
Returns: m_iterator
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 orm_pred(*m_iter) == true .
Returns: *this
Counting iterator
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.
Class template counting_iterator
template <
class Incrementable
, class CategoryOrTraversal = use_default
, class Difference = use_default
>
class counting_iterator
28{
public:
typedef Incrementable value_type;
typedef const Incrementable& reference;
typedef const Incrementable* pointer;
typedef /* see below */ difference_type;
typedef /* see below */ iterator_category;
counting_iterator();
counting_iterator(counting_iterator const& rhs);
explicit counting_iterator(Incrementable x);
Incrementable const& base() const;
reference operator*() const;
counting_iterator& operator++();
counting_iterator& operator--();
private:
Incrementable m_inc; // exposition
};
If the Difference argument is use_default then difference_type is an unspeciﬁed signed integral
type. Otherwise difference_type isDifference .
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&)
[Note: implementers are encouraged to provide an implementation of operator- and a dif-
ference_type that avoids overﬂows in the cases where std::numeric_limits<Incrementable>::is_specialized
is true.]
counting_iterator requirements
The Incrementable argument shall be Copy Constructible and Assignable.
Ifiterator_category is convertible to forward_iterator_tag orforward_traversal_tag , the
following must be well-formed:
Incrementable i, j;
++i; // pre-increment
i == j; // operator equal
Ifiterator_category is convertible to bidirectional_iterator_tag orbidirectional_traversal_tag ,
the following expression must also be well-formed:
--i
Ifiterator_category is convertible to random_access_iterator_tag orrandom_access_traversal_tag ,
the following must must also be valid:
29counting_iterator::difference_type n;
i += n;
n = i - j;
i < j;
counting_iterator models
Specializations of counting_iterator model Readable Lvalue Iterator. In addition, they model the con-
cepts corresponding to the iterator tags to which their iterator_category is convertible. Also, if Cat-
egoryOrTraversal 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
Xis interoperable with Y.
counting_iterator operations
In addition to the operations required by the concepts modeled by counting_iterator ,counting_iterator
provides the following operations.
counting_iterator();
Requires: Incrementable is Default Constructible.
Eﬀects: Default construct the member m_inc .
counting_iterator(counting_iterator const& rhs);
Eﬀects: Construct member m_inc from rhs.m_inc .
explicit counting_iterator(Incrementable x);
Eﬀects: Construct member m_inc from x.
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
Function output iterator
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.
30Class template function_output_iterator
Header
#include <boost/function_output_iterator.hpp>
template <class UnaryFunction>
class function_output_iterator {
public:
typedef std::output_iterator_tag iterator_category;
typedef void value_type;
typedef void difference_type;
typedef void pointer;
typedef void reference;
explicit function_output_iterator();
explicit function_output_iterator(const UnaryFunction& f);
/* see below */ operator*();
function_output_iterator& operator++();
function_output_iterator& operator++(int);
private:
UnaryFunction m_f; // exposition only
};
function_output_iterator requirements
UnaryFunction must be Assignable and Copy Constructible.
function_output_iterator models
function_output_iterator is a model of the Writable and Incrementable Iterator concepts.
function_output_iterator operations
explicit function_output_iterator(const UnaryFunction& f = UnaryFunction());
Eﬀects: Constructs an instance of function_output_iterator with m_fconstructed from
f.
operator*();
Returns: An object rof unspeciﬁed type such that r = t is equivalent to m_f(t) for all
t.
function_output_iterator& operator++();
Returns: *this
function_output_iterator& operator++(int);
Returns: *this
31