iterator/doc/facade-and-adaptor.html

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<meta name="author" content="David Abrahams, Jeremy Siek, Thomas Witt" />
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<div class="document" id="iterator-facade-and-adaptor">
<h1 class="title">Iterator Facade and Adaptor</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr><th class="docinfo-name">Author:</th>
<td>David Abrahams, Jeremy Siek, Thomas Witt</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first reference" href="mailto:dave&#64;boost-consulting.com">dave&#64;boost-consulting.com</a>, <a class="reference" href="mailto:jsiek&#64;osl.iu.edu">jsiek&#64;osl.iu.edu</a>, <a class="last reference" href="mailto:witt&#64;acm.org">witt&#64;acm.org</a></td></tr>
<tr><th class="docinfo-name">Organization:</th>
<td><a class="first reference" href="http://www.boost-consulting.com">Boost Consulting</a>, Indiana University <a class="reference" href="http://www.osl.iu.edu">Open Systems
Lab</a>, University of Hanover <a class="last reference" href="http://www.ive.uni-hannover.de">Institute for Transport
Railway Operation and Construction</a></td></tr>
<tr><th class="docinfo-name">Date:</th>
<td>2003-11-24</td></tr>
<tr class="field"><th class="docinfo-name">Number:</th><td class="field-body">This is a revised version of <a class="reference" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1530.html">N1530</a>=03-0113, which was
accepted for Technical Report 1 by the C++ standard
committee's library working group.</td>
</tr>
</tbody>
</table>
<!-- Version 1.9 of this ReStructuredText document corresponds to
n1530_, the paper accepted by the LWG. -->
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">copyright:</th><td class="field-body">Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. All rights reserved</td>
</tr>
</tbody>
</table>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">abstract:</th><td class="field-body">We propose a set of class templates that help programmers
build standard-conforming iterators, both from scratch and
by adapting other iterators.</td>
</tr>
</tbody>
</table>
<div class="contents topic" id="table-of-contents">
<p class="topic-title"><a name="table-of-contents">Table of Contents</a></p>
<ul class="simple">
<li><a class="reference" href="#motivation" id="id13" name="id13">Motivation</a></li>
<li><a class="reference" href="#impact-on-the-standard" id="id14" name="id14">Impact on the Standard</a></li>
<li><a class="reference" href="#design" id="id15" name="id15">Design</a><ul>
<li><a class="reference" href="#iterator-concepts" id="id16" name="id16">Iterator Concepts</a></li>
<li><a class="reference" href="#interoperability" id="id17" name="id17">Interoperability</a></li>
<li><a class="reference" href="#iterator-facade" id="id18" name="id18">Iterator Facade</a><ul>
<li><a class="reference" href="#usage" id="id19" name="id19">Usage</a></li>
<li><a class="reference" href="#iterator-core-access" id="id20" name="id20">Iterator Core Access</a></li>
<li><a class="reference" href="#operator" id="id21" name="id21"><tt class="literal"><span class="pre">operator[]</span></tt></a></li>
<li><a class="reference" href="#id6" id="id22" name="id22"><tt class="literal"><span class="pre">operator-&gt;</span></tt></a></li>
</ul>
</li>
<li><a class="reference" href="#iterator-adaptor" id="id23" name="id23">Iterator Adaptor</a></li>
<li><a class="reference" href="#specialized-adaptors" id="id24" name="id24">Specialized Adaptors</a></li>
</ul>
</li>
<li><a class="reference" href="#proposed-text" id="id25" name="id25">Proposed Text</a><ul>
<li><a class="reference" href="#header-iterator-helper-synopsis-lib-iterator-helper-synopsis" id="id26" name="id26">Header <tt class="literal"><span class="pre">&lt;iterator_helper&gt;</span></tt> synopsis    [lib.iterator.helper.synopsis]</a></li>
<li><a class="reference" href="#iterator-facade-lib-iterator-facade" id="id27" name="id27">Iterator facade [lib.iterator.facade]</a><ul>
<li><a class="reference" href="#class-template-iterator-facade" id="id28" name="id28">Class template <tt class="literal"><span class="pre">iterator_facade</span></tt></a><ul>
<li><a class="reference" href="#iterator-facade-usage" id="id29" name="id29"><tt class="literal"><span class="pre">iterator_facade</span></tt> usage</a></li>
<li><a class="reference" href="#iterator-facade-iterator-category" id="id30" name="id30"><tt class="literal"><span class="pre">iterator_facade</span></tt> iterator category</a></li>
<li><a class="reference" href="#iterator-facade-operations" id="id31" name="id31"><tt class="literal"><span class="pre">iterator_facade</span></tt> operations</a></li>
</ul>
</li>
</ul>
</li>
<li><a class="reference" href="#iterator-adaptor-lib-iterator-adaptor" id="id32" name="id32">Iterator adaptor [lib.iterator.adaptor]</a><ul>
<li><a class="reference" href="#class-template-iterator-adaptor" id="id33" name="id33">Class template <tt class="literal"><span class="pre">iterator_adaptor</span></tt></a></li>
<li><a class="reference" href="#iterator-adaptor-base-class-parameters" id="id34" name="id34"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> base class parameters</a></li>
<li><a class="reference" href="#iterator-adaptor-usage" id="id35" name="id35"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> usage</a></li>
<li><a class="reference" href="#iterator-adaptor-public-operations" id="id36" name="id36"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> public operations</a></li>
<li><a class="reference" href="#iterator-adaptor-protected-member-functions" id="id37" name="id37"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> protected member functions</a></li>
<li><a class="reference" href="#iterator-adaptor-private-member-functions" id="id38" name="id38"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> private member functions</a></li>
</ul>
</li>
<li><a class="reference" href="#specialized-adaptors-lib-iterator-special-adaptors" id="id39" name="id39">Specialized adaptors [lib.iterator.special.adaptors]</a><ul>
<li><a class="reference" href="#indirect-iterator" id="id40" name="id40">Indirect iterator</a><ul>
<li><a class="reference" href="#class-template-indirect-iterator" id="id41" name="id41">Class template <tt class="literal"><span class="pre">indirect_iterator</span></tt></a></li>
<li><a class="reference" href="#indirect-iterator-requirements" id="id42" name="id42"><tt class="literal"><span class="pre">indirect_iterator</span></tt> requirements</a></li>
<li><a class="reference" href="#indirect-iterator-operations" id="id43" name="id43"><tt class="literal"><span class="pre">indirect_iterator</span></tt> operations</a></li>
</ul>
</li>
<li><a class="reference" href="#reverse-iterator" id="id44" name="id44">Reverse iterator</a><ul>
<li><a class="reference" href="#class-template-reverse-iterator" id="id45" name="id45">Class template <tt class="literal"><span class="pre">reverse_iterator</span></tt></a></li>
<li><a class="reference" href="#reverse-iterator-requirements" id="id46" name="id46"><tt class="literal"><span class="pre">reverse_iterator</span></tt> requirements</a></li>
</ul>
</li>
<li><a class="reference" href="#transform-iterator" id="id47" name="id47">Transform iterator</a><ul>
<li><a class="reference" href="#class-template-transform-iterator" id="id48" name="id48">Class template <tt class="literal"><span class="pre">transform_iterator</span></tt></a></li>
<li><a class="reference" href="#transform-iterator-requirements" id="id49" name="id49"><tt class="literal"><span class="pre">transform_iterator</span></tt> requirements</a></li>
<li><a class="reference" href="#transform-iterator-public-operations" id="id50" name="id50"><tt class="literal"><span class="pre">transform_iterator</span></tt> public operations</a></li>
<li><a class="reference" href="#transform-iterator-private-operations" id="id51" name="id51"><tt class="literal"><span class="pre">transform_iterator</span></tt> private operations</a></li>
</ul>
</li>
<li><a class="reference" href="#filter-iterator" id="id52" name="id52">Filter iterator</a><ul>
<li><a class="reference" href="#class-template-filter-iterator" id="id53" name="id53">Class template <tt class="literal"><span class="pre">filter_iterator</span></tt></a></li>
</ul>
</li>
<li><a class="reference" href="#filter-iterator-requirements" id="id54" name="id54"><tt class="literal"><span class="pre">filter_iterator</span></tt> requirements</a></li>
<li><a class="reference" href="#filter-iterator-operations" id="id55" name="id55"><tt class="literal"><span class="pre">filter_iterator</span></tt> operations</a></li>
<li><a class="reference" href="#counting-iterator" id="id56" name="id56">Counting iterator</a><ul>
<li><a class="reference" href="#class-template-counting-iterator" id="id57" name="id57">Class template <tt class="literal"><span class="pre">counting_iterator</span></tt></a></li>
</ul>
</li>
<li><a class="reference" href="#counting-iterator-requirements" id="id58" name="id58"><tt class="literal"><span class="pre">counting_iterator</span></tt> requirements</a></li>
<li><a class="reference" href="#counting-iterator-operations" id="id59" name="id59"><tt class="literal"><span class="pre">counting_iterator</span></tt> operations</a></li>
<li><a class="reference" href="#function-output-iterator" id="id60" name="id60">Function output iterator</a><ul>
<li><a class="reference" href="#class-template-function-output-iterator" id="id61" name="id61">Class template <tt class="literal"><span class="pre">function_output_iterator</span></tt></a></li>
</ul>
</li>
<li><a class="reference" href="#function-output-iterator-requirements" id="id62" name="id62"><tt class="literal"><span class="pre">function_output_iterator</span></tt> requirements</a></li>
<li><a class="reference" href="#function-output-iterator-operations" id="id63" name="id63"><tt class="literal"><span class="pre">function_output_iterator</span></tt> operations</a></li>
<li><a class="reference" href="#function-output-iterator-output-proxy-operations" id="id64" name="id64"><tt class="literal"><span class="pre">function_output_iterator::output_proxy</span></tt> operations</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</div>
<div class="section" id="motivation">
<h1><a class="toc-backref" href="#id13" name="motivation">Motivation</a></h1>
<p>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 <a class="footnote-reference" href="#mutable" id="id1" name="id1"><sup>2</sup></a>, and also reverse versions of those
same iterators which traverse the container in the opposite direction.
The Standard also supplies <tt class="literal"><span class="pre">istream_iterator</span></tt> and
<tt class="literal"><span class="pre">ostream_iterator</span></tt> for reading from and writing to streams,
<tt class="literal"><span class="pre">insert_iterator</span></tt>, <tt class="literal"><span class="pre">front_insert_iterator</span></tt> and
<tt class="literal"><span class="pre">back_insert_iterator</span></tt> for inserting elements into containers, and
<tt class="literal"><span class="pre">raw_storage_iterator</span></tt> for initializing raw memory [7].</p>
<p>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.</p>
<p>Although it is easy to create iterators that <em>almost</em> conform to the
standard, the iterator requirements contain subtleties which can make
creating an iterator which <em>actually</em> 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
<tt class="literal"><span class="pre">iterator_facade</span></tt>, 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
<tt class="literal"><span class="pre">iterator_facade</span></tt> 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 <tt class="literal"><span class="pre">iterator_facade</span></tt>.</p>
<p>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 <tt class="literal"><span class="pre">reverse_iterator</span></tt>, 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:</p>
<ul class="simple">
<li>Checked iter[13] adds bounds-checking to an existing iterator.</li>
<li>The iterators of the View Template Library[14], which adapts
containers, are themselves adaptors over the underlying iterators.</li>
<li>Smart iterators [5] adapt an iterator's dereferencing behavior by
applying a function object to the object being referenced and
returning the result.</li>
<li>Custom iterators [4], in which a variety of adaptor types are enumerated.</li>
<li>Compound iterators [1], which access a slice out of a container of containers.</li>
<li>Several iterator adaptors from the MTL [12].  The MTL contains a
strided iterator, where each call to <tt class="literal"><span class="pre">operator++()</span></tt> moves the
iterator ahead by some constant factor, and a scaled iterator, which
multiplies the dereferenced value by some constant.</li>
</ul>
<table class="footnote" frame="void" id="concept" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a name="concept">[1]</a></td><td>We use the term concept to mean a set of requirements
that a type must satisfy to be used with a particular template
parameter.</td></tr>
</tbody>
</table>
<table class="footnote" frame="void" id="mutable" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1" name="mutable">[2]</a></td><td>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.</td></tr>
</tbody>
</table>
<p>To fulfill the need for constructing adaptors, we propose the
<tt class="literal"><span class="pre">iterator_adaptor</span></tt> class template.  Instantiations of
<tt class="literal"><span class="pre">iterator_adaptor</span></tt> 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 <tt class="literal"><span class="pre">transform_iterator</span></tt> that
applies some user-specified function during the dereference of the
iterator.</p>
</div>
<div class="section" id="impact-on-the-standard">
<h1><a class="toc-backref" href="#id14" name="impact-on-the-standard">Impact on the Standard</a></h1>
<p>This proposal is purely an addition to the C++ standard library.
However, note that this proposal relies on the proposal for New
Iterator Concepts.</p>
</div>
<div class="section" id="design">
<h1><a class="toc-backref" href="#id15" name="design">Design</a></h1>
<div class="section" id="iterator-concepts">
<h2><a class="toc-backref" href="#id16" name="iterator-concepts">Iterator Concepts</a></h2>
<p>This proposal is formulated in terms of the new <tt class="literal"><span class="pre">iterator</span> <span class="pre">concepts</span></tt>
as proposed in <a class="reference" href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/papers/2003/n1550.html">n1550</a>, since user-defined and especially adapted
iterators suffer from the well known categorization problems that are
inherent to the current iterator categories.</p>
<p>This proposal does not strictly depend on proposal <a class="reference" href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/papers/2003/n1550.html">n1550</a>, as there
is a direct mapping between new and old categories. This proposal
could be reformulated using this mapping if <a class="reference" href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/papers/2003/n1550.html">n1550</a> was not accepted.</p>
</div>
<div class="section" id="interoperability">
<h2><a class="toc-backref" href="#id17" name="interoperability">Interoperability</a></h2>
<p>The question of iterator interoperability is poorly addressed in the
current standard.  There are currently two defect reports that are
concerned with interoperability issues.</p>
<p>Issue <a class="reference" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html#179">179</a> 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
<a class="reference" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html#179">179</a>, 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.</p>
<p>Issue <a class="reference" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-active.html#280">280</a> 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.</p>
</div>
<div class="section" id="iterator-facade">
<h2><a class="toc-backref" href="#id18" name="iterator-facade">Iterator Facade</a></h2>
<!-- Version 1.1 of this ReStructuredText document corresponds to
n1530_, the paper accepted by the LWG for TR1. -->
<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. All
rights reserved -->
<p>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:</p>
<ul class="simple">
<li>dereferencing</li>
<li>incrementing</li>
<li>decrementing</li>
<li>equality comparison</li>
<li>random-access motion</li>
<li>distance measurement</li>
</ul>
<p>In addition to the behaviors listed above, the core interface elements
include the associated types exposed through iterator traits:
<tt class="literal"><span class="pre">value_type</span></tt>, <tt class="literal"><span class="pre">reference</span></tt>, <tt class="literal"><span class="pre">difference_type</span></tt>, and
<tt class="literal"><span class="pre">iterator_category</span></tt>.</p>
<p>Iterator facade uses the Curiously Recurring Template Pattern (CRTP)
<a class="citation-reference" href="#cop95" id="id4" name="id4">[Cop95]</a> so that the user can specify the behavior of
<tt class="literal"><span class="pre">iterator_facade</span></tt> in a derived class.  Former designs used policy
objects to specify the behavior.  <tt class="literal"><span class="pre">iterator_facade</span></tt> does not use policy
objects for several reasons:</p>
<blockquote>
<ol class="arabic simple">
<li>the creation and eventual copying of the policy object may create
overhead that can be avoided with the current approach.</li>
<li>The policy object approach does not allow for custom constructors
on the created iterator types, an essential feature if
<tt class="literal"><span class="pre">iterator_facade</span></tt> should be used in other library
implementations.</li>
<li>Without the use of CRTP, the standard requirement that an
iterator's <tt class="literal"><span class="pre">operator++</span></tt> returns the iterator type itself means
that all iterators generated by <tt class="literal"><span class="pre">iterator_facade</span></tt> would be
specializations of <tt class="literal"><span class="pre">iterator_facade</span></tt>.  Cumbersome type generator
metafunctions would be needed to build new parameterized
iterators, and a separate <tt class="literal"><span class="pre">iterator_adaptor</span></tt> layer would be
impossible.</li>
</ol>
</blockquote>
<div class="section" id="usage">
<h3><a class="toc-backref" href="#id19" name="usage">Usage</a></h3>
<p>The user of <tt class="literal"><span class="pre">iterator_facade</span></tt> derives his iterator class from an
specialization of <tt class="literal"><span class="pre">iterator_facade</span></tt> which takes the derived iterator
class as the first template parameter.  The order of the other
template parameters to <tt class="literal"><span class="pre">iterator_facade</span></tt> 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 <tt class="literal"><span class="pre">value_type</span></tt> as <tt class="literal"><span class="pre">iterator_facade</span></tt>'s <tt class="literal"><span class="pre">Value</span></tt>
parameter and omit the <tt class="literal"><span class="pre">Reference</span></tt> parameter which follows.</p>
<p>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.</p>
<blockquote>
<table class="table" frame="border" rules="all">
<colgroup>
<col width="44%" />
<col width="56%" />
</colgroup>
<thead valign="bottom">
<tr><th>Expression</th>
<th>Effects</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">i.dereference()</span></tt></td>
<td>Access the value referred to</td>
</tr>
<tr><td><tt class="literal"><span class="pre">i.equal(j)</span></tt></td>
<td>Compare for equality with <tt class="literal"><span class="pre">j</span></tt></td>
</tr>
<tr><td><tt class="literal"><span class="pre">i.increment()</span></tt></td>
<td>Advance by one position</td>
</tr>
<tr><td><tt class="literal"><span class="pre">i.decrement()</span></tt></td>
<td>Retreat by one position</td>
</tr>
<tr><td><tt class="literal"><span class="pre">i.advance(n)</span></tt></td>
<td>Advance by <tt class="literal"><span class="pre">n</span></tt> positions</td>
</tr>
<tr><td><tt class="literal"><span class="pre">i.distance_to(j)</span></tt></td>
<td>Measure the distance to <tt class="literal"><span class="pre">j</span></tt></td>
</tr>
</tbody>
</table>
</blockquote>
<!-- Should we add a comment that a zero overhead implementation of iterator_facade
is possible with proper inlining? -->
<p>In addition to implementing the core interface functions, an iterator
derived from <tt class="literal"><span class="pre">iterator_facade</span></tt> 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 <tt class="literal"><span class="pre">X</span></tt> is meant to be automatically interoperate with another
iterator type <tt class="literal"><span class="pre">Y</span></tt> (as with constant and mutable iterators) then
there must be an implicit conversion from <tt class="literal"><span class="pre">X</span></tt> to <tt class="literal"><span class="pre">Y</span></tt> or from <tt class="literal"><span class="pre">Y</span></tt>
to <tt class="literal"><span class="pre">X</span></tt> (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.</p>
</div>
<div class="section" id="iterator-core-access">
<h3><a class="toc-backref" href="#id20" name="iterator-core-access">Iterator Core Access</a></h3>
<p><tt class="literal"><span class="pre">iterator_facade</span></tt> 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.</p>
<p>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 <tt class="literal"><span class="pre">iterator_facade</span></tt> without fear of
breaking code that was accessing the public core member functions
directly.</p>
<p>In a naive implementation, keeping the derived class' core member
functions private would require it to grant friendship to
<tt class="literal"><span class="pre">iterator_facade</span></tt> and each of the seven operators.  In order to
reduce the burden of limiting access, <tt class="literal"><span class="pre">iterator_core_access</span></tt> 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 <tt class="literal"><span class="pre">iterator_core_access</span></tt> to make his core
member functions available to the library.</p>
<!-- This is no long uptodate -thw  -->
<!-- Yes it is; I made sure of it! -DWA -->
<p><tt class="literal"><span class="pre">iterator_core_access</span></tt> 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
<tt class="literal"><span class="pre">iterator_core_access</span></tt> used public member functions it would not
open a safety loophole, as every core member function preserves the
invariants of the iterator.</p>
</div>
<div class="section" id="operator">
<h3><a class="toc-backref" href="#id21" name="operator"><tt class="literal"><span class="pre">operator[]</span></tt></a></h3>
<p>The indexing operator for a generalized iterator presents special
challenges.  A random access iterator's <tt class="literal"><span class="pre">operator[]</span></tt> is only
required to return something convertible to its <tt class="literal"><span class="pre">value_type</span></tt>.
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. <a class="reference" href="#counting"><tt class="literal"><span class="pre">counting_iterator</span></tt></a>), because <tt class="literal"><span class="pre">*(p+n)</span></tt> is a reference
into the temporary iterator <tt class="literal"><span class="pre">p+n</span></tt>, which is destroyed when
<tt class="literal"><span class="pre">operator[]</span></tt> returns.</p>
<p>Writable iterators built with <tt class="literal"><span class="pre">iterator_facade</span></tt> implement the
semantics required by the preferred resolution to <a class="reference" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-active.html#299">issue 299</a> and
adopted by proposal <a class="reference" href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/papers/2003/n1550.html">n1550</a>: the result of <tt class="literal"><span class="pre">p[n]</span></tt> is a proxy object
containing a copy of <tt class="literal"><span class="pre">p+n</span></tt>, and <tt class="literal"><span class="pre">p[n]</span> <span class="pre">=</span> <span class="pre">x</span></tt> is equivalent to <tt class="literal"><span class="pre">*(p</span>
<span class="pre">+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">x</span></tt>.  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 <tt class="literal"><span class="pre">operator[]</span></tt> which returns an lvalue in the derived
iterator class; it will hide the one supplied by <tt class="literal"><span class="pre">iterator_facade</span></tt>
from clients of her iterator.</p>
<a class="target" id="operator-arrow" name="operator-arrow"></a></div>
<div class="section" id="id6">
<h3><a class="toc-backref" href="#id22" name="id6"><tt class="literal"><span class="pre">operator-&gt;</span></tt></a></h3>
<p>The <tt class="literal"><span class="pre">reference</span></tt> 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 <tt class="literal"><span class="pre">value_type</span></tt>.  When the <tt class="literal"><span class="pre">value_type</span></tt>
is a class, however, it must still be possible to access members
through <tt class="literal"><span class="pre">operator-&gt;</span></tt>.  Therefore, an iterator whose <tt class="literal"><span class="pre">reference</span></tt>
type is not in fact a reference must return a proxy containing a copy
of the referenced value from its <tt class="literal"><span class="pre">operator-&gt;</span></tt>.</p>
<p>The return type for <tt class="literal"><span class="pre">operator-&gt;</span></tt> and <tt class="literal"><span class="pre">operator[]</span></tt> is not
explicitly specified. Instead it requires each <tt class="literal"><span class="pre">iterator_facade</span></tt>
specialization to meet the requirements of its <tt class="literal"><span class="pre">iterator_category</span></tt>.</p>
<table class="citation" frame="void" id="cop95" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<col />
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id4" name="cop95">[Cop95]</a></td><td>[Coplien, 1995] Coplien, J., Curiously Recurring Template
Patterns, C++ Report, February 1995, pp. 24-27.</td></tr>
</tbody>
</table>
</div>
</div>
<div class="section" id="iterator-adaptor">
<h2><a class="toc-backref" href="#id23" name="iterator-adaptor">Iterator Adaptor</a></h2>
<!-- Version 1.2 of this ReStructuredText document corresponds to
n1530_, the paper accepted by the LWG for TR1. -->
<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. All
rights reserved -->
<p>The <tt class="literal"><span class="pre">iterator_adaptor</span></tt> class template adapts some <tt class="literal"><span class="pre">Base</span></tt> <a class="footnote-reference" href="#base" id="id7" name="id7"><sup>3</sup></a>
type to create a new iterator.  Instantiations of <tt class="literal"><span class="pre">iterator_adaptor</span></tt>
are derived from a corresponding instantiation of <tt class="literal"><span class="pre">iterator_facade</span></tt>
and implement the core behaviors in terms of the <tt class="literal"><span class="pre">Base</span></tt> type. In
essence, <tt class="literal"><span class="pre">iterator_adaptor</span></tt> merely forwards all operations to an
instance of the <tt class="literal"><span class="pre">Base</span></tt> type, which it stores as a member.</p>
<table class="footnote" frame="void" id="base" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id7" name="base">[3]</a></td><td>The term &quot;Base&quot; 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 <tt class="literal"><span class="pre">reverse_iterator</span></tt> adaptor.</td></tr>
</tbody>
</table>
<p>The user of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> creates a class derived from an
instantiation of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> and then selectively
redefines some of the core member functions described in the table
above. The <tt class="literal"><span class="pre">Base</span></tt> type need not meet the full requirements for an
iterator. It need only support the operations used by the core
interface functions of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> that have not been
redefined in the user's derived class.</p>
<p>Several of the template parameters of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> default
to <tt class="literal"><span class="pre">use_default</span></tt>. 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
<tt class="literal"><span class="pre">use_default</span></tt> can help to simplify the implementation.  Finally,
the identity of the <tt class="literal"><span class="pre">use_default</span></tt> type is not left unspecified
because specification helps to highlight that the <tt class="literal"><span class="pre">Reference</span></tt>
template parameter may not always be identical to the iterator's
<tt class="literal"><span class="pre">reference</span></tt> type, and will keep users from making mistakes based on
that assumption.</p>
</div>
<div class="section" id="specialized-adaptors">
<h2><a class="toc-backref" href="#id24" name="specialized-adaptors">Specialized Adaptors</a></h2>
<p>This proposal also contains several examples of specialized adaptors
which were easily implemented using <tt class="literal"><span class="pre">iterator_adaptor</span></tt>:</p>
<ul class="simple">
<li><tt class="literal"><span class="pre">indirect_iterator</span></tt>, which iterates over iterators, pointers,
or smart pointers and applies an extra level of dereferencing.</li>
<li>A new <tt class="literal"><span class="pre">reverse_iterator</span></tt>, 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).</li>
<li><tt class="literal"><span class="pre">transform_iterator</span></tt>, which applies a user-defined function object
to the underlying values when dereferenced.</li>
<li><tt class="literal"><span class="pre">projection_iterator</span></tt>, which is similar to <tt class="literal"><span class="pre">transform_iterator</span></tt>
except that when dereferenced it returns a reference instead of
a value.</li>
<li><tt class="literal"><span class="pre">filter_iterator</span></tt>, which provides a view of an iterator range in
which some elements of the underlying range are skipped.</li>
</ul>
<a class="target" id="counting" name="counting"></a><ul class="simple">
<li><tt class="literal"><span class="pre">counting_iterator</span></tt>, which adapts any incrementable type
(e.g. integers, iterators) so that incrementing/decrementing the
adapted iterator and dereferencing it produces successive values of
the Base type.</li>
<li><tt class="literal"><span class="pre">function_output_iterator</span></tt>, which makes it easier to create custom
output iterators.</li>
</ul>
<p>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).</p>
</div>
</div>
<div class="section" id="proposed-text">
<h1><a class="toc-backref" href="#id25" name="proposed-text">Proposed Text</a></h1>
<div class="section" id="header-iterator-helper-synopsis-lib-iterator-helper-synopsis">
<h2><a class="toc-backref" href="#id26" name="header-iterator-helper-synopsis-lib-iterator-helper-synopsis">Header <tt class="literal"><span class="pre">&lt;iterator_helper&gt;</span></tt> synopsis    [lib.iterator.helper.synopsis]</a></h2>
<pre class="literal-block">
struct use_default;

struct iterator_core_access { /* implementation detail */ };

template &lt;
    class Derived
  , class Value
  , class CategoryOrTraversal
  , class Reference  = Value&amp;
  , class Difference = ptrdiff_t
&gt;
class iterator_facade;

template &lt;
    class Derived
  , class Base
  , class Value      = use_default
  , class CategoryOrTraversal  = use_default
  , class Reference  = use_default
  , class Difference = use_default
&gt;
class iterator_adaptor;

template &lt;
    class Iterator
  , class Value = use_default
  , class CategoryOrTraversal = use_default
  , class Reference = use_default
  , class Difference = use_default
&gt;
class indirect_iterator;

template &lt;class Iterator&gt;
class reverse_iterator;

template &lt;
    class UnaryFunction
  , class Iterator
  , class Reference = use_default
  , class Value = use_default
&gt;
class transform_iterator;

template &lt;class Predicate, class Iterator&gt;
class filter_iterator;

template &lt;
    class Incrementable
  , class CategoryOrTraversal  = use_default
  , class Difference = use_default
&gt;
class counting_iterator

template &lt;class UnaryFunction&gt;
class function_output_iterator;
</pre>
</div>
<div class="section" id="iterator-facade-lib-iterator-facade">
<h2><a class="toc-backref" href="#id27" name="iterator-facade-lib-iterator-facade">Iterator facade [lib.iterator.facade]</a></h2>
<p><tt class="literal"><span class="pre">iterator_facade</span></tt> 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.</p>
<div class="section" id="class-template-iterator-facade">
<h3><a class="toc-backref" href="#id28" name="class-template-iterator-facade">Class template <tt class="literal"><span class="pre">iterator_facade</span></tt></a></h3>
<!-- Version 1.3 of this ReStructuredText document corresponds to
n1530_, the paper accepted by the LWG for TR1. -->
<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. All
rights reserved -->
<pre class="literal-block">
template &lt;
    class Derived
  , class Value
  , class CategoryOrTraversal
  , class Reference  = Value&amp;
  , class Difference = ptrdiff_t
&gt;
class iterator_facade {
public:
    typedef remove_const&lt;Value&gt;::type value_type;
    typedef Reference reference;
    typedef Value* pointer;
    typedef Difference difference_type;
    typedef /* see <a class="reference" href="#facade-iterator-category">below</a> */ iterator_category;

    reference operator*() const;
    /* see <a class="reference" href="#operator-arrow">below</a> */ operator-&gt;() const;
    /* see <a class="reference" href="#brackets">below</a> */ operator[](difference_type n) const;
    Derived&amp; operator++();
    Derived operator++(int);
    Derived&amp; operator--();
    Derived operator--(int);
    Derived&amp; operator+=(difference_type n);
    Derived&amp; operator-=(difference_type n);
    Derived operator-(difference_type n) const;
};

// Comparison operators
template &lt;class Dr1, class V1, class TC1, class R1, class D1,
          class Dr2, class V2, class TC2, class R2, class D2&gt;
typename enable_if_interoperable&lt;Dr1, Dr2, bool&gt;::type // exposition
operator ==(iterator_facade&lt;Dr1, V1, TC1, R1, D1&gt; const&amp; lhs,
            iterator_facade&lt;Dr2, V2, TC2, R2, D2&gt; const&amp; rhs);

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

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

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

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

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

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

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

// Iterator addition
template &lt;class Derived, class V, class TC, class R, class D&gt;
Derived operator+ (iterator_facade&lt;Derived, V, TC, R, D&gt; const&amp;,
                   typename Derived::difference_type n)
</pre>
<p>The <tt class="literal"><span class="pre">enable_if_interoperable</span></tt> template used above is for exposition
purposes.  The member operators should be only be in an overload set
provided the derived types <tt class="literal"><span class="pre">Dr1</span></tt> and <tt class="literal"><span class="pre">Dr2</span></tt> are interoperable,
meaning that at least one of the types is convertible to the other.  The
<tt class="literal"><span class="pre">enable_if_interoperable</span></tt> approach uses SFINAE to take the operators
out of the overload set when the types are not interoperable.
The operators should behave <em>as-if</em> <tt class="literal"><span class="pre">enable_if_interoperable</span></tt>
were defined to be:</p>
<pre class="literal-block">
template &lt;bool, typename&gt; enable_if_interoperable_impl
{};

template &lt;typename T&gt; enable_if_interoperable_impl&lt;true,T&gt;
{ typedef T type; };

template&lt;typename Dr1, typename Dr2, typename T&gt;
struct enable_if_interoperable
  : enable_if_interoperable_impl&lt;
        is_convertible&lt;Dr1,Dr2&gt;::value || is_convertible&lt;Dr2,Dr1&gt;::value
      , T
    &gt;
{};
</pre>
<div class="section" id="iterator-facade-usage">
<h4><a class="toc-backref" href="#id29" name="iterator-facade-usage"><tt class="literal"><span class="pre">iterator_facade</span></tt> usage</a></h4>
<p>The following table describes the typical valid expressions on
<tt class="literal"><span class="pre">iterator_facade</span></tt>'s <tt class="literal"><span class="pre">Derived</span></tt> 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
<tt class="literal"><span class="pre">iterator_core_access</span></tt>.</p>
<p>In the table below, <tt class="literal"><span class="pre">F</span></tt> is <tt class="literal"><span class="pre">iterator_facade&lt;X,V,C,R,D&gt;</span></tt>, <tt class="literal"><span class="pre">a</span></tt> is an
object of type <tt class="literal"><span class="pre">X</span></tt>, <tt class="literal"><span class="pre">b</span></tt> and <tt class="literal"><span class="pre">c</span></tt> are objects of type <tt class="literal"><span class="pre">const</span> <span class="pre">X</span></tt>,
<tt class="literal"><span class="pre">n</span></tt> is an object of <tt class="literal"><span class="pre">F::difference_type</span></tt>, <tt class="literal"><span class="pre">y</span></tt> is a constant
object of a single pass iterator type interoperable with <tt class="literal"><span class="pre">X</span></tt>, and <tt class="literal"><span class="pre">z</span></tt>
is a constant object of a random access traversal iterator type
interoperable with <tt class="literal"><span class="pre">X</span></tt>.</p>
<table class="table" frame="border" rules="all">
<colgroup>
<col width="19%" />
<col width="21%" />
<col width="35%" />
<col width="25%" />
</colgroup>
<thead valign="bottom">
<tr><th>Expression</th>
<th>Return Type</th>
<th>Assertion/Note</th>
<th>Used to implement Iterator
Concept(s)</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">c.dereference()</span></tt></td>
<td><tt class="literal"><span class="pre">F::reference</span></tt></td>
<td>&nbsp;</td>
<td>Readable Iterator, Writable
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">c.equal(b)</span></tt></td>
<td>convertible to bool</td>
<td>true iff <tt class="literal"><span class="pre">b</span></tt> and <tt class="literal"><span class="pre">c</span></tt> are
equivalent.</td>
<td>Single Pass Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">c.equal(y)</span></tt></td>
<td>convertible to bool</td>
<td>true iff <tt class="literal"><span class="pre">c</span></tt> and <tt class="literal"><span class="pre">y</span></tt> refer to the
same position.  Implements <tt class="literal"><span class="pre">c</span> <span class="pre">==</span> <span class="pre">y</span></tt>
and <tt class="literal"><span class="pre">c</span> <span class="pre">!=</span> <span class="pre">y</span></tt>.</td>
<td>Single Pass Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">a.advance(n)</span></tt></td>
<td>unused</td>
<td>&nbsp;</td>
<td>Random Access Traversal
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">a.increment()</span></tt></td>
<td>unused</td>
<td>&nbsp;</td>
<td>Incrementable Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">a.decrement()</span></tt></td>
<td>unused</td>
<td>&nbsp;</td>
<td>Bidirectional Traversal
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">c.distance_to(b)</span></tt></td>
<td>convertible to
<tt class="literal"><span class="pre">F::difference_type</span></tt></td>
<td>equivalent to <tt class="literal"><span class="pre">distance(c,</span> <span class="pre">b)</span></tt></td>
<td>Random Access Traversal
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">c.distance_to(z)</span></tt></td>
<td>convertible to
<tt class="literal"><span class="pre">F::difference_type</span></tt></td>
<td>equivalent to <tt class="literal"><span class="pre">distance(c,</span> <span class="pre">z)</span></tt>.
Implements <tt class="literal"><span class="pre">c</span> <span class="pre">-</span> <span class="pre">z</span></tt>, <tt class="literal"><span class="pre">c</span> <span class="pre">&lt;</span> <span class="pre">z</span></tt>, <tt class="literal"><span class="pre">c</span>
<span class="pre">&lt;=</span> <span class="pre">z</span></tt>, <tt class="literal"><span class="pre">c</span> <span class="pre">&gt;</span> <span class="pre">z</span></tt>, and <tt class="literal"><span class="pre">c</span> <span class="pre">&gt;=</span> <span class="pre">c</span></tt>.</td>
<td>Random Access Traversal
Iterator</td>
</tr>
</tbody>
</table>
<a class="target" id="facade-iterator-category" name="facade-iterator-category"></a></div>
<div class="section" id="iterator-facade-iterator-category">
<h4><a class="toc-backref" href="#id30" name="iterator-facade-iterator-category"><tt class="literal"><span class="pre">iterator_facade</span></tt> iterator category</a></h4>
<p>The <tt class="literal"><span class="pre">iterator_category</span></tt> member of <tt class="literal"><span class="pre">iterator_facade&lt;X,V,R,C,D&gt;</span></tt>
is a type which satisfies the following conditions:</p>
<blockquote>
<ul>
<li><p class="first">if <tt class="literal"><span class="pre">C</span></tt> is convertible to <tt class="literal"><span class="pre">std::input_iterator_tag</span></tt> or
<tt class="literal"><span class="pre">C</span></tt> is convertible to <tt class="literal"><span class="pre">std::output_iterator_tag</span></tt>,
<tt class="literal"><span class="pre">iterator_category</span></tt> is the same as <tt class="literal"><span class="pre">C</span></tt>.</p>
</li>
<li><p class="first">Otherwise, if <tt class="literal"><span class="pre">C</span></tt> is not convertible to
<tt class="literal"><span class="pre">incrementable_traversal_tag</span></tt>, the program is ill-formed</p>
</li>
<li><p class="first">Otherwise:</p>
<ul>
<li><p class="first"><tt class="literal"><span class="pre">iterator_category</span></tt> 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:</p>
<pre class="literal-block">
if (R is a reference type
    &amp;&amp; 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
        &amp;&amp; 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
}
</pre>
</li>
<li><p class="first"><tt class="literal"><span class="pre">iterator_traversal&lt;X&gt;::type</span></tt> is convertible to the most
derived traversal tag type to which <tt class="literal"><span class="pre">C</span></tt> is also
convertible, and not to any more-derived traversal tag type.</p>
</li>
</ul>
</li>
</ul>
</blockquote>
</div>
<div class="section" id="iterator-facade-operations">
<h4><a class="toc-backref" href="#id31" name="iterator-facade-operations"><tt class="literal"><span class="pre">iterator_facade</span></tt> operations</a></h4>
<p>The operations in this section are described in terms of operations on
the core interface of <tt class="literal"><span class="pre">Derived</span></tt> which may be inaccessible
(i.e. private).  The implementation should access these operations
through member functions of class <tt class="literal"><span class="pre">iterator_core_access</span></tt>.</p>
<p><tt class="literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">static_cast&lt;Derived</span> <span class="pre">const*&gt;(this)-&gt;dereference()</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">operator-&gt;()</span> <span class="pre">const;</span></tt> (see <a class="reference" href="#operator-arrow">below</a>)</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">If <tt class="literal"><span class="pre">reference</span></tt> is a reference type, an object
of type <tt class="literal"><span class="pre">pointer</span></tt> equal to:</p>
<pre class="literal-block">
&amp;static_cast&lt;Derived const*&gt;(this)-&gt;dereference()
</pre>
<p class="last">Otherwise returns an object of unspecified type such that,
<tt class="literal"><span class="pre">(*static_cast&lt;Derived</span> <span class="pre">const*&gt;(this))-&gt;m</span></tt> is equivalent to <tt class="literal"><span class="pre">(w</span> <span class="pre">=</span> <span class="pre">**static_cast&lt;Derived</span> <span class="pre">const*&gt;(this),</span>
<span class="pre">w.m)</span></tt> for some temporary object <tt class="literal"><span class="pre">w</span></tt> of type <tt class="literal"><span class="pre">value_type</span></tt>.</p>
</td>
</tr>
</tbody>
</table>
<a class="target" id="brackets" name="brackets"></a><p><em>unspecified</em> <tt class="literal"><span class="pre">operator[](difference_type</span> <span class="pre">n)</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">an object convertible to <tt class="literal"><span class="pre">reference</span></tt> and holding a copy
<em>p</em> of <tt class="literal"><span class="pre">*static_cast&lt;Derived</span> <span class="pre">const*&gt;(this)</span> <span class="pre">+</span> <span class="pre">n</span></tt> such that, for a constant object <tt class="literal"><span class="pre">v</span></tt> of type
<tt class="literal"><span class="pre">value_type</span></tt>, <tt class="literal"><span class="pre">(*static_cast&lt;Derived</span> <span class="pre">const*&gt;(this))[n]</span> <span class="pre">=</span> <span class="pre">v</span></tt> is equivalent
to <tt class="literal"><span class="pre">p</span> <span class="pre">=</span> <span class="pre">v</span></tt>.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived&amp;</span> <span class="pre">operator++();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
static_cast&lt;Derived*&gt;(this)-&gt;increment();
return *static_cast&lt;Derived*&gt;(this);
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived</span> <span class="pre">operator++(int);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
Derived tmp(static_cast&lt;Derived const*&gt;(this));
++*this;
return tmp;
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived&amp;</span> <span class="pre">operator--();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
static_cast&lt;Derived*&gt;(this)-&gt;decrement();
return static_cast&lt;Derived*&gt;(this);
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived</span> <span class="pre">operator--(int);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
Derived tmp(static_cast&lt;Derived const*&gt;(this));
--*this;
return tmp;
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived&amp;</span> <span class="pre">operator+=(difference_type</span> <span class="pre">n);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
static_cast&lt;Derived*&gt;(this)-&gt;advance(n);
return static_cast&lt;Derived*&gt;(this);
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived&amp;</span> <span class="pre">operator-=(difference_type</span> <span class="pre">n);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
static_cast&lt;Derived*&gt;(this)-&gt;advance(-n);
return static_cast&lt;Derived*&gt;(this);
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived</span> <span class="pre">operator-(difference_type</span> <span class="pre">n)</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
Derived tmp(static_cast&lt;Derived const*&gt;(this));
return tmp -= n;
</pre>
</td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
<div class="section" id="iterator-adaptor-lib-iterator-adaptor">
<h2><a class="toc-backref" href="#id32" name="iterator-adaptor-lib-iterator-adaptor">Iterator adaptor [lib.iterator.adaptor]</a></h2>
<!-- Version 1.1 of this ReStructuredText document corresponds to
n1530_, the paper accepted by the LWG. -->
<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. All
rights reserved -->
<p>Each specialization of the <tt class="literal"><span class="pre">iterator_adaptor</span></tt> class template is derived from
a specialization of <tt class="literal"><span class="pre">iterator_facade</span></tt>. The core interface functions
expected by <tt class="literal"><span class="pre">iterator_facade</span></tt> are implemented in terms of the
<tt class="literal"><span class="pre">iterator_adaptor</span></tt>'s <tt class="literal"><span class="pre">Base</span></tt> template parameter. A class derived
from <tt class="literal"><span class="pre">iterator_adaptor</span></tt> typically redefines some of the core
interface functions to adapt the behavior of the <tt class="literal"><span class="pre">Base</span></tt> type.
Whether the derived class models any of the standard iterator concepts
depends on the operations supported by the <tt class="literal"><span class="pre">Base</span></tt> type and which
core interface functions of <tt class="literal"><span class="pre">iterator_facade</span></tt> are redefined in the
<tt class="literal"><span class="pre">Derived</span></tt> class.</p>
<div class="section" id="class-template-iterator-adaptor">
<h3><a class="toc-backref" href="#id33" name="class-template-iterator-adaptor">Class template <tt class="literal"><span class="pre">iterator_adaptor</span></tt></a></h3>
<!-- Version 1.4 of this ReStructuredText document corresponds to
n1530_, the paper accepted by the LWG for TR1. -->
<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. All
rights reserved. -->
<pre class="literal-block">
template &lt;
    class Derived
  , class Base
  , class Value        = use_default
  , class CategoryOrTraversal  = use_default
  , class Reference  = use_default
  , class Difference = use_default
&gt;
class iterator_adaptor
  : public iterator_facade&lt;Derived, <em>V</em>, <em>C</em>, <em>R</em>, <em>D</em>&gt; // see <a class="reference" href="#base-parameters">details</a>
{
    friend class iterator_core_access;
 public:
    iterator_adaptor();
    explicit iterator_adaptor(Base iter);
    Base base() const;
 protected:
    Base const&amp; base_reference() const;
    Base&amp; base_reference();
 private: // Core iterator interface for iterator_facade.
    typename iterator_adaptor::reference dereference() const;

    template &lt;
    class OtherDerived, class OtherIterator, class V, class C, class R, class D
    &gt;
    bool equal(iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; x) const;

    void advance(typename iterator_adaptor::difference_type n);
    void increment();
    void decrement();

    template &lt;
        class OtherDerived, class OtherIterator, class V, class C, class R, class D
    &gt;
    typename iterator_adaptor::difference_type distance_to(
        iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; y) const;

 private:
    Base m_iterator; // exposition only
};
</pre>
<a class="target" id="base-parameters" name="base-parameters"></a></div>
<div class="section" id="iterator-adaptor-base-class-parameters">
<h3><a class="toc-backref" href="#id34" name="iterator-adaptor-base-class-parameters"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> base class parameters</a></h3>
<p>The <em>V</em>, <em>C</em>, <em>R</em>, and <em>D</em> parameters of the <tt class="literal"><span class="pre">iterator_facade</span></tt>
used as a base class in the summary of <tt class="literal"><span class="pre">iterator_adaptor</span></tt>
above are defined as follows:</p>
<pre class="literal-block">
<em>V</em> = if (Value is use_default)
          return iterator_traits&lt;Base&gt;::value_type
      else
          return Value

<em>C</em> = if (CategoryOrTraversal is use_default)
          return iterator_traversal&lt;Base&gt;::type
      else
          return CategoryOrTraversal

<em>R</em> = if (Reference is use_default)
          if (Value is use_default)
              return iterator_traits&lt;Base&gt;::reference
          else
              return Value&amp;
      else
          return Reference

<em>D</em> = if (Difference is use_default)
          return iterator_traits&lt;Base&gt;::difference_type
      else
          return Difference
</pre>
</div>
<div class="section" id="iterator-adaptor-usage">
<h3><a class="toc-backref" href="#id35" name="iterator-adaptor-usage"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> usage</a></h3>
<p>The <tt class="literal"><span class="pre">Derived</span></tt> template parameter must be a publicly derived from
<tt class="literal"><span class="pre">iterator_adaptor</span></tt>.  In order for <tt class="literal"><span class="pre">Derived</span></tt> to model the
iterator concepts corresponding to
<tt class="literal"><span class="pre">iterator_traits&lt;Derived&gt;::iterator_category</span></tt>, the expressions
involving <tt class="literal"><span class="pre">m_iterator</span></tt> in the specifications of those private
member functions of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> that may be called by
<tt class="literal"><span class="pre">iterator_facade&lt;Derived,</span> <span class="pre">``\</span> <span class="pre">*V*\</span></tt>, <tt class="literal"><span class="pre">\</span> <span class="pre">*C*\</span></tt>, <tt class="literal"><span class="pre">\</span> <span class="pre">*R*\</span></tt>, <tt class="literal"><span class="pre">\</span>
<span class="pre">*D*\</span></tt>&gt;`` in evaluating any valid expression involving <tt class="literal"><span class="pre">Derived</span></tt>
in those concepts' requirements.</p>
</div>
<div class="section" id="iterator-adaptor-public-operations">
<h3><a class="toc-backref" href="#id36" name="iterator-adaptor-public-operations"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> public operations</a></h3>
<p><tt class="literal"><span class="pre">iterator_adaptor();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Requires:</th><td class="field-body">The <tt class="literal"><span class="pre">Base</span></tt> type must be Default Constructible.</td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> with
<tt class="literal"><span class="pre">m_iterator</span></tt> default constructed.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">explicit</span> <span class="pre">iterator_adaptor(Base</span> <span class="pre">iter);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> with
<tt class="literal"><span class="pre">m_iterator</span></tt> copy constructed from <tt class="literal"><span class="pre">iter</span></tt>.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Base</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">m_iterator</span></tt></td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="iterator-adaptor-protected-member-functions">
<h3><a class="toc-backref" href="#id37" name="iterator-adaptor-protected-member-functions"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> protected member functions</a></h3>
<p><tt class="literal"><span class="pre">Base</span> <span class="pre">const&amp;</span> <span class="pre">base_reference()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">A const reference to <tt class="literal"><span class="pre">m_iterator</span></tt>.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Base&amp;</span> <span class="pre">base_reference();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">A non-const reference to <tt class="literal"><span class="pre">m_iterator</span></tt>.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="iterator-adaptor-private-member-functions">
<h3><a class="toc-backref" href="#id38" name="iterator-adaptor-private-member-functions"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> private member functions</a></h3>
<p><tt class="literal"><span class="pre">typename</span> <span class="pre">iterator_adaptor::reference</span> <span class="pre">dereference()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">*m_iterator</span></tt></td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template &lt;
class OtherDerived, class OtherIterator, class V, class C, class R, class D
&gt;
bool equal(iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; x) const;
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">m_iterator</span> <span class="pre">==</span> <span class="pre">x.base()</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">void</span> <span class="pre">advance(typename</span> <span class="pre">iterator_adaptor::difference_type</span> <span class="pre">n);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="literal"><span class="pre">m_iterator</span> <span class="pre">+=</span> <span class="pre">n;</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">void</span> <span class="pre">increment();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="literal"><span class="pre">++m_iterator;</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">void</span> <span class="pre">decrement();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="literal"><span class="pre">--m_iterator;</span></tt></td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template &lt;
    class OtherDerived, class OtherIterator, class V, class C, class R, class D
&gt;
typename iterator_adaptor::difference_type distance_to(
    iterator_adaptor&lt;OtherDerived, OtherIterator, V, C, R, D&gt; const&amp; y) const;
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">y.base()</span> <span class="pre">-</span> <span class="pre">m_iterator</span></tt></td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="section" id="specialized-adaptors-lib-iterator-special-adaptors">
<h2><a class="toc-backref" href="#id39" name="specialized-adaptors-lib-iterator-special-adaptors">Specialized adaptors [lib.iterator.special.adaptors]</a></h2>
<p>The <tt class="literal"><span class="pre">enable_if_convertible&lt;X,Y&gt;::type</span></tt> 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 <tt class="literal"><span class="pre">X</span></tt> is implicitly convertible to an object of
type <tt class="literal"><span class="pre">Y</span></tt>.
The signatures involving <tt class="literal"><span class="pre">enable_if_convertible</span></tt> should behave
<em>as-if</em> <tt class="literal"><span class="pre">enable_if_convertible</span></tt> were defined to be:</p>
<pre class="literal-block">
template &lt;bool&gt; enable_if_convertible_impl
{};

template &lt;&gt; enable_if_convertible_impl&lt;true&gt;
{ struct type; };

template&lt;typename From, typename To&gt;
struct enable_if_convertible
  : enable_if_convertible_impl&lt;is_convertible&lt;From,To&gt;::value&gt;
{};
</pre>
<p>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 <tt class="literal"><span class="pre">enable_if_convertible</span></tt>, the program is ill-formed, no
diagnostic required.</p>
<p>[<em>Note:</em> The <tt class="literal"><span class="pre">enable_if_convertible</span></tt> approach uses SFINAE to
take the constructor out of the overload set when the types are not
implicitly convertible.
]</p>
<div class="section" id="indirect-iterator">
<h3><a class="toc-backref" href="#id40" name="indirect-iterator">Indirect iterator</a></h3>
<p>The indirect iterator adapts an iterator by applying an <em>extra</em>
dereference inside of <tt class="literal"><span class="pre">operator*()</span></tt>. For example, this iterator
adaptor makes it possible to view a container of pointers
(e.g. <tt class="literal"><span class="pre">list&lt;foo*&gt;</span></tt>) as if it were a container of the pointed-to type
(e.g. <tt class="literal"><span class="pre">list&lt;foo&gt;</span></tt>) .</p>
<!-- At some point we should add the capability to handle
iterators over smart pointers, which the impl handles. -JGS -->
<div class="section" id="class-template-indirect-iterator">
<h4><a class="toc-backref" href="#id41" name="class-template-indirect-iterator">Class template <tt class="literal"><span class="pre">indirect_iterator</span></tt></a></h4>
<pre class="literal-block">
template &lt;
    class Iterator
  , class Value = use_default
  , class CategoryOrTraversal = use_default
  , class Reference = use_default
  , class Difference = use_default
&gt;
class indirect_iterator
  : public iterator_adaptor&lt;
               indirect_iterator&lt;Iterator, Value, Access, Traversal,
                                 Reference, Difference&gt;,
               Iterator,
               /* Value = see below */,
               CategoryOrTraversal,
               Reference,
               Difference&gt;
{
    friend class iterator_core_access;
 public:
    indirect_iterator();
    indirect_iterator(Iterator x);

    template &lt;
        class Iterator2, class Value2, class Category2
      , class Reference2, class Difference2
    &gt;
    indirect_iterator(
        indirect_iterator&lt;
             Iterator2, Value2, Category2, Reference2, Difference2
        &gt; const&amp; y
      , typename enable_if_convertible&lt;Iterator2, Iterator&gt;::type* = 0 // exposition
    );
private: // as-if specification
    typename indirect_iterator::reference dereference() const
    {
        return **this-&gt;base();
    }
};

template &lt;class Dereferenceable&gt;
struct referent {
  typedef /* see below */ type;
};
</pre>
<p>If <tt class="literal"><span class="pre">Value</span></tt> is not <tt class="literal"><span class="pre">use_default</span></tt> then the the argument for the
<tt class="literal"><span class="pre">iterator_adaptor</span></tt> base class' <tt class="literal"><span class="pre">Value</span></tt> parameter is <tt class="literal"><span class="pre">Value</span></tt> with
cv-qualifiers removed. If <tt class="literal"><span class="pre">Value</span></tt> is <tt class="literal"><span class="pre">use_default</span></tt>, then the
argument for the <tt class="literal"><span class="pre">iterator_adaptor</span></tt> base class' <tt class="literal"><span class="pre">Value</span></tt> parameter
is computed as follows. We use the abbreviation
<tt class="literal"><span class="pre">V=iterator_traits&lt;Iterator&gt;::value_type</span></tt> and <tt class="literal"><span class="pre">v</span></tt> is an object of
type <tt class="literal"><span class="pre">V</span></tt>.:</p>
<pre class="literal-block">
if (*v returns a constant lvalue or an rvalue) then
    referent&lt;V&gt;::type
else
    add_const&lt;referent&lt;V&gt;::type&gt;::type
</pre>
<p>The algorithm for the <tt class="literal"><span class="pre">type</span></tt> member of <tt class="literal"><span class="pre">referent</span></tt> traits class is
as follows:</p>
<pre class="literal-block">
if (Dereferenceable is a class and has member element_type)
    Dereferenceable::element_type
else
    iterator_traits&lt;Dereferenceable&gt;::value_type
</pre>
</div>
<div class="section" id="indirect-iterator-requirements">
<h4><a class="toc-backref" href="#id42" name="indirect-iterator-requirements"><tt class="literal"><span class="pre">indirect_iterator</span></tt> requirements</a></h4>
<p>The <tt class="literal"><span class="pre">Iterator</span></tt> type must meet the requirements of Readable
Iterator. Also, the following requirements are placed on
<tt class="literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt>. Let <tt class="literal"><span class="pre">i</span></tt> be an object of
type <tt class="literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt>.  Then <tt class="literal"><span class="pre">*i</span></tt> must be a
valid expression, and the type of <tt class="literal"><span class="pre">*i</span></tt> must be the same as the
<tt class="literal"><span class="pre">iterator_adaptor::reference</span></tt>. Also, there are further requirements
on the <tt class="literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt> if the <tt class="literal"><span class="pre">Value</span></tt>
parameter is not <tt class="literal"><span class="pre">use_default</span></tt>, as implied by the algorithm for deducing
the default.</p>
</div>
<div class="section" id="indirect-iterator-operations">
<h4><a class="toc-backref" href="#id43" name="indirect-iterator-operations"><tt class="literal"><span class="pre">indirect_iterator</span></tt> operations</a></h4>
<p><tt class="literal"><span class="pre">indirect_iterator();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="literal"><span class="pre">Iterator</span></tt> must be Default Constructible.</td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">indirect_iterator</span></tt> with
a default-constructed <tt class="literal"><span class="pre">iterator_adaptor</span></tt> subobject.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">indirect_iterator(Iterator</span> <span class="pre">x);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">indirect_iterator</span></tt> with
the <tt class="literal"><span class="pre">iterator_adaptor</span></tt> subobject copy constructed from <tt class="literal"><span class="pre">x</span></tt>.</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template &lt;
    class Iterator2, class Value2, unsigned Access, class Traversal
  , class Reference2, class Difference2
&gt;
indirect_iterator(
    indirect_iterator&lt;
         Iterator2, Value2, Access, Traversal, Reference2, Difference2
    &gt; const&amp; y
  , typename enable_if_convertible&lt;Iterator2, Iterator&gt;::type* = 0 // exposition
);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="literal"><span class="pre">Iterator2</span></tt> is implicitly convertible to <tt class="literal"><span class="pre">Iterator</span></tt>.</td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">indirect_iterator</span></tt> whose
<tt class="literal"><span class="pre">iterator_adaptor</span></tt> subobject is constructed from <tt class="literal"><span class="pre">y.base()</span></tt>.</td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="section" id="reverse-iterator">
<h3><a class="toc-backref" href="#id44" name="reverse-iterator">Reverse iterator</a></h3>
<!-- I think we'd better strike the old reverse_iterator text from the standard, eh? -->
<p>The reverse iterator adaptor flips the direction of a base iterator's
motion. Invoking <tt class="literal"><span class="pre">operator++()</span></tt> moves the base iterator backward and
invoking <tt class="literal"><span class="pre">operator--()</span></tt> moves the base iterator forward.</p>
<div class="section" id="class-template-reverse-iterator">
<h4><a class="toc-backref" href="#id45" name="class-template-reverse-iterator">Class template <tt class="literal"><span class="pre">reverse_iterator</span></tt></a></h4>
<pre class="literal-block">
template &lt;class Iterator&gt;
class reverse_iterator :
  public iterator_adaptor&lt; reverse_iterator&lt;Iterator&gt;, Iterator &gt;
{
  friend class iterator_core_access;
public:
  reverse_iterator() {}
  explicit reverse_iterator(Iterator x) ;

  template&lt;class OtherIterator&gt;
  reverse_iterator(
      reverse_iterator&lt;OtherIterator&gt; const&amp; r
    , typename enable_if_convertible&lt;OtherIterator, Iterator&gt;::type* = 0 // exposition
  );

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

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

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

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

};
</pre>
</div>
<div class="section" id="reverse-iterator-requirements">
<h4><a class="toc-backref" href="#id46" name="reverse-iterator-requirements"><tt class="literal"><span class="pre">reverse_iterator</span></tt> requirements</a></h4>
<p>The base <tt class="literal"><span class="pre">Iterator</span></tt> must be a model of Bidirectional Traversal
Iterator. The resulting <tt class="literal"><span class="pre">reverse_iterator</span></tt> will be a model of the
most refined standard traversal and access concepts that are modeled
by <tt class="literal"><span class="pre">Iterator</span></tt>.</p>
<p><tt class="literal"><span class="pre">reverse_iterator();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="literal"><span class="pre">Iterator</span></tt> must be Default Constructible.</td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">reverse_iterator</span></tt> with a
default constructed base object.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">explicit</span> <span class="pre">reverse_iterator(Iterator</span> <span class="pre">x);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">reverse_iterator</span></tt> with a
base object copy constructed from <tt class="literal"><span class="pre">x</span></tt>.</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template&lt;class OtherIterator&gt;
reverse_iterator(
    reverse_iterator&lt;OtherIterator&gt; const&amp; r
  , typename enable_if_convertible&lt;OtherIterator, Iterator&gt;::type* = 0 // exposition
);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="literal"><span class="pre">OtherIterator</span></tt> is implicitly convertible to <tt class="literal"><span class="pre">Iterator</span></tt>.</td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">reverse_iterator</span></tt> that is a copy of <tt class="literal"><span class="pre">r</span></tt>.</td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="section" id="transform-iterator">
<h3><a class="toc-backref" href="#id47" name="transform-iterator">Transform iterator</a></h3>
<p>The transform iterator adapts an iterator by applying some function
object to the result of dereferencing the iterator. In other words,
the <tt class="literal"><span class="pre">operator*</span></tt> of the transform iterator first dereferences the
base iterator, passes the result of this to the function object, and
then returns the result.</p>
<div class="section" id="class-template-transform-iterator">
<h4><a class="toc-backref" href="#id48" name="class-template-transform-iterator">Class template <tt class="literal"><span class="pre">transform_iterator</span></tt></a></h4>
<!-- Version 1.3 of this document was accepted for TR1 -->
<pre class="literal-block">
template &lt;class UnaryFunction,
          class Iterator,
          class Reference = use_default,
          class Value = use_default&gt;
class transform_iterator
  : public iterator_adaptor&lt;/* see discussion */&gt;
{
  friend class iterator_core_access;
public:
  transform_iterator();
  transform_iterator(Iterator const&amp; x, UnaryFunction f);

  template&lt;class F2, class I2, class R2, class V2&gt;
  transform_iterator(
        transform_iterator&lt;F2, I2, R2, V2&gt; const&amp; t
      , typename enable_if_convertible&lt;I2, Iterator&gt;::type* = 0      // exposition
      , typename enable_if_convertible&lt;F2, UnaryFunction&gt;::type* = 0 // exposition
  );

  UnaryFunction functor() const;
private:
  typename transform_iterator::value_type dereference() const;
  UnaryFunction m_f;
};
</pre>
</div>
<div class="section" id="transform-iterator-requirements">
<h4><a class="toc-backref" href="#id49" name="transform-iterator-requirements"><tt class="literal"><span class="pre">transform_iterator</span></tt> requirements</a></h4>
<p>The type <tt class="literal"><span class="pre">UnaryFunction</span></tt> must be Assignable, Copy Constructible, and
the expression <tt class="literal"><span class="pre">f(*i)</span></tt> must be valid where <tt class="literal"><span class="pre">f</span></tt> is an object of
type <tt class="literal"><span class="pre">UnaryFunction</span></tt>, <tt class="literal"><span class="pre">i</span></tt> is an object of type <tt class="literal"><span class="pre">Iterator</span></tt>, and
where the type of <tt class="literal"><span class="pre">f(*i)</span></tt> must be
<tt class="literal"><span class="pre">result_of&lt;UnaryFunction(iterator_traits&lt;Iterator&gt;::reference)&gt;::type</span></tt>.</p>
<p>The type <tt class="literal"><span class="pre">Iterator</span></tt> must at least model Readable Iterator.  The
resulting <tt class="literal"><span class="pre">transform_iterator</span></tt> models the most refined of the
following options that is also modeled by <tt class="literal"><span class="pre">Iterator</span></tt>.</p>
<blockquote>
<ul class="simple">
<li>Writable Lvalue Iterator if <tt class="literal"><span class="pre">result_of&lt;UnaryFunction(iterator_traits&lt;Iterator&gt;::reference)&gt;::type</span></tt> is a non-const reference.</li>
<li>Readable Lvalue Iterator if <tt class="literal"><span class="pre">result_of&lt;UnaryFunction(iterator_traits&lt;Iterator&gt;::reference)&gt;::type</span></tt> is a const
reference.</li>
<li>Readable Iterator otherwise.</li>
</ul>
</blockquote>
<p>The <tt class="literal"><span class="pre">transform_iterator</span></tt> models the most refined standard traversal
concept that is modeled by <tt class="literal"><span class="pre">Iterator</span></tt>.</p>
<p>The <tt class="literal"><span class="pre">reference</span></tt> type of <tt class="literal"><span class="pre">transform_iterator</span></tt> is
<tt class="literal"><span class="pre">result_of&lt;UnaryFunction(iterator_traits&lt;Iterator&gt;::reference)&gt;::type</span></tt>.
The <tt class="literal"><span class="pre">value_type</span></tt> is <tt class="literal"><span class="pre">remove_cv&lt;remove_reference&lt;reference&gt;</span> <span class="pre">&gt;::type</span></tt>.</p>
</div>
<div class="section" id="transform-iterator-public-operations">
<h4><a class="toc-backref" href="#id50" name="transform-iterator-public-operations"><tt class="literal"><span class="pre">transform_iterator</span></tt> public operations</a></h4>
<p><tt class="literal"><span class="pre">transform_iterator();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">transform_iterator</span></tt> with <tt class="literal"><span class="pre">m_f</span></tt>
and <tt class="literal"><span class="pre">m_iterator</span></tt> default constructed.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">transform_iterator(Iterator</span> <span class="pre">const&amp;</span> <span class="pre">x,</span> <span class="pre">UnaryFunction</span> <span class="pre">f);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">transform_iterator</span></tt> with <tt class="literal"><span class="pre">m_f</span></tt>
initialized to <tt class="literal"><span class="pre">f</span></tt> and <tt class="literal"><span class="pre">m_iterator</span></tt> initialized to <tt class="literal"><span class="pre">x</span></tt>.</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template&lt;class OtherIterator, class R2, class V2&gt;
transform_iterator(
      transform_iterator&lt;UnaryFunction, OtherIterator, R2, V2&gt; const&amp; t
    , typename enable_if_convertible&lt;OtherIterator, Iterator&gt;::type* = 0 // exposition
);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">transform_iterator</span></tt> that is a copy of <tt class="literal"><span class="pre">t</span></tt>.</td>
</tr>
<tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="literal"><span class="pre">OtherIterator</span></tt> is implicitly convertible to <tt class="literal"><span class="pre">Iterator</span></tt>.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">UnaryFunction</span> <span class="pre">functor()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">m_f</span></tt></td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="transform-iterator-private-operations">
<h4><a class="toc-backref" href="#id51" name="transform-iterator-private-operations"><tt class="literal"><span class="pre">transform_iterator</span></tt> private operations</a></h4>
<p><tt class="literal"><span class="pre">typename</span> <span class="pre">transform_iterator::value_type</span> <span class="pre">dereference()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">m_f(transform_iterator::dereference());</span></tt></td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="section" id="filter-iterator">
<h3><a class="toc-backref" href="#id52" name="filter-iterator">Filter iterator</a></h3>
<p>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 <tt class="literal"><span class="pre">true</span></tt> then the
element is retained and if it returns <tt class="literal"><span class="pre">false</span></tt> 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.</p>
<div class="section" id="class-template-filter-iterator">
<h4><a class="toc-backref" href="#id53" name="class-template-filter-iterator">Class template <tt class="literal"><span class="pre">filter_iterator</span></tt></a></h4>
<pre class="literal-block">
template &lt;class Predicate, class Iterator&gt;
class filter_iterator
    : public iterator_adaptor&lt;
          filter_iterator&lt;Predicate, Iterator&gt;, Iterator
        , use_default
        , /* see details */
      &gt;
{
 public:
    filter_iterator();
    filter_iterator(Predicate f, Iterator x, Iterator end = Iterator());
    filter_iterator(Iterator x, Iterator end = Iterator());
    template&lt;class OtherIterator&gt;
    filter_iterator(
        filter_iterator&lt;Predicate, OtherIterator&gt; const&amp; t
        , typename enable_if_convertible&lt;OtherIterator, Iterator&gt;::type* = 0 // exposition
        );
    Predicate predicate() const;
    Iterator end() const;

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

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

    Predicate m_predicate;
    Iterator m_end;
};
</pre>
</div>
</div>
<div class="section" id="filter-iterator-requirements">
<h3><a class="toc-backref" href="#id54" name="filter-iterator-requirements"><tt class="literal"><span class="pre">filter_iterator</span></tt> requirements</a></h3>
<p>The base <tt class="literal"><span class="pre">Iterator</span></tt> parameter must be a model of Readable
Iterator and Single Pass Iterator. The resulting
<tt class="literal"><span class="pre">filter_iterator</span></tt> will be a model of Forward Traversal Iterator
if <tt class="literal"><span class="pre">Iterator</span></tt> is, otherwise the <tt class="literal"><span class="pre">filter_iterator</span></tt> will be a
model of Single Pass Iterator. The access category of the
<tt class="literal"><span class="pre">filter_iterator</span></tt> will be the same as the access category of
<tt class="literal"><span class="pre">Iterator</span></tt>.</p>
<!-- Thomas is going to try implementing filter_iterator so that
it will be bidirectional if the underlying iterator is. -JGS -->
<p>The <tt class="literal"><span class="pre">Predicate</span></tt> must be Assignable, Copy Constructible, and the
expression <tt class="literal"><span class="pre">p(x)</span></tt> must be valid where <tt class="literal"><span class="pre">p</span></tt> is an object of type
<tt class="literal"><span class="pre">Predicate</span></tt>, <tt class="literal"><span class="pre">x</span></tt> is an object of type
<tt class="literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt>, and where the type of
<tt class="literal"><span class="pre">p(x)</span></tt> must be convertible to <tt class="literal"><span class="pre">bool</span></tt>.</p>
</div>
<div class="section" id="filter-iterator-operations">
<h3><a class="toc-backref" href="#id55" name="filter-iterator-operations"><tt class="literal"><span class="pre">filter_iterator</span></tt> operations</a></h3>
<p><tt class="literal"><span class="pre">filter_iterator();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="literal"><span class="pre">Predicate</span></tt> and <tt class="literal"><span class="pre">Iterator</span></tt> must be Default Constructible.</td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">a <tt class="literal"><span class="pre">filter_iterator</span></tt> whose
predicate is a default constructed <tt class="literal"><span class="pre">Predicate</span></tt> and
whose <tt class="literal"><span class="pre">end</span></tt> is a default constructed <tt class="literal"><span class="pre">Iterator</span></tt>.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">filter_iterator(Predicate</span> <span class="pre">f,</span> <span class="pre">Iterator</span> <span class="pre">x,</span> <span class="pre">Iterator</span> <span class="pre">end</span> <span class="pre">=</span> <span class="pre">Iterator());</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">A <tt class="literal"><span class="pre">filter_iterator</span></tt> at position <tt class="literal"><span class="pre">x</span></tt> that filters according
to predicate <tt class="literal"><span class="pre">f</span></tt> and that will not increment past <tt class="literal"><span class="pre">end</span></tt>.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">filter_iterator(Iterator</span> <span class="pre">x,</span> <span class="pre">Iterator</span> <span class="pre">end</span> <span class="pre">=</span> <span class="pre">Iterator());</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="literal"><span class="pre">Predicate</span></tt> must be Default Constructible.</td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">A <tt class="literal"><span class="pre">filter_iterator</span></tt> at position <tt class="literal"><span class="pre">x</span></tt> that filters
according to a default constructed <tt class="literal"><span class="pre">Predicate</span></tt>
and that will not increment past <tt class="literal"><span class="pre">end</span></tt>.</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template &lt;class OtherIterator&gt;
filter_iterator(
    filter_iterator&lt;Predicate, OtherIterator&gt; const&amp; t
    , typename enable_if_convertible&lt;OtherIterator, Iterator&gt;::type* = 0 // exposition
    );``
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="literal"><span class="pre">OtherIterator</span></tt> is implicitly convertible to <tt class="literal"><span class="pre">Iterator</span></tt>.</td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">A copy of iterator <tt class="literal"><span class="pre">t</span></tt>.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Predicate</span> <span class="pre">predicate()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">A copy of the predicate object used to construct <tt class="literal"><span class="pre">*this</span></tt>.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Iterator</span> <span class="pre">end()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">The object <tt class="literal"><span class="pre">end</span></tt> used to construct <tt class="literal"><span class="pre">*this</span></tt>.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="counting-iterator">
<h3><a class="toc-backref" href="#id56" name="counting-iterator">Counting iterator</a></h3>
<p>The counting iterator adaptor implements dereference by returning a
reference to the base object. The other operations are implemented by
the base <tt class="literal"><span class="pre">m_iterator</span></tt>, as per the inheritance from
<tt class="literal"><span class="pre">iterator_adaptor</span></tt>.</p>
<div class="section" id="class-template-counting-iterator">
<h4><a class="toc-backref" href="#id57" name="class-template-counting-iterator">Class template <tt class="literal"><span class="pre">counting_iterator</span></tt></a></h4>
<pre class="literal-block">
template &lt;
    class Incrementable
  , unsigned Access = use_default_access
  , class Traversal = use_default
  , class Difference = use_default
&gt;
class counting_iterator
  : public iterator_adaptor&lt;
        counting_iterator&lt;Incrementable, Access, Traversal, Difference&gt;
      , Incrementable
      , Incrementable
      , Access
      , /* see details for traversal category */
      , Incrementable const&amp;
      , Incrementable const*
      , /* distance = Difference or a signed integral type */&gt;
{
    friend class iterator_core_access;
 public:
    counting_iterator();
    counting_iterator(counting_iterator const&amp; rhs);
    counting_iterator(Incrementable x);
 private:
    typename counting_iterator::reference dereference() const
    {
        return this-&gt;base_reference();
    }
  };
</pre>
<dl>
<dt>[<em>Note:</em> implementers are encouraged to provide an implementation of</dt>
<dd><tt class="literal"><span class="pre">distance_to</span></tt> and a <tt class="literal"><span class="pre">difference_type</span></tt> that avoids overflows in
the cases when the <tt class="literal"><span class="pre">Incrementable</span></tt> type is a numeric type.]</dd>
</dl>
</div>
</div>
<div class="section" id="counting-iterator-requirements">
<h3><a class="toc-backref" href="#id58" name="counting-iterator-requirements"><tt class="literal"><span class="pre">counting_iterator</span></tt> requirements</a></h3>
<p>The <tt class="literal"><span class="pre">Incrementable</span></tt> type must be Default Constructible, Copy
Constructible, and Assignable.  The default distance is
an implementation defined signed integegral type.</p>
<p>The resulting <tt class="literal"><span class="pre">counting_iterator</span></tt> models Readable Lvalue Iterator.</p>
<p>Furthermore, if you wish to create a counting iterator that is a Forward
Traversal Iterator, then the following expressions must be valid:</p>
<pre class="literal-block">
Incrementable i, j;
++i         // pre-increment
i == j      // operator equal
</pre>
<p>If you wish to create a counting iterator that is a
Bidirectional Traversal Iterator, then pre-decrement is also required:</p>
<pre class="literal-block">
--i
</pre>
<p>If you wish to create a counting iterator that is a Random Access
Traversal Iterator, then these additional expressions are also
required:</p>
<pre class="literal-block">
counting_iterator::difference_type n;
i += n
n = i - j
i &lt; j
</pre>
</div>
<div class="section" id="counting-iterator-operations">
<h3><a class="toc-backref" href="#id59" name="counting-iterator-operations"><tt class="literal"><span class="pre">counting_iterator</span></tt> operations</a></h3>
<p><tt class="literal"><span class="pre">counting_iterator();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">A default constructed instance of <tt class="literal"><span class="pre">counting_iterator</span></tt>.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">counting_iterator(counting_iterator</span> <span class="pre">const&amp;</span> <span class="pre">rhs);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">counting_iterator</span></tt> that is a copy of <tt class="literal"><span class="pre">rhs</span></tt>.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">counting_iterator(Incrementable</span> <span class="pre">x);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">counting_iterator</span></tt> with its base
object copy constructed from <tt class="literal"><span class="pre">x</span></tt>.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="function-output-iterator">
<h3><a class="toc-backref" href="#id60" name="function-output-iterator">Function output iterator</a></h3>
<p>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.</p>
<div class="section" id="class-template-function-output-iterator">
<h4><a class="toc-backref" href="#id61" name="class-template-function-output-iterator">Class template <tt class="literal"><span class="pre">function_output_iterator</span></tt></a></h4>
<pre class="literal-block">
template &lt;class UnaryFunction&gt;
class function_output_iterator {
public:
  typedef iterator_tag&lt;
        writable_iterator
      , incrementable_traversal_tag
  &gt; iterator_category;
  typedef void                value_type;
  typedef void                difference_type;
  typedef void                pointer;
  typedef void                reference;

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

  struct output_proxy {
    output_proxy(UnaryFunction&amp; f);
    template &lt;class T&gt; output_proxy&amp; operator=(const T&amp; value);
  };
  output_proxy operator*();
  function_output_iterator&amp; operator++();
  function_output_iterator&amp; operator++(int);
};
</pre>
</div>
</div>
<div class="section" id="function-output-iterator-requirements">
<h3><a class="toc-backref" href="#id62" name="function-output-iterator-requirements"><tt class="literal"><span class="pre">function_output_iterator</span></tt> requirements</a></h3>
<p>The <tt class="literal"><span class="pre">UnaryFunction</span></tt> must be Assignable, Copy Constructible, and the
expression <tt class="literal"><span class="pre">f(x)</span></tt> must be valid, where <tt class="literal"><span class="pre">f</span></tt> is an object of type
<tt class="literal"><span class="pre">UnaryFunction</span></tt> and <tt class="literal"><span class="pre">x</span></tt> is an object of a type accepted by <tt class="literal"><span class="pre">f</span></tt>.
The resulting <tt class="literal"><span class="pre">function_output_iterator</span></tt> is a model of the Writable
and Incrementable Iterator concepts.</p>
</div>
<div class="section" id="function-output-iterator-operations">
<h3><a class="toc-backref" href="#id63" name="function-output-iterator-operations"><tt class="literal"><span class="pre">function_output_iterator</span></tt> operations</a></h3>
<p><tt class="literal"><span class="pre">explicit</span> <span class="pre">function_output_iterator(const</span> <span class="pre">UnaryFunction&amp;</span> <span class="pre">f</span> <span class="pre">=</span> <span class="pre">UnaryFunction());</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">function_output_iterator</span></tt> with
<tt class="literal"><span class="pre">f</span></tt> stored as a data member.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">output_proxy</span> <span class="pre">operator*();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">output_proxy</span></tt> constructed with
a copy of the unary function <tt class="literal"><span class="pre">f</span></tt>.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">function_output_iterator&amp;</span> <span class="pre">operator++();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">*this</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">function_output_iterator&amp;</span> <span class="pre">operator++(int);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">*this</span></tt></td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="function-output-iterator-output-proxy-operations">
<h3><a class="toc-backref" href="#id64" name="function-output-iterator-output-proxy-operations"><tt class="literal"><span class="pre">function_output_iterator::output_proxy</span></tt> operations</a></h3>
<p><tt class="literal"><span class="pre">output_proxy(UnaryFunction&amp;</span> <span class="pre">f);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">output_proxy</span></tt> with <tt class="literal"><span class="pre">f</span></tt> stored as
a data member.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">template</span> <span class="pre">&lt;class</span> <span class="pre">T&gt;</span> <span class="pre">output_proxy&amp;</span> <span class="pre">operator=(const</span> <span class="pre">T&amp;</span> <span class="pre">value);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
m_f(value);
return *this;
</pre>
</td>
</tr>
</tbody>
</table>
<!-- LocalWords:  Abrahams Siek Witt istream ostream iter MTL strided interoperate
LocalWords:  CRTP metafunctions inlining lvalue JGS incrementable BGL LEDA cv
LocalWords:  GraphBase struct ptrdiff UnaryFunction const int typename bool pp
LocalWords:  lhs rhs SFINAE markup iff tmp OtherDerived OtherIterator DWA foo
LocalWords:  dereferenceable subobject AdaptableUnaryFunction impl pre ifdef'd
LocalWords:  OtherIncrementable Coplien -->
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