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<h1>Boost Iterator Adaptor Library</h1>
<h2>Introduction</h2>
<p>The Iterator Adaptor library allows you transform an arbitrary ``base''
type into a standard-conforming iterator with the behaviors you choose.
Doing so is especially easy if the ``base'' type is itself an iterator. The
library also supplies several example <a href=
"../../more/generic_programming.html#adaptors">adaptors</a> which apply
specific useful behaviors to arbitrary base iterators.
<h2>Table of Contents</h2>
<ul>
<li>
Header <tt><a href=
"../../boost/iterator_adaptors.hpp">boost/iterator_adaptors.hpp</a></tt>
<ul>
<li>
Generalized Iterator Adaptor
<ul>
<li>Class template <tt><a href=
"#iterator_adaptor">iterator_adaptor</a></tt>
<li><a href="#template_parameters">Template Parameters</a>
<li><a href="#named_template_parameters">Named Template Parameters</a>
<li><a href="#policies">The Policies Class</a>
<li><a href="#additional_members">Additional Class Members</a>
<li><a href="#example">Example</a>
<li>(<tt>const</tt>/non-<tt>const</tt>) <a href=
"#iterator_interactions">Iterator Interactions</a>
<li><a href="#challenge">Challenge</a>
<li><a href="#concept_model">Concept Model</a>
<li><a href="#declaration_synopsis">Declaration Synopsis</a>
<li><a href="#notes">Notes</a>
</ul>
<li>
<a name="specialized_adaptors">Specialized Iterator Adaptors</a>
<ul>
<li><a href="indirect_iterator.htm">Indirect Iterator Adaptor</a>
<li><a href="reverse_iterator.htm">Reverse Iterator Adaptor</a>
<li><a href="transform_iterator.htm">Transform Iterator
Adaptor</a>
<li><a href="projection_iterator.htm">Projection Iterator
Adaptor</a>
<li><a href="filter_iterator.htm">Filter Iterator Adaptor</a>
</ul>
</ul>
<li>Header <tt><a href=
"../../boost/counting_iterator.hpp">boost/counting_iterator.hpp</a></tt><br>
<a href="counting_iterator.htm">Counting Iterator Adaptor</a>
<li>Header <tt><a href=
"../../boost/function_output_iterator.hpp">boost/function_output_iterator.hpp</a></tt><br>
<a href="function_output_iterator.htm">Function Output Iterator Adaptor</a>
</ul>
<p><b><a href="../../people/dave_abrahams.htm">Dave
Abrahams</a></b> started the library, applying <a href=
"../../more/generic_programming.html#policy">policy class</a> technique and
handling const/non-const iterator interactions. He also contributed the
<tt><a href="indirect_iterator.htm">indirect_</a></tt> and <tt><a href=
"reverse_iterator.htm">reverse_</a></tt> iterator generators, and expanded
<tt><a href="counting_iterator.htm">counting_iterator_generator</a></tt> to
cover all incrementable types. He edited most of the documentation,
sometimes heavily.<br>
<b><a href="../../people/jeremy_siek.htm">Jeremy
Siek</a></b> contributed the <a href="transform_iterator.htm">transform
iterator</a> adaptor, the integer-only version of <tt><a href=
"counting_iterator.htm">counting_iterator_generator</a></tt>,
the <a href="function_output_iterator.htm">function output iterator</a>
adaptor, and most of the documentation.<br>
<b><a href="http://www.boost.org/people/john_potter.htm">John
Potter</a></b> contributed the <tt><a href=
"projection_iterator.htm">projection_</a></tt> and <tt><a href=
"filter_iterator.htm">filter_</a></tt> iterator generators and made some
simplifications to the main <tt><a href=
"#iterator_adaptor">iterator_adaptor</a></tt> template.<br>
<h2><a name="iterator_adaptor">Class template</a>
<tt>iterator_adaptor</tt></h2>
Implementing standard conforming iterators is a non-trivial task. There are
some fine points such as the interactions between an iterator and its
corresponding const_iterator, and there are myriad operators that should be
implemented but are easily forgotten or mishandled, such as
<tt>operator-&gt;()</tt>. Using <tt>iterator_adaptor</tt>, you can easily
implement an iterator class, and even more easily extend and <a href=
"../../more/generic_programming.html#adaptors">adapt</a> existing iterator
types. Moreover, it is easy to make a pair of interoperable <tt>const</tt>
and <tt>non-const</tt> iterators.
<p><tt>iterator_adaptor</tt> is declared like this:
<pre>
template &lt;class Base, class Policies,
class ValueOrNamedParam = typename std::iterator_traits&lt;Base&gt;::value_type,
class ReferenceOrNamedParam = <i>...(see below)</i>,
class PointerOrNamedParam = <i>...(see below)</i>,
class CategoryOrNamedParam = typename std::iterator_traits&lt;Base&gt;::iterator_category,
class DistanceOrNamedParam = typename std::iterator_traits&lt;Base&gt;::difference_type&gt;
struct iterator_adaptor;
</pre>
<h3><a name="template_parameters">Template Parameters</a></h3>
<p>Although <tt>iterator_adaptor</tt> takes seven template parameters,
defaults have been carefully chosen to minimize the number of parameters
you must supply in most cases, especially if <tt>BaseType</tt> is an
iterator.
<table border="1" summary="iterator_adaptor template parameters">
<tr>
<th>Parameter
<th>Description
<tr>
<td><tt>BaseType</tt>
<td>The type being wrapped.
<tr>
<td><tt>Policies</tt>
<td>A <a href="../../more/generic_programming.html#policy">policy
class</a> that supplies core functionality to the resulting iterator. A
detailed description can be found <a href="#policies">below</a>.
<tr>
<td><tt>Value</tt>
<td>The <tt>value_type</tt> of the resulting iterator, unless const. If
Value is <tt>const X</tt> the
<tt>value_type</tt> will be (<i>non-</i><tt>const</tt>) <tt>X</tt><a href=
"#1">[1]</a>. If the <tt>value_type</tt> you wish to use is an abstract
base class see note <a href="#5">[5]</a>.<br>
<b>Default:</b>
<tt>std::iterator_traits&lt;BaseType&gt;::value_type</tt> <a href=
"#2">[2]</a>
<tr>
<td><tt>Reference</tt>
<td>The <tt>reference</tt> type of the resulting iterator, and in
particular, the result type of <tt>operator*()</tt>.<br>
<b>Default:</b> If <tt>Value</tt> is supplied, <tt>Value&amp;</tt> is
used. Otherwise
<tt>std::iterator_traits&lt;BaseType&gt;::reference</tt> is used.
<tr>
<td><tt>Pointer</tt>
<td>The <tt>pointer</tt> type of the resulting iterator, and in
particular, the result type of <tt>operator-&gt;()</tt>.<br>
<b>Default:</b> If <tt>Value</tt> was supplied, then <tt>Value*</tt>,
otherwise <tt>std::iterator_traits&lt;BaseType&gt;::pointer</tt>.
<tr>
<td><tt>Category</tt>
<td>The <tt>iterator_category</tt> type for the resulting iterator.<br>
<b>Default:</b>
<tt>std::iterator_traits&lt;BaseType&gt;::iterator_category</tt>
<tr>
<td><tt>Distance</tt>
<td>The <tt>difference_type</tt> for the resulting iterator.<br>
<b>Default:</b>
<tt>std::iterator_traits&lt;BaseType&gt;::difference_type</tt>
<tr>
<td><tt>NamedParam</tt>
<td>A named template parameter (see below).
</table>
<h3><a name="named_template_parameters">Named Template Parameters</a></h3>
With seven template parameters, providing arguments for
<tt>iterator_adaptor</tt> in the correct order can be challenging.
Also, often times one would like to specify the sixth or seventh
template parameter, but use the defaults for the third through
fifth. As a solution to these problems we provide a mechanism for
naming the last five template parameters, and providing them in
any order through a set of named template parameters. The following
classes are provided for specifying the parameters. Any of these
classes can be used for any of the last five template parameters
of <tt>iterator_adaptor</tt>.
<blockquote>
<pre>
template &lt;class Value&gt; struct value_type_is;
template &lt;class Reference&gt; struct reference_is;
template &lt;class Pointer&gt; struct pointer_is;
template &lt;class Distance&gt; struct difference_type_is;
template &lt;class Category&gt; struct iterator_category_is;
</pre>
</blockquote>
For example, the following adapts <tt>foo_iterator</tt> to create
an <a href=
"http://www.sgi.com/tech/stl/InputIterator.html">InputIterator</a>
with <tt>reference</tt> type <tt>foo</tt>, and whose other traits
are determined according to the defaults described <a
href="#template_parameters">above</a>.
<blockquote>
<pre>
typedef iterator_adaptor&lt;foo_iterator, foo_policies,
reference_is&lt;foo&gt;, iterator_category_is&lt;std::input_iterator_tag&gt;
&gt; MyIterator;
</pre>
</blockquote>
<h3><a name="policies">The Policies Class</a></h3>
<p>The main task in using <tt>iterator_adaptor</tt> is creating an
appropriate <tt>Policies</tt> class. The <tt>Policies</tt> class will become
the functional heart of the resulting iterator, supplying the core
operations that determine its behavior. The <tt>iterator_adaptor</tt>
template defines all of the operators required of a <a href=
"http://www.sgi.com/tech/stl/RandomAccessIterator.html">Random Access
Iterator</a> by dispatching to a <tt>Policies</tt> object. Your
<tt>Policies</tt> class must implement a subset of the core iterator
operations below corresponding to the iterator categories you want it to
support.<br>
<br>
<table border="1" summary="iterator_adaptor Policies operations">
<caption>
<b>Core Iterator Operations</b><br>
<tt>T</tt>: adapted iterator type; <tt>p</tt>: object of type T; <tt>n</tt>: <tt>T::size_type</tt>; <tt>x</tt>: <tt>T::difference_type</tt>; <tt>p1</tt>, <tt>p2</tt>: iterators
</caption>
<tr>
<th>Operation
<th>Effects
<th>Implements Operations
<th>Required for Iterator Categories
<tr>
<td><tt>initialize</tt>
<td>optionally modify base iterator during iterator construction
<td>constructors
<td rowspan="4"><a href=
"http://www.sgi.com/tech/stl/InputIterator.html">Input</a>/ <a href=
"http://www.sgi.com/tech/stl/OutputIterator.html">Output</a>/ <a href=
"http://www.sgi.com/tech/stl/ForwardIterator.html">Forward</a>/ <a
href=
"http://www.sgi.com/tech/stl/BidirectionalIterator.html">Bidirectional</a>/
<a href="http://www.sgi.com/tech/stl/RandomAccessIterator.html">Random
Access</a>
<tr>
<td><tt>dereference</tt>
<td>returns an element of the iterator's <tt>reference</tt> type
<td><tt>*p</tt>, <tt>p[n]</tt>
<tr>
<td><tt>equal</tt>
<td>tests the iterator for equality
<td><tt>p1&nbsp;==&nbsp;p2</tt>, <tt>p1&nbsp;!=&nbsp;p2</tt>
<tr>
<td><tt>increment</tt>
<td>increments the iterator
<td><tt>++p</tt>, <tt>p++</tt>
<tr>
<td><tt>decrement</tt>
<td>decrements the iterator
<td><tt>--p</tt>, <tt>p--</tt>
<td><a href=
"http://www.sgi.com/tech/stl/BidirectionalIterator.html">Bidirectional</a>/
<a href="http://www.sgi.com/tech/stl/RandomAccessIterator.html">Random
Access</a>
<tr>
<td><tt>less</tt>
<td>imposes a <a href=
"http://www.sgi.com/tech/stl/StrictWeakOrdering.html">Strict Weak
Ordering</a> relation on iterators
<td>
<tt>p1&nbsp;&lt;&nbsp;p2</tt>,
<tt>p1&nbsp;&lt;=&nbsp;p2</tt>,
<tt>p1&nbsp;&gt;&nbsp;p2</tt>,
<tt>p1&nbsp;&gt;=&nbsp;p2</tt>
<td rowspan="3"><a href=
"http://www.sgi.com/tech/stl/RandomAccessIterator.html">Random
Access</a>
<tr>
<td><tt>distance</tt>
<td>measures the distance between iterators
<td><tt>p1 - p2</tt>
<tr>
<td><tt>advance</tt>
<td>adds an integer offset to iterators
<td>
<tt>p&nbsp;+&nbsp;x</tt>,
<tt>x&nbsp;+&nbsp;p</tt>,
<tt>p&nbsp;+=&nbsp;x</tt>,
<tt>p&nbsp;-&nbsp;x</tt>,
<tt>p&nbsp;-=&nbsp;x</tt>
</table>
<p>The library also supplies a "trivial" policy class,
<tt>default_iterator_policies</tt>, which implements all seven of the core
operations in the usual way. If you wish to create an iterator adaptor that
only changes a few of the base type's behaviors, then you can derive your
new policy class from <tt>default_iterator_policies</tt> to avoid retyping
the usual behaviors. You should also look at
<tt>default_iterator_policies</tt> as the ``boilerplate'' for your own
policy classes, defining functions with the same interface. This is the
definition of <tt>default_iterator_policies</tt>:<br>
<br>
<blockquote>
<pre>
struct <a name="default_iterator_policies">default_iterator_policies</a>
{
template &lt;class BaseType&gt;
void initialize(BaseType&amp;)
{ }
template &lt;class Reference, class BaseType&gt;
Reference dereference(type&lt;Reference&gt;, const BaseType&amp; x) const
{ return *x; }
template &lt;class BaseType&gt;
void increment(BaseType&amp; x)
{ ++x; }
template &lt;class BaseType1, class BaseType2&gt;
bool equal(const BaseType1&amp; x, const BaseType2&amp; y) const
{ return x == y; }
template &lt;class BaseType&gt;
void decrement(BaseType&amp; x)
{ --x; }
template &lt;class BaseType, class DifferenceType&gt;
void advance(BaseType&amp; x, DifferenceType n)
{ x += n; }
template &lt;class Difference, class BaseType1, class BaseType2&gt;
Difference distance(type&lt;Difference&gt;, const BaseType1&amp; x, const BaseType2&amp; y) const
{ return y - x; }
template &lt;class BaseType1, class BaseType2&gt;
bool less(const BaseType1&amp; x, const BaseType2&amp; y) const
{ return x &lt; y; }
};
</pre>
</blockquote>
<p>Template member functions are used throughout
<tt>default_iterator_policies</tt> so that it can be employed with a wide
range of iterators. If we had used concrete types above, we'd have tied the
usefulness of <tt>default_iterator_policies</tt> to a particular range of
adapted iterators. If you follow the same pattern with your
<tt>Policies</tt> classes, you can use them to generate more specialized
adaptors along the lines of <a href="#specialized_adaptors">those supplied by this library</a>.
<h3><a name="additional_members">Additional Members</a></h3>
In addition to all of the member functions required of a <a href=
"http://www.sgi.com/tech/stl/RandomAccessIterator.html">Random Access
Iterator</a>, the <tt>iterator_adaptor</tt> class template defines the
following members. <br>
<br>
<table border="1" summary="additional iterator_adaptor members">
<tr>
<td><tt>explicit iterator_adaptor(const Base&amp;, const Policies&amp; =
Policies())</tt>
<br><br>
Construct an adapted iterator from a base object and a policies
object. As this constructor is <tt>explicit</tt>, it does not
provide for implicit conversions from the <tt>Base</tt> type to
the iterator adaptor.
<tr>
<td><tt>template &lt;class B, class V, class R, class P&gt;<br>
iterator_adaptor(const
iterator_adaptor&lt;B,Policies,V,R,P,Category,Distance&gt;&amp;)</tt>
<br><br>
This constructor allows for conversion from non-<tt>const</tt> to
constant adapted iterators. See <a href=
"#iterator_interactions">below</a> for more details.<br>
Requires: <tt>B</tt> is convertible to <tt>Base</tt>.
<tr>
<td><tt>base_type base() const;</tt>
<br><br>
Return a copy of the base object.
</table>
<h3><a name="example">Example</a></h3>
<p>It is often useful to automatically apply some function to the value
returned by dereferencing an iterator. The <a href=
"./transform_iterator.htm">transform iterator</a> makes it easy to create
an iterator adaptor which does just that. Here we will show how easy it is
to implement the transform iterator using the <tt>iterator_adaptor</tt>
template.
<p>We want to be able to adapt a range of iterators and functions, so the
policies class will have a template parameter for the function type and it
will have a data member of that type. We know that the function takes one
argument and that we'll need to be able to deduce the <tt>result_type</tt>
of the function so we can use it for the adapted iterator's
<tt>value_type</tt>. <a href=
"http://www.sgi.com/Technology/STL/AdaptableUnaryFunction.html">AdaptableUnaryFunction</a>
is the <a href="../../more/generic_programming.html#concept">Concept</a>
that fulfills those requirements.
<p>To implement a transform iterator we will only change one of the base
iterator's behaviors, so the <tt>transform_iterator_policies</tt> class can
inherit the rest from <tt>default_iterator_policies</tt>. We will define
the <tt>dereference()</tt> member function, which is used to implement
<tt>operator*()</tt> of the adapted iterator. The implementation will
dereference the base iterator and apply the function object. The
<tt>type&lt;Reference&gt;</tt> parameter is used to convey the appropriate
return type. The complete code for <tt>transform_iterator_policies</tt>
is:<br>
<br>
<blockquote>
<pre>
template &lt;class AdaptableUnaryFunction&gt;
struct transform_iterator_policies : public default_iterator_policies
{
transform_iterator_policies() { }
transform_iterator_policies(const AdaptableUnaryFunction&amp; f)
: m_f(f) { }
template &lt;class Reference, class BaseIterator&gt;
Reference dereference(type&lt;Reference&gt;, const BaseIterator&amp; i) const
{ return m_f(*i); }
AdaptableUnaryFunction m_f;
};
</pre>
</blockquote>
<p>The next step is to use the <tt>iterator_adaptor</tt> template to
construct the transform iterator type. The nicest way to package the
construction of the transform iterator is to create a <a href=
"../../more/generic_programming.html#type_generator">type generator</a>.
The first template parameter to the generator will be the type of the
function object and the second will be the base iterator type. We use
<tt>iterator_adaptor</tt> to define the transform iterator type as a nested
<tt>typedef</tt> inside the <tt>transform_iterator_generator</tt> class.
Because the function may return by-value, we must limit the
<tt>iterator_category</tt> to <a href=
"http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a>, and
the iterator's <tt>reference</tt> type cannot be a true reference (the
standard allows this for input iterators), so in this case we can use few
of <tt>iterator_adaptor</tt>'s default template arguments.<br>
<br>
<blockquote>
<pre>
template &lt;class AdaptableUnaryFunction, class Iterator&gt;
struct transform_iterator_generator
{
typedef typename AdaptableUnaryFunction::result_type value_type;
public:
typedef iterator_adaptor&lt;Iterator,
transform_iterator_policies&lt;AdaptableUnaryFunction&gt;,
value_type, value_type, value_type*, std::input_iterator_tag&gt;
type;
};
</pre>
</blockquote>
<p>As a finishing touch, we will create an <a href=
"../../more/generic_programming.html#object_generator">object generator</a>
for the transform iterator. This is a function that makes it more
convenient to create a transform iterator.<br>
<br>
<blockquote>
<pre>
template &lt;class AdaptableUnaryFunction, class Iterator&gt;
typename transform_iterator_generator&lt;AdaptableUnaryFunction,Iterator&gt;::type
make_transform_iterator(Iterator base,
const AdaptableUnaryFunction&amp; f = AdaptableUnaryFunction())
{
typedef typename transform_iterator_generator&lt;AdaptableUnaryFunction,
Iterator&gt;::type result_t;
return result_t(base, f);
}
</pre>
</blockquote>
<p>Here is an example that shows how to use a transform iterator to iterate
through a range of numbers, multiplying each of them by 2 and printing the
result to standard output.<br>
<br>
<blockquote>
<pre>
#include &lt;functional&gt;
#include &lt;algorithm&gt;
#include &lt;iostream&gt;
#include &lt;boost/iterator_adaptors.hpp&gt;
int main(int, char*[])
{
int x[] = { 1, 2, 3, 4, 5, 6, 7, 8 };
const int N = sizeof(x)/sizeof(int);
std::cout &lt;&lt; &quot;multiplying the array by 2:&quot; &lt;&lt; std::endl;
std::copy(boost::make_transform_iterator(x, std::bind1st(std::multiplies&lt;int&gt;(), 2)),
boost::make_transform_iterator(x + N, std::bind1st(std::multiplies&lt;int&gt;(), 2)),
std::ostream_iterator&lt;int&gt;(std::cout, &quot; &quot;));
std::cout &lt;&lt; std::endl;
return 0;
}
</pre>
This output is:
<pre>
2 4 6 8 10 12 14 16
</pre>
</blockquote>
<h3><a name="iterator_interactions">Iterator Interactions</a></h3>
<p>C++ allows <tt>const</tt> and non-<tt>const</tt> pointers to interact in
the following intuitive ways:
<ul>
<li>a non-<tt>const</tt> pointer to <tt>T</tt> can be implicitly
converted to a <tt>const</tt> pointer to <tt>T</tt>.
<li><tt>const</tt> and non-<tt>const</tt> pointers to <tt>T</tt> can be
freely mixed in comparison expressions.
<li><tt>const</tt> and non-<tt>const</tt> pointers to <tt>T</tt> can be
freely subtracted, in any order.
</ul>
Getting user-defined iterators to work together that way is nontrivial (see
<a href="reverse_iterator.htm#interactions">here</a> for an example of where
the C++ standard got it wrong), but <tt>iterator_adaptor</tt> can make it
easy. The rules are as follows:
<ul>
<li><a name="interoperable">Adapted iterators that share the same <tt>Policies</tt>,
<tt>Category</tt>, and <tt>Distance</tt> parameters are called
<i>interoperable</i>.</a>
<li>An adapted iterator can be implicitly converted to any other adapted
iterator with which it is interoperable, so long as the <tt>Base</tt>
type of the source iterator can be converted to the <tt>Base</tt> type of
the target iterator.
<li>Interoperable iterators can be freely mixed in comparison expressions
so long as the <tt>Policies</tt> class has <tt>equal</tt> (and, for
random access iterators, <tt>less</tt>) members that can accept both
<tt>Base</tt> types in either order.
<li>Interoperable iterators can be freely mixed in subtraction
expressions so long as the <tt>Policies</tt> class has a
<tt>distance</tt> member that can accept both <tt>Base</tt> types in
either order.
</ul>
<h4>Example</h4>
<p>The <a href="projection_iterator.htm">Projection Iterator</a> adaptor is similar to the <a
href="./transform_iterator.htm">transform iterator adaptor</a> in that
its <tt>operator*()</tt> applies some function to the result of
dereferencing the base iterator and then returns the result. The
difference is that the function must return a reference to some
existing object (for example, a data member within the
<tt>value_type</tt> of the base iterator).
<p>
The <a
href="projection_iterator.htm#projection_iterator_pair_generator">projection_iterator_pair_generator</a> template
is a special two-<a href="../../more/generic_programming.html#type_generator">type generator</a> for mutable and constant versions of a
projection iterator. It is defined as follows:
<blockquote>
<pre>
template &lt;class AdaptableUnaryFunction, class Iterator, class ConstIterator&gt;
struct projection_iterator_pair_generator {
typedef typename AdaptableUnaryFunction::result_type value_type;
typedef projection_iterator_policies&lt;AdaptableUnaryFunction&gt; policies;
public:
typedef iterator_adaptor&lt;Iterator,policies,value_type&gt; iterator;
typedef iterator_adaptor&lt;ConstIterator,policies,value_type,
const value_type&amp;,const value_type*&gt; const_iterator;
};
</pre>
</blockquote>
<p>It is assumed that the <tt>Iterator</tt> and <tt>ConstIterator</tt> arguments are corresponding mutable
and constant iterators. <ul>
<li>
Clearly, then, the
<tt>projection_iterator_pair_generator</tt>'s <tt>iterator</tt> and
<tt>const_iterator</tt> are <a href="#interoperable">interoperable</a>, since
they share the same <tt>Policies</tt> and since <tt>Category</tt> and
<tt>Distance</tt> as supplied by <tt>std::iterator_traits</tt> through the
<a href="#template_parameters">default template parameters</a> to
<tt>iterator_adaptor</tt> should be the same.
<li>Since <tt>Iterator</tt> can presumably be converted to
<tt>ConstIterator</tt>, the projection <tt>iterator</tt> will be convertible to
the projection <tt>const_iterator</tt>.
<li> Since <tt>projection_iterator_policies</tt> implements only the
<tt>dereference</tt> operation, and inherits all other behaviors from <tt><a
href="#default_iterator_policies">default_iterator_policies</a></tt>, which has
fully-templatized <tt>equal</tt>, <tt>less</tt>, and <tt>distance</tt>
operations, the <tt>iterator</tt> and <tt>const_iterator</tt> can be freely
mixed in comparison and subtraction expressions.
</ul>
<h3><a name="challenge">Challenge</a></h3>
<p>There is an unlimited number of ways the <tt>iterator_adaptors</tt>
class can be used to create iterators. One interesting exercise would be to
re-implement the iterators of <tt>std::list</tt> and <tt>std::slist</tt>
using <tt>iterator_adaptors</tt>, where the adapted <tt>Iterator</tt> types
would be node pointers.
<h3><a name="concept_model">Concept Model</a></h3>
Depending on the <tt>Base</tt> and <tt>Policies</tt> template parameters,
an <tt>iterator_adaptor</tt> can be a <a href=
"http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a>, <a
href="http://www.sgi.com/tech/stl/ForwardIterator.html">Forward
Iterator</a>, <a href=
"http://www.sgi.com/tech/stl/BidirectionalIterator.html">Bidirectional
Iterator</a>, or <a href=
"http://www.sgi.com/tech/stl/RandomAccessIterator.html">Random Access
Iterator</a>.
<h3><a name="declaration_synopsis">Declaration Synopsis</a></h3>
<pre>
template &lt;class Base, class Policies,
class Value = typename std::iterator_traits&lt;Base&gt;::value_type,
class Reference = <i>...(see below)</i>,
class Pointer = <i>...(see below)</i>,
class Category = typename std::iterator_traits&lt;Base&gt;::iterator_category,
class Distance = typename std::iterator_traits&lt;Base&gt;::difference_type
&gt;
struct iterator_adaptor
{
typedef Distance difference_type;
typedef typename boost::remove_const&lt;Value&gt;::type value_type;
typedef Pointer pointer;
typedef Reference reference;
typedef Category iterator_category;
typedef Base base_type;
typedef Policies policies_type;
iterator_adaptor();
explicit iterator_adaptor(const Base&amp;, const Policies&amp; = Policies());
base_type base() const;
template &lt;class B, class V, class R, class P&gt;
iterator_adaptor(
const iterator_adaptor&lt;B,Policies,V,R,P,Category,Distance&gt;&amp;);
reference operator*() const; <a href="#6">[6]</a>
<i>operator_arrow_result_type</i> operator-&gt;() const; <a href=
"#3">[3]</a>
<i>value_type</i> operator[](difference_type n) const; <a href="#3">[4]</a>, <a href="#6">[6]</a>
iterator_adaptor&amp; operator++();
iterator_adaptor&amp; operator++(int);
iterator_adaptor&amp; operator--();
iterator_adaptor&amp; operator--(int);
iterator_adaptor&amp; operator+=(difference_type n);
iterator_adaptor&amp; operator-=(difference_type n);
iterator_adaptor&amp; operator-(Distance x) const;
};
template &lt;class B, class P, class V, class R, class Ptr,
class C, class D1, class D2&gt;
iterator_adaptor&lt;B,P,V,R,Ptr,C,D1&gt;
operator+(iterator_adaptor&lt;B,P,V,R,Ptr,C,D1&gt;, D2);
template &lt;class B, class P, class V, class R, class Ptr,
class C, class D1, class D2&gt;
iterator_adaptor&lt;B,P,V,R,P,C,D1&gt;
operator+(D2, iterator_adaptor&lt;B,P,V,R,Ptr,C,D1&gt; p);
template &lt;class B1, class B2, class P, class V1, class V2,
class R1, class R2, class P1, class P2, class C, class D&gt;
Distance operator-(const iterator_adaptor&lt;B1,P,V1,R1,P1,C,D&gt;&amp;,
const iterator_adaptor&lt;B2,P,V2,R2,P2,C,D&gt;&amp;);
template &lt;class B1, class B2, class P, class V1, class V2,
class R1, class R2, class P1, class P2, class C, class D&gt;
bool operator==(const iterator_adaptor&lt;B1,P,V1,R1,P1,C,D&gt;&amp;,
const iterator_adaptor&lt;B2,P,V2,R2,P2,C,D&gt;&amp;);
// and similarly for operators !=, &lt;, &lt;=, &gt;=, &gt;
</pre>
<h3><a name="notes">Notes</a></h3>
<p><a name="1">[1]</a> The standard specifies that the <tt>value_type</tt>
of <tt>const</tt> iterators to <tt>T</tt> (e.g. <tt>const T*</tt>) is
<tt><i>non-</i>const T</tt>, while the <tt>pointer</tt> and
<tt>reference</tt> types for all <a href=
"http://www.sgi.com/tech/stl/ForwardIterator.html">Forward Iterators</a> are
<tt>const T*</tt> and <tt>const T&amp;</tt>, respectively. Stripping the
<tt>const</tt>-ness of <tt>Value</tt> allows you to easily
make a <tt>const</tt> iterator adaptor by supplying a <tt>const</tt> type
for <tt>Value</tt>, and allowing the defaults for the <tt>Pointer</tt> and
<tt>Reference</tt> parameters to take effect. Although compilers that don't
support partial specialization won't strip <tt>const</tt> for you, having a
<tt>const value_type</tt> is often harmless in practice.
<p><a name="2">[2]</a> If your compiler does not support partial
specialization and the base iterator is a builtin pointer type, you
will not be able to use the default for <tt>Value</tt> and will have to
specify this type explicitly.
<p><a name="3">[3]</a> The result type for the <tt>operator-&gt;()</tt>
depends on the category and value type of the iterator and is somewhat
complicated to describe. But be assured, it works in a stardard conforming
fashion, providing access to members of the objects pointed to by the
iterator.
<p><a name="4">[4]</a> The result type of <tt>operator[]()</tt> is
<tt>value_type</tt> instead of <tt>reference</tt> as might be expected.
There are two reasons for this choice. First, the C++ standard only
requires that the return type of an arbitrary <a href=
"http://www.sgi.com/tech/stl/RandomAccessIterator.html">Random Access
Iterator</a>'s <tt>operator[]</tt>be ``convertible to T'' (Table 76), so
when adapting an arbitrary base iterator we may not have a reference to
return. Second, and more importantly, for certain kinds of iterators,
returning a reference could cause serious memory problems due to the
reference being bound to a temporary object whose lifetime ends inside of
the <tt>operator[]</tt>.
<p><a name="5">[5]</a>
The <tt>value_type</tt> of an iterator may not be
an abstract base class, however many common uses of iterators
never need the <tt>value_type</tt>, only the <tt>reference</tt> type.
If you wish to create such an iterator adaptor, use a dummy
type such as <tt>char</tt> for the <tt>Value</tt> parameter,
and use a reference to your abstract base class for
the <tt>Reference</tt> parameter. Note that such an iterator
does not fulfill the C++ standards requirements for a
<a href= "http://www.sgi.com/tech/stl/ForwardIterator.html">
Forward Iterator</a>, so you will need to use a less restrictive
iterator category such as <tt>std::input_iterator_tag</tt>.
<p><a name="6">[6]</a>
There is a common misconception that an iterator should have two
versions of <tt>operator*</tt> and of <tt>operator[]</tt>, one
version that is a <tt>const</tt> member function and one version
that is non-<tt>const</tt>. Perhaps the source of this
misconception is that containers typically have const and
non-const versions of many of their member functions. Iterators,
however, are different. A particular iterator type can be either
<i>mutable</i> or <i>constant</i> (but not both). One can assign
to and change the object pointed to by a mutable iterator whereas a
constant iterator returns constant objects when dereferenced. Whether
the iterator object itself is <tt>const</tt> has nothing to do with
whether the iterator is mutable or constant. This is analogous to
the way built-in pointer types behave. For example, one can
modify objects pointed to by a <tt>const</tt> pointer
<pre>
int* const x = new int;
int i = 3;
*x = i;
</pre>
but one cannot modify objects pointed to by a pointer
to <tt>const</tt>
<pre>
int const* x = new int;
int i = 3;
*x = i;
</pre>
<hr>
<p>Revised
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %b %Y" startspan -->18 Sep 2001<!--webbot bot="Timestamp" endspan i-checksum="14941" -->
<p>&copy; Copyright Dave Abrahams and Jeremy Siek 2001. Permission to copy,
use, modify, sell and distribute this document is granted provided this
copyright notice appears in all copies. This document is provided "as is"
without express or implied warranty, and with no claim as to its
suitability for any purpose.
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