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<title>Boost Iterator Adaptor Library</title>
<img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" align=
@ -15,13 +14,12 @@
<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.
<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>
@ -32,32 +30,46 @@
<ul>
<li>Generalized Iterator Adaptor
<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="#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>(<tt>const</tt>/non-<tt>const</tt>) <a href=
"#iterator_interactions">Iterator Interactions</a>
<li><a href="#challenge">Challenge</a>
<li><a href="#model_of">Model of</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>Specialized Iterator Adaptors
<li>
Specialized Iterator Adaptors
<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="transform_iterator.htm">Transform Iterator
Adaptor</a>
<li><a href="projection_iterator.htm">Projection Iterator Adaptor</a>
<li><a href="projection_iterator.htm">Projection Iterator
Adaptor</a>
<li><a href="filter_iterator.htm">Filter Iterator Adaptor</a>
</ul>
@ -71,8 +83,8 @@
<p><b><a href="http://www.boost.org/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
"../../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
@ -91,7 +103,8 @@
"#iterator_adaptor">iterator_adaptor</a></tt> template.<br>
<h2><a name="iterator_adaptor">Class template</a> <tt>iterator_adaptor</tt></h2>
<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
@ -116,10 +129,11 @@ struct iterator_adaptor;
<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.
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>
<table border="1" summary="iterator_adaptor template parameters">
<tr>
<th>Parameter
@ -133,16 +147,17 @@ struct iterator_adaptor;
<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
<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>, a conforming compiler makes the
<tt>value_type</tt> <tt><i>non-</i>const X</tt><a href="#1">[1]</a>.<br>
<td>The <tt>value_type</tt> of the resulting iterator, unless const. If
Value is <tt>const X</tt>, a conforming compiler makes the
<tt>value_type</tt> <tt><i>non-</i>const X</tt><a href=
"#1">[1]</a>.<br>
<b>Default:</b>
<tt>std::iterator_traits&lt;BaseType&gt;::value_type</tt>
@ -181,66 +196,95 @@ struct iterator_adaptor;
<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.
appropriate <tt>Policies</tt> class. The <tt>Policies</tt> class will
become the functional heart of the iterator adaptor, supplying the core
iterator operations that will determine how your new adaptor class will
behave. 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>. Your <tt>Policies</tt> class must implement three, four, or
seven of the core iterator operations below depending on the iterator
categories you want it to support.
The <tt>Policies</tt> class will become the functional heart of the iterator
adaptor, supplying the core iterator operations that will determine how your
new adaptor class will behave. 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>.
Your <tt>Policies</tt> class must implement three, four, or seven of the core
iterator operations below depending on the iterator categories you want it to support.
<table border="1" summary="iterator_adaptor Policies operations">
<caption>
<b>Core Iterator Operations</b>
</caption>
<table border>
<caption><b>Core Iterator Operations</b></caption>
<tr><th>Operation <th>Effects <th>Required for Iterator Categories
<tr>
<td><tt>dereference</tt> <td>returns an element of the iterator's
<tt>reference</tt> type <td rowspan="3"><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>
<th>Operation
<tr><td><tt>equal</tt> <td>tests the iterator for equality
<th>Effects
<tr><td><tt>increment</tt> <td>increments the iterator
<th>Required for Iterator Categories
<tr><td><tt>decrement</tt> <td>decrements the iterator <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>dereference</tt>
<tr><td><tt>less</tt> <td>imposes a <a
href="http://www.sgi.com/tech/stl/StrictWeakOrdering.html">Strict Weak
Ordering</a> relation on the iterator's <tt>reference</tt> type
<td
rowspan="3"><a
href="http://www.sgi.com/tech/stl/RandomAccessIterator.html">Random
<td>returns an element of the iterator's <tt>reference</tt> type
<td rowspan="3"><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>distance</tt> <td>measures the distance between iterators
<tr>
<td><tt>equal</tt>
<tr><td><tt>advance</tt> <td>adds an integer offset to iterators
<td>tests the iterator for equality
<tr>
<td><tt>increment</tt>
<td>increments the iterator
<tr>
<td><tt>decrement</tt>
<td>decrements the iterator
<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 the iterator's <tt>reference</tt> type
<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
<tr>
<td><tt>advance</tt>
<td>adds an integer offset to iterators
</table>
<p>
The library also supplies a &quot;trivial&quot; policy class,
<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
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>:
policy classes, defining functions with the same interface. This is the
definition of <tt>default_iterator_policies</tt>:<br>
<br>
<p>
<blockquote>
<pre>
struct default_iterator_policies
@ -277,78 +321,75 @@ struct default_iterator_policies
</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 may achieve the same sort of reusability.
<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 may achieve the same sort of reusability.
<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>
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.
<p>
<table border>
<tr><td><tt>
iterator_adaptor(const Base&amp;, const Policies&amp; = Policies())
</tt><br>
Construct an adapted iterator from a base object and a policies object.
</td></tr>
<table border="1" summary="additional iterator_adaptor members">
<tr>
<td><tt>iterator_adaptor(const Base&amp;, const Policies&amp; =
Policies())</tt> <br>
Construct an adapted iterator from a base object and a policies
object.
<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>
<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>.
</td></tr>
<tr><td><tt>
base_type base() const;
</tt><br><br>
<tr>
<td><tt>base_type base() const;</tt> <br>
<br>
Return a copy of the base object.
</td></tr>
</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>
<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.
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 implement
<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 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:
<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>
<p>
<blockquote>
<pre>
template &lt;class AdaptableUnaryFunction&gt;
@ -368,23 +409,23 @@ type. The complete code for <tt>transform_iterator_policies</tt> is:
</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
<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.
<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>
<p>
<blockquote>
<pre>
template &lt;class AdaptableUnaryFunction, class Iterator&gt;
@ -400,11 +441,13 @@ public:
</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.
<p>
<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;
@ -419,12 +462,12 @@ make_transform_iterator(Iterator base,
</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.
<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>
<p>
<blockquote>
<pre>
#include &lt;functional&gt;
@ -452,64 +495,61 @@ This output is:
<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:
<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>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, but
<tt>iterator_adaptor</tt> can make it easy. The rules are as follows:
<ul>
<li>Adapted iterators that share the same <tt>Policies</tt>, <tt>Category</tt>, and <tt>Distance</tt>
parameters are called <i>interoperable</i>.
<li>Adapted iterators that share the same <tt>Policies</tt>,
<tt>Category</tt>, and <tt>Distance</tt> parameters are called
<i>interoperable</i>.
<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.
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.
<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>
<h3><a name="challenge">Challenge</a></h3>
<p>There is an unlimited number of ways the 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.
<p>There is an unlimited number of ways the 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="model_of">Model of</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
<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>.
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>
@ -540,7 +580,8 @@ struct iterator_adaptor
const iterator_adaptor&lt;B,Policies,V,R,P,Category,Distance&gt;&amp;);
reference operator*() const;
<i>operator_arrow_result_type</i> operator-&gt;() const; <a href="#3">[3]</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>
iterator_adaptor&amp; operator++();
@ -574,68 +615,72 @@ template &lt;class B1, class B2, class P, class V1, class V2,
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 !=, <, <=, >=, >
// and similarly for operators !=, &lt;, &lt;=, &gt;=, &gt;
</pre>
<h3><a name="notes">Notes</a></h3>
<p><a name="1">[1]</a> If your compiler does not support partial
specialization and the base iterator is a builtin pointer type,
then you will not be able to use the default for <tt>Value</tt>
and will need to explicitly specify this type.
specialization and the base iterator is a builtin pointer type, then you
will not be able to use the default for <tt>Value</tt> and will need to
explicitly specify this type.
<p><a name="2">[2]</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> is
designed to allow 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
do this for you, having a <tt>const value_type</tt> is often harmless in
practice.
<p><a name="3">[3]</a> The result type for the <tt>operator->()</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>.
<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> is designed to allow 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 do this for you, having a
<tt>const value_type</tt> is often harmless in practice.
<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>.
<hr>
<p>Revised
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %b %Y" startspan -->15
Feb 2001<!--webbot bot="Timestamp" endspan i-checksum="14373" -->
<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.
</body>
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