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Jeremy Siek ec8d069057 more work on the specification of iterator_category 
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<title>Indirect Iterator</title>
<meta name="author" content="David Abrahams, Jeremy Siek, Thomas Witt" />
<meta name="organization" content="Boost Consulting, Indiana University Open Systems Lab, University of Hanover Institute for Transport Railway Operation and Construction" />
<meta name="date" content="2004-01-13" />
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<div class="document" id="indirect-iterator">
<h1 class="title">Indirect Iterator</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;ive.uni-hannover.de">witt&#64;ive.uni-hannover.de</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>2004-01-13</td></tr>
<tr><th class="docinfo-name">Copyright:</th>
<td>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"><p class="first">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 -->
</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="#indirect-iterator-synopsis" id="id2" name="id2"><tt class="literal"><span class="pre">indirect_iterator</span></tt> synopsis</a></li>
<li><a class="reference" href="#indirect-iterator-requirements" id="id3" name="id3"><tt class="literal"><span class="pre">indirect_iterator</span></tt> requirements</a></li>
<li><a class="reference" href="#indirect-iterator-models" id="id4" name="id4"><tt class="literal"><span class="pre">indirect_iterator</span></tt> models</a></li>
<li><a class="reference" href="#indirect-iterator-operations" id="id5" name="id5"><tt class="literal"><span class="pre">indirect_iterator</span></tt> operations</a></li>
<li><a class="reference" href="#example" id="id6" name="id6">Example</a></li>
</ul>
</div>
<div class="section" id="indirect-iterator-synopsis">
<h1><a class="toc-backref" href="#id2" name="indirect-iterator-synopsis"><tt class="literal"><span class="pre">indirect_iterator</span></tt> synopsis</a></h1>
<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:
typedef /* see below */ value_type;
typedef /* see below */ reference;
typedef /* see below */ pointer;
typedef /* see below */ difference_type;
typedef /* see below */ iterator_category;
indirect_iterator();
indirect_iterator(Iterator x);
template &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
);
Iterator base() const;
reference operator*() const;
indirect_iterator&amp; operator++();
indirect_iterator&amp; operator--();
private:
Iterator m_iterator; // exposition
};
</pre>
<p>The member types of <tt class="literal"><span class="pre">indirect_iterator</span></tt> are defined according to the
following pseudo-code. We use the abbreviation
<tt class="literal"><span class="pre">V=iterator_traits&lt;Iterator&gt;::value_type</span></tt>.:</p>
<pre class="literal-block">
if (Value is use_default) then
typedef iterator_traits&lt;V&gt;::value_type value_type;
else
typedef remove_const&lt;Value&gt;::type value_type;
if (Reference is use_default) then
if (Value is use_default) then
typedef iterator_traits&lt;V&gt;::reference reference;
else
typedef Value&amp; reference;
else
typedef Reference reference;
if (Value is use_default) then
typedef iterator_traits&lt;V&gt;::value_type* pointer;
else
typedef Value* pointer;
if (Difference is use_default)
typedef iterator_traits&lt;Iterator&gt;::difference_type difference_type;
else
typedef Difference difference_type;
</pre>
<p>If <tt class="literal"><span class="pre">CategoryOrTraversal</span></tt> is not <tt class="literal"><span class="pre">use_default</span></tt> then
<tt class="literal"><span class="pre">iterator_category</span></tt> is <tt class="literal"><span class="pre">CategoryOrTraversal</span></tt>. Otherwise
<tt class="literal"><span class="pre">iterator_category</span></tt> is a type convertible to the tag determined by
the following algorithm. Let <tt class="literal"><span class="pre">C</span></tt> be <tt class="literal"><span class="pre">traveral_category&lt;Iterator&gt;::type</span></tt>.</p>
<pre class="literal-block">
if (reference is a reference to value_type) then
if (C is convertible to random_access_traversal_tag) then
random_access_iterator_tag
else if (C is convertible to bidirectional_traversal_tag) then
bidirectional_iterator_tag
else
forward_iterator_tag
else
input_iterator_tag
</pre>
</div>
<div class="section" id="indirect-iterator-requirements">
<h1><a class="toc-backref" href="#id3" name="indirect-iterator-requirements"><tt class="literal"><span class="pre">indirect_iterator</span></tt> requirements</a></h1>
<p>The <tt class="literal"><span class="pre">Iterator</span></tt> argument shall meet the requirements of Readable
Iterator. The <tt class="literal"><span class="pre">CategoryOrTraversal</span></tt> argument shall be one of the
standard iterator tags or <tt class="literal"><span class="pre">use_default</span></tt>. If <tt class="literal"><span class="pre">CategoryOrTraversal</span></tt>
is an iterator tag, the template parameter <tt class="literal"><span class="pre">Iterator</span></tt> argument shall
meet the requirements corresponding to the iterator tag.</p>
<p>The expression <tt class="literal"><span class="pre">*v</span></tt>, where <tt class="literal"><span class="pre">v</span></tt> is an object of type
<tt class="literal"><span class="pre">iterator_traits&lt;Iterator&gt;::value_type</span></tt>, must be a valid expression
and must be convertible to <tt class="literal"><span class="pre">indirect_iterator::reference</span></tt>. Also
<tt class="literal"><span class="pre">indirect_iterator::reference</span></tt> must be convertible to
<tt class="literal"><span class="pre">indirect_iterator::value</span></tt>. 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 for the <tt class="literal"><span class="pre">value_type</span></tt> member.</p>
</div>
<div class="section" id="indirect-iterator-models">
<h1><a class="toc-backref" href="#id4" name="indirect-iterator-models"><tt class="literal"><span class="pre">indirect_iterator</span></tt> models</a></h1>
<p>If <tt class="literal"><span class="pre">CategoryOrTraversal</span></tt> is a standard iterator tag,
<tt class="literal"><span class="pre">indirect_iterator</span></tt> is a model of the iterator concept corresponding
to the tag, otherwise <tt class="literal"><span class="pre">indirect_iterator</span></tt> satisfies the requirements
of the most refined standard traversal concept that is satisfied by
the <tt class="literal"><span class="pre">Iterator</span></tt> argument.</p>
<p><tt class="literal"><span class="pre">indirect_iterator</span></tt> models Readable Iterator. If
<tt class="literal"><span class="pre">indirect_iterator::reference(*v)</span> <span class="pre">=</span> <span class="pre">t</span></tt> is a valid expression (where
<tt class="literal"><span class="pre">t</span></tt> is an object of type <tt class="literal"><span class="pre">indirect_iterator::value_type</span></tt>) then
<tt class="literal"><span class="pre">indirect_iterator</span></tt> models Writable Iterator. If
<tt class="literal"><span class="pre">indirect_iterator::reference</span></tt> is a reference then
<tt class="literal"><span class="pre">indirect_iterator</span></tt> models Lvalue Iterator.</p>
</div>
<div class="section" id="indirect-iterator-operations">
<h1><a class="toc-backref" href="#id5" name="indirect-iterator-operations"><tt class="literal"><span class="pre">indirect_iterator</span></tt> operations</a></h1>
<p>In addition to the operations required by the concepts modeled by
<tt class="literal"><span class="pre">indirect_iterator</span></tt>, <tt class="literal"><span class="pre">indirect_iterator</span></tt> provides the following
operations.</p>
<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">m_iterator</span></tt>.</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
<tt class="literal"><span class="pre">m_iterator</span></tt> 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">m_iterator</span></tt> subobject is constructed from <tt class="literal"><span class="pre">y.base()</span></tt>.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Iterator</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>
<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">**m_iterator</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">indirect_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">Effects:</th><td class="field-body"><tt class="literal"><span class="pre">++m_iterator</span></tt></td>
</tr>
<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">indirect_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">Effects:</th><td class="field-body"><tt class="literal"><span class="pre">--m_iterator</span></tt></td>
</tr>
<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="example">
<h1><a class="toc-backref" href="#id6" name="example">Example</a></h1>
<p>This example prints an array of characters, using
<tt class="literal"><span class="pre">indirect_iterator</span></tt> to access the array of characters through an
array of pointers. Next <tt class="literal"><span class="pre">indirect_iterator</span></tt> is used with the
<tt class="literal"><span class="pre">transform</span></tt> algorithm to copy the characters (incremented by one) to
another array. A constant indirect iterator is used for the source and
a mutable indirect iterator is used for the destination. The last part
of the example prints the original array of characters, but this time
using the <tt class="literal"><span class="pre">make_indirect_iterator</span></tt> helper function.</p>
<pre class="literal-block">
char characters[] = &quot;abcdefg&quot;;
const int N = sizeof(characters)/sizeof(char) - 1; // -1 since characters has a null char
char* pointers_to_chars[N]; // at the end.
for (int i = 0; i &lt; N; ++i)
pointers_to_chars[i] = &amp;characters[i];
// Example of using indirect_iterator
boost::indirect_iterator&lt;char**, char&gt;
indirect_first(pointers_to_chars), indirect_last(pointers_to_chars + N);
std::copy(indirect_first, indirect_last, std::ostream_iterator&lt;char&gt;(std::cout, &quot;,&quot;));
std::cout &lt;&lt; std::endl;
// Example of making mutable and constant indirect iterators
char mutable_characters[N];
char* pointers_to_mutable_chars[N];
for (int j = 0; j &lt; N; ++j)
pointers_to_mutable_chars[j] = &amp;mutable_characters[j];
boost::indirect_iterator&lt;char* const*&gt; mutable_indirect_first(pointers_to_mutable_chars),
mutable_indirect_last(pointers_to_mutable_chars + N);
boost::indirect_iterator&lt;char* const*, char const&gt; const_indirect_first(pointers_to_chars),
const_indirect_last(pointers_to_chars + N);
std::transform(const_indirect_first, const_indirect_last,
mutable_indirect_first, std::bind1st(std::plus&lt;char&gt;(), 1));
std::copy(mutable_indirect_first, mutable_indirect_last,
std::ostream_iterator&lt;char&gt;(std::cout, &quot;,&quot;));
std::cout &lt;&lt; std::endl;
// Example of using make_indirect_iterator()
std::copy(boost::make_indirect_iterator(pointers_to_chars),
boost::make_indirect_iterator(pointers_to_chars + N),
std::ostream_iterator&lt;char&gt;(std::cout, &quot;,&quot;));
std::cout &lt;&lt; std::endl;
</pre>
<p>The output is:</p>
<pre class="literal-block">
a,b,c,d,e,f,g,
b,c,d,e,f,g,h,
a,b,c,d,e,f,g,
</pre>
<p>The source code for this example can be found <a class="reference" href="../example/indirect_iterator_example.cpp">here</a>.</p>
</div>
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