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[SVN r10575]
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Dave Abrahams 2001-07-09 23:50:55 +00:00
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operators.htm
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@ -50,6 +50,7 @@ provided by the class.</p>
<ul> <ul>
<li><a href="#dereference">Dereference operators</a></li> <li><a href="#dereference">Dereference operators</a></li>
<li><a href="#iterator">Iterator Helpers</a></li> <li><a href="#iterator">Iterator Helpers</a></li>
<li><a href="#iterator_helpers_notes">Iterator Helper Notes</a></li>
<li><a href="#i_demo">Iterator Demonstration and Test <li><a href="#i_demo">Iterator Demonstration and Test
Program</a></li> Program</a></li>
</ul></li> </ul></li>
@ -889,7 +890,7 @@ href="#chaining">base class chaining</a>.</p>
<h3><a name="iterator">Iterator</a> Helpers</h3> <h3><a name="iterator">Iterator</a> Helpers</h3>
<p>There are three separate iterator helper classes, each for a <p>There are five separate iterator helper classes, each for a
different category of iterator. Here is a summary of the core set of different category of iterator. Here is a summary of the core set of
operators that the custom iterator must define, and the extra operators operators that the custom iterator must define, and the extra operators
that are created by the helper classes. These classes cannot be used for <a that are created by the helper classes. These classes cannot be used for <a
@ -931,11 +932,13 @@ C++ standard (<code>iterator_category</code>, <code>value_type</code>,
</ul></td> </ul></td>
</tr> </tr>
<tr valign="baseline"> <tr valign="baseline">
<td><code><a name="output_iterator_helper">output_iterator_helper&lt;T, V, D, P, R&gt;</a></code></td> <td><code><a name="output_iterator_helper">output_iterator_helper&lt;T&gt;</a></code></td>
<td>Supports the operations and has the requirements of <td>Supports the operations and has the requirements of
<ul> <ul>
<li><code><a href="#incrementable">incrementable&lt;T&gt;</a></code></li> <li><code><a href="#incrementable">incrementable&lt;T&gt;</a></code></li>
</ul></td> </ul>
See also [<a href="#1">1</a>], [<a href="#2">2</a>].
</td>
</tr> </tr>
<tr valign="baseline"> <tr valign="baseline">
<td><code><a name="forward_iterator_helper">forward_iterator_helper&lt;T, V, D, P, R&gt;</a></code></td> <td><code><a name="forward_iterator_helper">forward_iterator_helper&lt;T, V, D, P, R&gt;</a></code></td>
@ -975,6 +978,53 @@ C++ standard (<code>iterator_category</code>, <code>value_type</code>,
</tr> </tr>
</table> </table>
<h3><a name="iterator_helpers_notes">Iterator Helper Notes</a></h3>
<p><a name="1">[1]</a> Unlike other iterator helpers templates,
<code>output_iterator_helper</code> takes only one template parameter - the type of
its target class. Although to some it might seem like an unnecessary
restriction, the standard requires <code>difference_type</code> and
<code>value_type</code> of any output iterator to be
<code>void</code> (24.3.1 [lib.iterator.traits]), and
<code>output_iterator_helper</code> template respects this
requirement. Also, output iterators in the standard have void <tt>pointer</tt> and
<tt>reference</tt> types, so the <tt>output_iterator_helper</tt> does the
same.
<p><a name="2">[2]</a> As self-proxying is the easiest and most common way to
implement output iterators (see, for example, insert [24.4.2] and stream
iterators [24.5] in the standard library), <code>output_iterator_helper</code>
supports the idiom by defining <code>operator*</code>
and <code>operator++</code> member functions which just return a
non-const reference to the iterator itself. Support for
self-proxying allows us, in many cases, to reduce the task of writing an output
iterator to writing just two member functions - an appropriate
constructor and a copy-assignment operator. For example, here is a possible
implementation of <code><a href="function_output_iterator.htm">boost::function_output_iterator</a></code>
adaptor:</p>
<pre>
template&lt;class UnaryFunction&gt;
struct function_output_iterator
: boost::output_iterator_helper&lt; function_output_iterator&lt;UnaryFunction&gt; &gt;
{
explicit function_output_iterator(UnaryFunction const& f = UnaryFunction())
: func(f) {}
template&lt;typename T&gt;
function_output_iterator& operator=(T const& value)
{
this->func(value);
return *this;
}
private:
UnaryFunction func;
};
</pre>
<p>Note that support for self-proxying does not prevent you from using <code>output_iterator_helper</code> to ease any other, different kind of output iterator's implementation. If <code>output_iterator_helper</code>'s target type provides its own definition of <code>operator*</code> or/and <code>operator++</code>, then these operators will get used and the ones supplied by <code>output_iterator_helper</code> will never be instantiated.</p>
<h3><a name="i_demo">Iterator Demonstration</a> and Test Program</h3> <h3><a name="i_demo">Iterator Demonstration</a> and Test Program</h3>
<p>The <cite><a href="iterators_test.cpp">iterators_test.cpp</a></cite> <p>The <cite><a href="iterators_test.cpp">iterators_test.cpp</a></cite>
@ -1092,7 +1142,7 @@ the library remain backward-compatible.</p>
<hr> <hr>
<p>Revised: 20 May 2001</p> <p>Revised: 25 Jun 2001</p>
<p>Copyright &copy; David Abrahams and Beman Dawes 1999-2001. <p>Copyright &copy; David Abrahams and Beman Dawes 1999-2001.
Permission to copy, use, modify, sell and distribute this document is Permission to copy, use, modify, sell and distribute this document is