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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>
<li><a href="#dereference">Dereference operators</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
Program</a></li>
</ul></li>
@ -889,7 +890,7 @@ href="#chaining">base class chaining</a>.</p>
<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
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
@ -931,11 +932,13 @@ C++ standard (<code>iterator_category</code>, <code>value_type</code>,
</ul></td>
</tr>
<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
<ul>
<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 valign="baseline">
<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>
</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>
<p>The <cite><a href="iterators_test.cpp">iterators_test.cpp</a></cite>
@ -1092,7 +1142,7 @@ the library remain backward-compatible.</p>
<hr>
<p>Revised: 20 May 2001</p>
<p>Revised: 25 Jun 2001</p>
<p>Copyright &copy; David Abrahams and Beman Dawes 1999-2001.
Permission to copy, use, modify, sell and distribute this document is