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added section on "tag dispatching"

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[SVN r9154]
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Jeremy Siek 2001-02-12 05:04:28 +00:00
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@ -20,6 +20,8 @@
<ul>
<li><a href="#traits">Traits</a>
<li><a href="#tag_dispatching">Tag Dispatching</a>
<li><a href="#type_generator">Type Generators</a>
<li><a href="#object_generator">Object Generators</a>
@ -65,6 +67,88 @@ struct iterator_traits {
standard is <tt>std::numeric_limits&lt;T&gt;</tt> which provides constants
describing the range and capabilities of numeric types.
<h2><a name="tag_dispatching">Tag Dispatching</a></h2>
<p>
A technique that often goes hand in hand with traits classes is
tag dispatching, which is a way of using function overloading to
dispatch based on properties of a type. A good example of this is
the implementation of the <a
href="http://www.sgi.com/tech/stl/advance.html"><tt>std::advance()</tt></a>
function in the C++ Standard Library, which increments an
iterator <tt>n</tt> times. Depending on the kind of iterator,
there are different optimizations that can be applied in the
implementation. If the iterator is <a
href="http://www.sgi.com/tech/stl/RandomAccessIterator.html">random
access</a> (can jump forward and backward arbitrary distances),
then the <tt>advance()</tt> function can simply be implemented
with <tt>i += n</tt>, and is very efficient: constant time. If
the iterator is <a
href="http://www.sgi.com/tech/stl/BidirectionalIterator.html">bidirectional</a>,
then it makes sense for <tt>n</tt> to be negative, we can
decrement the iterator <tt>n</tt> times.
</p>
<p>
The relation between tag dispatching and traits classes is
that the property used for dispatching (in this case the
<tt>iterator_category</tt>) is accessed through a traits class.
The main <tt>advance()</tt> function uses the <a
href="http://www.sgi.com/tech/stl/iterator_traits.html"><tt>iterator_traits</tt></a>
class to get the <tt>iterator_category</tt>. It then makes a call
the the overloaded <tt>advance_dispatch()</tt> function.
The
appropriate <tt>advance_dispatch()</tt> is selected by the
compiler based on whatever type the <tt>iterator_category</tt>
resolves to, either <a
href="http://www.sgi.com/tech/stl/input_iterator_tag.html">
<tt>input_iterator_tag</tt></a>, <a
href="http://www.sgi.com/tech/stl/bidirectional_iterator_tag.html">
<tt>bidirectional_iterator_tag</tt></a>, or <a
href="http://www.sgi.com/tech/stl/random_access_iterator_tag.html">
<tt>random_access_iterator_tag</tt></a>. A <b><i>tag</i></b> is
simply a class whose only purpose is to convey some property for
use in tag dispatching and similar techniques. Refer to <a
href="http://www.sgi.com/tech/stl/iterator_tags.html">this
page</a> for a more detailed description of iterator tags.
</p>
<blockquote>
<pre>
namespace std {
struct input_iterator_tag { };
struct bidirectional_iterator_tag { };
struct random_access_iterator_tag { };
namespace detail {
template &lt;class InputIterator, class Distance&gt;
void advance_dispatch(InputIterator&amp; i, Distance n, <b>input_iterator_tag</b>) {
while (n--) ++i;
}
template &lt;class BidirectionalIterator, class Distance&gt;
void advance_dispatch(BidirectionalIterator&amp; i, Distance n,
<b>bidirectional_iterator_tag</b>) {
if (n &gt;= 0)
while (n--) ++i;
else
while (n++) --i;
}
template &lt;class RandomAccessIterator, class Distance&gt;
void advance_dispatch(RandomAccessIterator&amp; i, Distance n,
<b>random_access_iterator_tag</b>) {
i += n;
}
}
template &lt;class InputIterator, class Distance&gt;
void advance(InputIterator&amp; i, Distance n) {
typename <b>iterator_traits&lt;InputIterator&gt;::iterator_category</b> category;
detail::advance_dispatch(i, n, <b>category</b>);
}
}
</pre>
</blockquote>
<h2><a name="type_generator">Type Generators</a></h2>
<p>A <i>type generator</i> is a template whose only purpose is to
@ -202,3 +286,9 @@ void append_sequence(Container&amp; c, Iterator start, Iterator finish)
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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