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A few fixes for the "tag dispatching" section.

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[SVN r9513]
This commit is contained in:
Dave Abrahams 2001-03-09 02:07:39 +00:00
parent 4f44c61501
commit 1a251e261a
1 changed files with 10 additions and 8 deletions
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18
generic_programming.html
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@ -206,8 +206,8 @@ struct iterator_traits {
<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=
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
@ -215,14 +215,16 @@ struct iterator_traits {
<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=
and is very efficient: constant time. Other iterators must be
<tt>advance</tt>d in steps, making the operation linear in n. 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.
then it makes sense for <tt>n</tt> to be negative, so we must decide whether
to increment or decrement the iterator.
<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
is often 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
@ -256,9 +258,9 @@ namespace std {
void advance_dispatch(BidirectionalIterator&amp; i, Distance n,
<b>bidirectional_iterator_tag</b>) {
if (n &gt;= 0)
while (n--) ++i;
while (n--) ++i;
else
while (n++) --i;
while (n++) --i;
}
template &lt;class RandomAccessIterator, class Distance&gt;