iterator_tags.html

指导程序员合理、高效的进行标准模板库编程。

This example does exactly the same thing, using <tt><A href="iterator_traits.html">iterator_traits</A></tt>
instead.  Note how much simpler it is: the auxiliary function is
no longer required.
<pre>
    template &lt;class <A href="ForwardIterator.html">ForwardIterator</A>1, class <A href="ForwardIterator.html" tppabs="http://www.sgi.com/Technology/STL/ForwardIterator.shtml">ForwardIterator</A>2&gt;
    inline void iter_swap(ForwardIterator1 a, ForwardIterator2 b) {
        <A href="iterator_traits.html">iterator_traits</A>&lt;ForwardIterator1&gt;::value_type tmp = *a;
        *a = *b;
        *b = tmp;    
    }
</pre>
<P>
This example uses the <tt><A href="iterator_category.html">iterator_category</A></tt>
iterator tag function: <tt><A href="reverse.html">reverse</A></tt> can be implemented for either
<A href="BidirectionalIterator.html">Bidirectional Iterator</A>s or for <A href="RandomAccessIterator.html" tppabs="http://www.sgi.com/Technology/STL/RandomAccessIterator.shtml">Random Access Iterators</A>,
but the algorithm for <A href="RandomAccessIterator.html">Random Access Iterators</A> is more efficient.
Consequently, <tt><A href="reverse.html">reverse</A></tt> is written to dispatch on the iterator
category.  This dispatch takes place at compile time, and should not
incur any run-time penalty.
<pre>
    template &lt;class <A href="BidirectionalIterator.html">BidirectionalIterator</A>&gt;
    void __reverse(BidirectionalIterator first, BidirectionalIterator last, 
		   <A href="bidirectional_iterator_tag.html">bidirectional_iterator_tag</A>) {
	while (true)
	    if (first == last || first == --last)
		return;
	    else
		iter_swap(first++, last);
    }

    template &lt;class <A href="RandomAccessIterator.html">RandomAccessIterator</A>&gt;
    void __reverse(RandomAccessIterator first, RandomAccessIterator last,
		   <A href="random_access_iterator_tag.html">random_access_iterator_tag</A>) {
	while (first &lt; last) iter_swap(first++, --last);
    }

    template &lt;class <A href="BidirectionalIterator.html">BidirectionalIterator</A>&gt;
    inline void <A href="reverse.html">reverse</A>(BidirectionalIterator first, BidirectionalIterator last) {
	__reverse(first, last, <A href="iterator_category.html">iterator_category</A>(first));
    }
</pre>
<P>
In this case, <tt><A href="iterator_traits.html">iterator_traits</A></tt> would not be different in any
substantive way: it would still be necessary to use auxiliary 
functions to dispatch on the iterator category.  The only difference
is changing the top-level function to 
<pre>
    template &lt;class <A href="BidirectionalIterator.html">BidirectionalIterator</A>&gt;
    inline void <A href="reverse.html">reverse</A>(BidirectionalIterator first, BidirectionalIterator last) {
	__reverse(first, last, 
                  <A href="iterator_traits.html">iterator_traits</A>&lt;first&gt;::iterator_category());
    }
</pre>
<h3>Concepts</h3>
<h3>Types</h3>
<UL>
<LI>
<tt><A href="output_iterator.html">output_iterator</A></tt>
<LI>
<tt><A href="input_iterator.html">input_iterator</A></tt>
<LI>
<tt><A href="forward_iterator.html">forward_iterator</A></tt>
<LI>
<tt><A href="bidirectional_iterator.html">bidirectional_iterator</A></tt>
<LI>
<tt><A href="random_access_iterator.html">random_access_iterator</A></tt>
</UL>
<UL>
<LI>
<tt><A href="output_iterator_tag.html">output_iterator_tag</A></tt>
<LI>
<tt><A href="input_iterator_tag.html">input_iterator_tag</A></tt>
<LI>
<tt><A href="forward_iterator_tag.html">forward_iterator_tag</A></tt>
<LI>
<tt><A href="bidirectional_iterator_tag.html">bidirectional_iterator_tag</A></tt>
<LI>
<tt><A href="random_access_iterator_tag.html">random_access_iterator_tag</A></tt>
</UL>
<UL>
<LI>
<tt><A href="iterator_traits.html">iterator_traits</A></tt>
</UL>
<h3>Functions</h3>
<UL>
<LI>
<tt><A href="iterator_category.html">iterator_category</A></tt>
<LI>
<tt><A href="value_type.html">value_type</A></tt>
<LI>
<tt><A href="distance_type.html">distance_type</A></tt>
</UL>
<h3>Notes</h3>
<P><A name="1">[1]</A>
 <A href="OutputIterator.html">Output Iterators</A> have neither a distance type nor a value
type; in many ways, in fact, <A href="OutputIterator.html">Output Iterators</A> aren't really
iterators.  Output iterators do not have a value type, because it is
impossible to obtain a value from an output iterator but only to write
a value through it.  They do not have a distance type, similarly,
because it is impossible to find the distance from one output iterator
to another.  Finding a distance requires a comparison for equality,
and output iterators do not support <tt>operator==</tt>.
<P><A name="2">[2]</A>
The <tt><A href="iterator_traits.html">iterator_traits</A></tt> class
relies on a C++ feature known as <i>partial specialization</i>.  Many of
today's compilers don't implement the complete standard; in
particular, many compilers do not support partial specialization.  If
your compiler does not support partial specialization, then you will
not be able to use <tt><A href="iterator_traits.html">iterator_traits</A></tt>, and you will have to 
continue to use the older iterator tag functions.
<P><A name="3">[3]</A>
Note that <A href="trivial.html">Trivial Iterator</A> does not appear in this list.
The <A href="trivial.html">Trivial Iterator</A> concept is introduced solely for conceptual
clarity; the STL does not actually define any <A href="trivial.html">Trivial Iterator</A>
types, so there is no need for a <A href="trivial.html">Trivial Iterator</A> tag.  There
is, in fact, a strong reason not to define one: the C++ type system
does not provide any way to distinguish between a pointer that is
being used as a trivial iterator (that is, a pointer to an object
that isn't part of an array) and a pointer that is being used as a
<A href="RandomAccessIterator.html">Random Access Iterator</A> into an array.
<h3>See also</h3>
<A href="InputIterator.html">Input Iterator</A>, <A href="OutputIterator.html" tppabs="http://www.sgi.com/Technology/STL/OutputIterator.shtml">Output Iterator</A>, <A href="ForwardIterator.html" tppabs="http://www.sgi.com/Technology/STL/ForwardIterator.shtml">Forward Iterator</A>, 
<A href="BidirectionalIterator.html">Bidirectional Iterator</A>, <A href="RandomAccessIterator.html" tppabs="http://www.sgi.com/Technology/STL/RandomAccessIterator.shtml">Random Access Iterator</A>,
<tt><A href="iterator_traits.html">iterator_traits</A></tt>, <A href="Iterators.html" tppabs="http://www.sgi.com/Technology/STL/Iterators.shtml">Iterator Overview</A>

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