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<h1>Reverse Iterator Adaptor</h1>
Defined in header <a href=
"../../boost/iterator_adaptors.hpp">boost/iterator_adaptors.hpp</a>
<p>The reverse iterator adaptor flips the direction of a base iterator's
motion. Invoking <tt>operator++()</tt> moves the base iterator backward and
invoking <tt>operator--()</tt> moves the base iterator forward. The Boost
reverse iterator adaptor is better to use than the
<tt>std::reverse_iterator</tt> class in situations where pairs of
mutable/constant iterators are needed (e.g., in containers) because
comparisons and conversions between the mutable and const versions are
implemented correctly.
<h2>Synopsis</h2>
<pre>
namespace boost {
template <class <a href=
"http://www.sgi.com/tech/stl/BidirectionalIterator.html">BidirectionalIterator</a>,
class Value, class Reference, class Pointer, class Category, class Distance>
struct reverse_iterator_generator;
template <class <a href=
"http://www.sgi.com/tech/stl/BidirectionalIterator.html">BidirectionalIterator</a>>
typename reverse_iterator_generator<BidirectionalIterator>::type
make_reverse_iterator(BidirectionalIterator base)
}
</pre>
<hr>
<h2><a name="reverse_iterator_generator">The Reverse Iterator Type
Generator</a></h2>
The <tt>reverse_iterator_generator</tt> template is a <a href=
"../../more/generic_programming.html#type_generator">generator</a> of
reverse iterator types. The main template parameter for this class is the
base <tt>BidirectionalIterator</tt> type that is being adapted. In most
cases the associated types of the base iterator can be deduced using
<tt>std::iterator_traits</tt>, but in some situations the user may want to
override these types, so there are also template parameters for the base
iterator's associated types.
<blockquote>
<pre>
template <class <a href=
"http://www.sgi.com/tech/stl/BidirectionalIterator.html">BidirectionalIterator</a>,
class Value, class Reference, class Pointer, class Category, class Distance>
class reverse_iterator_generator
{
public:
typedef <tt><a href=
"./iterator_adaptors.htm#iterator_adaptor">iterator_adaptor</a><...></tt> type; // the resulting reverse iterator type
};
</pre>
</blockquote>
<h3>Example</h3>
In this example we sort a sequence of letters and then output the sequence
in descending order using reverse iterators.
<blockquote>
<pre>
#include <boost/config.hpp>
#include <iostream>
#include <algorithm>
#include <boost/iterator_adaptors.hpp>
int main(int, char*[])
{
char letters[] = "hello world!";
const int N = sizeof(letters)/sizeof(char) - 1;
std::cout << "original sequence of letters:\t"
<< letters << std::endl;
std::sort(letters, letters + N);
// Use reverse_iterator_generator to print a sequence
// of letters in reverse order.
boost::reverse_iterator_generator<char*>::type
reverse_letters_first(letters + N),
reverse_letters_last(letters);
std::cout << "letters in descending order:\t";
std::copy(reverse_letters_first, reverse_letters_last,
std::ostream_iterator<char>(std::cout));
std::cout << std::endl;
// to be continued...
</pre>
</blockquote>
The output is:
<blockquote>
<pre>
original sequence of letters: hello world!
letters in descending order: wroolllhed!
</pre>
</blockquote>
<h3>Template Parameters</h3>
<table border>
<tr>
<th>Parameter
<th>Description
<tr>
<td><tt><a href=
"http://www.sgi.com/tech/stl/BidirectionalIterator.html">BidirectionalIterator</a></tt>
<td>The iterator type being wrapped.
<tr>
<td><tt>Value</tt>
<td>The value-type of the base iterator and the resulting reverse
iterator.<br>
<b>Default:</b><tt>std::iterator_traits<BidirectionalIterator>::value_type</tt>
<tr>
<td><tt>Reference</tt>
<td>The <tt>reference</tt> type of the resulting iterator, and in
particular, the result type of <tt>operator*()</tt>.<br>
<b>Default:</b> If <tt>Value</tt> is supplied, <tt>Value&</tt> is
used. Otherwise
<tt>std::iterator_traits<BidirectionalIterator>::reference</tt>
is used.
<tr>
<td><tt>Pointer</tt>
<td>The <tt>pointer</tt> type of the resulting iterator, and in
particular, the result type of <tt>operator->()</tt>.<br>
<b>Default:</b> If <tt>Value</tt> was supplied, then <tt>Value*</tt>,
otherwise
<tt>std::iterator_traits<BidirectionalIterator>::pointer</tt>.
<tr>
<td><tt>Category</tt>
<td>The <tt>iterator_category</tt> type for the resulting iterator.<br>
<b>Default:</b>
<tt>std::iterator_traits<BidirectionalIterator>::iterator_category</tt>
<tr>
<td><tt>Distance</tt>
<td>The <tt>difference_type</tt> for the resulting iterator.<br>
<b>Default:</b>
<tt>std::iterator_traits<BidirectionalIterator&gt::difference_type</tt>
</table>
<h3>Concept Model</h3>
The indirect iterator will model whichever <a href=
"http://www.sgi.com/tech/stl/Iterators.html">standard iterator concept
category</a> is modeled by the base iterator. Thus, if the base iterator is
a model of <a href=
"http://www.sgi.com/tech/stl/RandomAccessIterator.html">Random Access
Iterator</a> then so is the resulting indirect iterator. If the base
iterator models a more restrictive concept, the resulting indirect iterator
will model the same concept. The base iterator must be at least a <a href=
"http://www.sgi.com/tech/stl/BidirectionalIterator.html">Bidirectional
Iterator</a>
<h3>Members</h3>
The reverse iterator type implements the member functions and operators
required of the <a href=
"http://www.sgi.com/tech/stl/RandomAccessIterator.html">Random Access
Iterator</a> concept. In addition it has the following constructor:
<blockquote>
<pre>
reverse_iterator_generator::type(const BidirectionalIterator& it)
</pre>
</blockquote>
<br>
<br>
<hr>
<p>
<h2><a name="make_reverse_iterator">The Reverse Iterator Object
Generator</a></h2>
The <tt>make_reverse_iterator()</tt> function provides a more convenient
way to create reverse iterator objects. The function saves the user the
trouble of explicitly writing out the iterator types.
<blockquote>
<pre>
template <class BidirectionalIterator>
typename reverse_iterator_generator<BidirectionalIterator>::type
make_reverse_iterator(BidirectionalIterator base);
</pre>
</blockquote>
<h3>Example</h3>
In this part of the example we use <tt>make_reverse_iterator()</tt> to
print the sequence of letters in reverse-reverse order, which is the
original order.
<blockquote>
<pre>
// continuing from the previous example...
std::cout << "letters in ascending order:\t";
std::copy(boost::make_reverse_iterator(reverse_letters_last),
boost::make_reverse_iterator(reverse_letters_first),
std::ostream_iterator<char>(std::cout));
std::cout << std::endl;
return 0;
}
</pre>
</blockquote>
The output is:
<blockquote>
<pre>
letters in ascending order: !dehllloorw
</pre>
</blockquote>
<hr>
<h2><a name="interactions">Constant/Mutable Iterator Interactions</a></h2>
<p>One failing of the standard <tt><a
href="http://www.sgi.com/tech/stl/ReverseIterator.html">reverse_iterator</a></tt>
adaptor is that it doesn't properly support interactions between adapted
<tt>const</tt> and non-<tt>const</tt> iterators. For example:
<blockquote>
<pre>
#include <vector>
template <class T> void convert(T x) {}
// Test interactions of a matched pair of random access iterators
template <class Iterator, class ConstIterator>
void test_interactions(Iterator i, ConstIterator ci)
{
bool eq = i == ci; // comparisons
bool ne = i != ci;
bool lt = i < ci;
bool le = i <= ci;
bool gt = i > ci;
bool ge = i >= ci;
std::size_t distance = i - ci; // difference
ci = i; // assignment
ConstIterator ci2(i); // construction
convert<ConstIterator>(i); // implicit conversion
}
void f()
{
typedef std::vector<int> vec;
vec v;
const vec& cv;
test_interactions(v.begin(), cv.begin()); // <font color="#007F00">OK</font>
test_interactions(v.rbegin(), cv.rbegin()); // <font color="#FF0000">ERRORS ON EVERY TEST!!</font>
</pre>
</blockquote>
Reverse iterators created with <tt>boost::reverse_iterator_generator</tt> don't have this problem, though:
<blockquote>
<pre>
typedef boost::reverse_iterator_generator<vec::iterator>::type ri;
typedef boost::reverse_iterator_generator<vec::const_iterator>::type cri;
test_interactions(ri(v.begin()), cri(cv.begin())); // <font color="#007F00">OK!!</font>
</pre>
</blockquote>
Or, more simply,
<blockquote>
<pre>
test_interactions(
boost::make_reverse_iterator(v.begin()),
boost::make_reverse_iterator(cv.begin())); // <font color="#007F00">OK!!</font>
}
</pre>
</blockquote>
<p>If you are wondering why there is no
<tt>reverse_iterator_pair_generator</tt> in the manner of <tt><a
href="projection_iterator.htm#projection_iterator_pair_generator">projection_iterator_pair_generator</a></tt>,
the answer is simple: we tried it, but found that in practice it took
<i>more</i> typing to use <tt>reverse_iterator_pair_generator</tt> than to
simply use <tt>reverse_iterator_generator</tt> twice!<br><br>
<hr>
<p>Revised
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %b %Y" startspan -->19 Aug 2001<!--webbot bot="Timestamp" endspan i-checksum="14767" -->
<p>© Copyright Jeremy Siek 2000. Permission to copy, use, modify, sell
and distribute this document is granted provided this copyright notice
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.
<!-- LocalWords: html charset alt gif hpp BidirectionalIterator const namespace struct
-->
<!-- LocalWords: ConstPointer ConstReference typename iostream int abcdefg
-->
<!-- LocalWords: sizeof PairGen pre Siek wroolllhed dehllloorw
-->
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