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Describe const/mutable interactions and give rationale for no

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reverse_iterator_pair_generator.


[SVN r9285]
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Dave Abrahams 2001-02-20 03:04:03 +00:00
parent d9d58ea66e
commit 168012b465
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reverse_iterator.htm
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@ -23,7 +23,7 @@
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/const iterators are needed (e.g., in containers) because
mutable/constant iterators are needed (e.g., in containers) because
comparisons and conversions between the mutable and const versions are
implemented correctly.
@ -194,6 +194,8 @@ letters in descending order: wroolllhed!
reverse_iterator_generator::type(const BidirectionalIterator&amp; it)
</pre>
</blockquote>
<br>
<br>
@ -243,6 +245,72 @@ letters in ascending order: !dehllloorw
</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 &lt;vector&gt;
template &lt;class T&gt; void convert(T x) {}
// Test interactions of a matched pair of random access iterators
template &lt;class Iterator, class ConstIterator&gt;
void test_interactions(Iterator i, ConstIterator ci)
{
bool eq = i == ci; // comparisons
bool ne = i != ci;
bool lt = i &lt; ci;
bool le = i &lt;= ci;
bool gt = i &gt; ci;
bool ge = i &gt;= ci;
std::size_t distance = i - ci; // difference
ci = i; // assignment
ConstIterator ci2(i); // construction
convert&lt;ConstIterator&gt;(i); // implicit conversion
}
void f()
{
typedef std::vector&lt;int&gt; vec;
vec v;
const vec&amp; 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&lt;vec::iterator&gt;::type ri;
typedef boost::reverse_iterator_generator&lt;vec::const_iterator&gt;::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 -->15
Feb 2001<!--webbot bot="Timestamp" endspan i-checksum="14373" -->