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<title>Problem with is_writable and is_swappable in N1550</title>
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<div class="document" id="problem-with-is-writable-and-is-swappable-in-n1550">
<h1 class="title">Problem with <tt class="literal"><span class="pre">is_writable</span></tt> and <tt class="literal"><span class="pre">is_swappable</span></tt> in <a class="reference" href="http://www.boost-consulting.com/writing/n1550.html">N1550</a></h1>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Author:</th><td class="field-body">David Abrahams and Jeremy Siek</td>
</tr>
<tr class="field"><th class="field-name">Contact:</th><td class="field-body"><a class="reference" href="mailto:dave&#64;boost-consulting.com">dave&#64;boost-consulting.com</a>, <a class="reference" href="mailto:jsiek&#64;osl.iu.edu">jsiek&#64;osl.iu.edu</a></td>
</tr>
<tr class="field"><th class="field-name">Organization:</th><td class="field-body"><a class="reference" href="http://www.boost-consulting.com">Boost Consulting</a>, Indiana University Bloomington</td>
</tr>
<tr class="field"><th class="field-name">date:</th><td class="field-body">$Date$</td>
</tr>
<tr class="field"><th class="field-name">Copyright:</th><td class="field-body">Copyright David Abrahams, Jeremy Siek 2003. Use, modification and
distribution is subject to the Boost Software License,
Version 1.0. (See accompanying file LICENSE_1_0.txt or copy
at <a class="reference" href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</td>
</tr>
</tbody>
</table>
<div class="contents topic" id="table-of-contents">
<p class="topic-title"><a name="table-of-contents">Table of Contents</a></p>
<ul class="simple">
<li><a class="reference" href="#introduction" id="id3" name="id3">Introduction</a></li>
<li><a class="reference" href="#proposed-resolution" id="id4" name="id4">Proposed Resolution</a></li>
<li><a class="reference" href="#rationale" id="id5" name="id5">Rationale</a></li>
</ul>
</div>
<div class="section" id="introduction">
<h1><a class="toc-backref" href="#id3" name="introduction">Introduction</a></h1>
<p>The <tt class="literal"><span class="pre">is_writable</span></tt> and <tt class="literal"><span class="pre">is_swappable</span></tt> traits classes in <a class="reference" href="http://www.boost-consulting.com/writing/n1550.html">N1550</a>
provide a mechanism for determining at compile time if an iterator
type is a model of the new Writable Iterator and Swappable Iterator
concepts, analogous to <tt class="literal"><span class="pre">iterator_traits&lt;X&gt;::iterator_category</span></tt>
for the old iterator concepts. For backward compatibility,
<tt class="literal"><span class="pre">is_writable</span></tt> and <tt class="literal"><span class="pre">is_swappable</span></tt> not only work with new
iterators, but they also are intended to work for old
iterators (iterators that meet the requirements for one of the
iterator concepts in the current standard). In the case of old
iterators, the writability and swapability is deduced based on the
<tt class="literal"><span class="pre">iterator_category</span></tt> and also the <tt class="literal"><span class="pre">reference</span></tt> type. The
specification for this deduction gives false positives for forward
iterators that have non-assignable value types.</p>
<p>To review, the part of the <tt class="literal"><span class="pre">is_writable</span></tt> trait definition which
applies to old iterators is:</p>
<pre class="literal-block">
if (cat is convertible to output_iterator_tag)
return true;
else if (cat is convertible to forward_iterator_tag
and iterator_traits&lt;Iterator&gt;::reference is a
mutable reference)
return true;
else
return false;
</pre>
<p>Suppose the <tt class="literal"><span class="pre">value_type</span></tt> of the iterator <tt class="literal"><span class="pre">It</span></tt> has a private
assignment operator:</p>
<pre class="literal-block">
class B {
public:
...
private:
B&amp; operator=(const B&amp;);
};
</pre>
<p>and suppose the <tt class="literal"><span class="pre">reference</span></tt> type of the iterator is <tt class="literal"><span class="pre">B&amp;</span></tt>. In
that case, <tt class="literal"><span class="pre">is_writable&lt;It&gt;::value</span></tt> will be true when in fact
attempting to write into <tt class="literal"><span class="pre">B</span></tt> will cause an error.</p>
<p>The same problem applies to <tt class="literal"><span class="pre">is_swappable</span></tt>.</p>
</div>
<div class="section" id="proposed-resolution">
<h1><a class="toc-backref" href="#id4" name="proposed-resolution">Proposed Resolution</a></h1>
<ol class="arabic">
<li><p class="first">Remove the <tt class="literal"><span class="pre">is_writable</span></tt> and <tt class="literal"><span class="pre">is_swappable</span></tt> traits, and remove the
requirements in the Writable Iterator and Swappable Iterator concepts
that require their models to support these traits.</p>
</li>
<li><p class="first">Change the <tt class="literal"><span class="pre">is_readable</span></tt> specification to be:
<tt class="literal"><span class="pre">is_readable&lt;X&gt;::type</span></tt> is <tt class="literal"><span class="pre">true_type</span></tt> if the
result type of <tt class="literal"><span class="pre">X::operator*</span></tt> is convertible to
<tt class="literal"><span class="pre">iterator_traits&lt;X&gt;::value_type</span></tt> and is <tt class="literal"><span class="pre">false_type</span></tt>
otherwise.</p>
<p>Remove the requirement for support of the <tt class="literal"><span class="pre">is_readable</span></tt> trait from
the Readable Iterator concept.</p>
</li>
</ol>
<!-- We should give some attention to the UnaryTypeTrait concept,
which requires the trait to be derived from either true_type or
false_type (as of the last LWG meeting). -->
<ol class="arabic" start="3">
<li><p class="first">Change <tt class="literal"><span class="pre">iterator_tag</span></tt> to:</p>
<pre class="literal-block">
template &lt;class Value, class Reference, class Traversal&gt;
struct iterator_tag;
</pre>
<p>The argument for <tt class="literal"><span class="pre">Value</span></tt> must be the <tt class="literal"><span class="pre">value_type</span></tt> of the
iterator, possibly const-qualified, <tt class="literal"><span class="pre">Reference</span></tt> must be the
return type of <tt class="literal"><span class="pre">operator*</span></tt> <a class="footnote-reference" href="#id2" id="id1" name="id1"><sup>*</sup></a>, and <tt class="literal"><span class="pre">Traversal</span></tt> the
traversal tag for the iterator.</p>
</li>
</ol>
<!-- I think the language above is still too informal. There is no
"the iterator", when considering iterator_tag in isolation.
Perhaps that language belongs in a non-normative note
``iterator_tag<Value,Reference,Traversal>`` is required to be
convertible to both ``Traversal`` tag and also to the
``iterator_category`` type specified by the following
pseudo-code::
old-category(Value, Reference, Traversal) =
if (Reference is a reference
and Traversal is convertible to forward_traversal_tag)
{
if (Traversal is convertible to random_access_traversal_tag)
return random_access_iterator_tag;
else if (Traversal is convertible to bidirectional_traversal_tag)
return bidirectional_iterator_tag;
else
return forward_iterator_tag;
}
else if (Traversal is convertible to single_pass_traversal_tag
and Reference is convertible to Value)
{
if (Value is const)
return input_iterator_tag;
else
return input_output_iterator_tag;
} else
return output_iterator_tag; -->
<!-- I reformatted the code for legibility; sorry. -->
<table class="footnote" frame="void" id="id2" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1" name="id2">[*]</a></td><td>Instead of saying &quot;return type of operator*&quot;, we could have
said <tt class="literal"><span class="pre">iterator_traits&lt;X&gt;::reference</span></tt>. However, the standard
specifies nothing about <tt class="literal"><span class="pre">iterator_traits&lt;X&gt;::reference</span></tt> in
many cases, which we believe is a defect. Furthermore, in some
cases it explicitly differs from the return type of
<tt class="literal"><span class="pre">operator*</span></tt>, for example see <tt class="literal"><span class="pre">istreambuf_iterator</span></tt>.</td></tr>
</tbody>
</table>
<ol class="arabic" start="4">
<li><p class="first">Change the specification of <tt class="literal"><span class="pre">traversal_category</span></tt> to:</p>
<pre class="literal-block">
traversal-category(Iterator) =
let cat = iterator_traits&lt;Iterator&gt;::iterator_category
if (cat is convertible to incrementable_iterator_tag)
return cat; // Iterator is a new iterator
else if (cat is convertible to random_access_iterator_tag)
return random_access_traversal_tag;
else if (cat is convertible to bidirectional_iterator_tag)
return bidirectional_traversal_tag;
else if (cat is convertible to forward_iterator_tag)
return forward_traversal_tag;
else if (cat is convertible to input_iterator_tag)
return single_pass_iterator_tag;
else if (cat is convertible to output_iterator_tag)
return incrementable_iterator_tag;
else
return null_category_tag;
</pre>
</li>
</ol>
</div>
<div class="section" id="rationale">
<h1><a class="toc-backref" href="#id5" name="rationale">Rationale</a></h1>
<ol class="arabic simple">
<li>There are two reasons for removing <tt class="literal"><span class="pre">is_writable</span></tt>
and <tt class="literal"><span class="pre">is_swappable</span></tt>. The first is that we do not know of
a way to fix the specification so that it gives the correct
answer for all iterators. Second, there was only a weak
motivation for having <tt class="literal"><span class="pre">is_writable</span></tt> and <tt class="literal"><span class="pre">is_swappable</span></tt>
there in the first place. The main motivation was simply
uniformity: we have tags for the old iterator categories
so we should have tags for the new iterator categories.
While having tags and the capability to dispatch based
on the traversal categories is often used, we see
less of a need for dispatching based on writability
and swappability, since typically algorithms
that need these capabilities have no alternative if
they are not provided.</li>
<li>We discovered that the <tt class="literal"><span class="pre">is_readable</span></tt> trait can be implemented
using only the iterator type itself and its <tt class="literal"><span class="pre">value_type</span></tt>.
Therefore we remove the requirement for <tt class="literal"><span class="pre">is_readable</span></tt> from the
Readable Iterator concept, and change the definition of
<tt class="literal"><span class="pre">is_readable</span></tt> so that it works for any iterator type.</li>
<li>With <tt class="literal"><span class="pre">is_writable</span></tt> and <tt class="literal"><span class="pre">is_swappable</span></tt> gone, and
<tt class="literal"><span class="pre">is_readable</span></tt> no longer in need of special hints,
there is no reason for the <tt class="literal"><span class="pre">iterator_tag</span></tt> class to provide
information about the access capabilities of an iterator.
This new version provides only information about the traversal
capabilities and the old iterator category tag. Instead of accessing
the traversal category as a nested typedef <tt class="literal"><span class="pre">::traversal</span></tt>,
the <tt class="literal"><span class="pre">iterator_tag</span></tt> itself will be convertible to the traversal
tag. The <tt class="literal"><span class="pre">access_bits</span></tt> parameter is no longer needed for
specifying the access member (which is now gone). However,
some access information is still needed so that we can
deduce the appropriate old iterator category. The
<tt class="literal"><span class="pre">Value</span></tt> and <tt class="literal"><span class="pre">Reference</span></tt> parameters fill this need.
Note that this solution cleans up the issue that John
Maddock raised on the reflector (<tt class="literal"><span class="pre">c++std-lib-12187</span></tt>) about the non-uniformity
of the lvalue bit.</li>
<li>The changes to the specification of <tt class="literal"><span class="pre">traversal_category</span></tt> are a
direct result of the changes to <tt class="literal"><span class="pre">iterator_tag</span></tt>.</li>
</ol>
</div>
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