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<title>Problem with is_writable and is_swappable in N1550</title> <title>Problem with is_writable and is_swappable in N1550</title>
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<body> <body>
<div class="document" id="problem-with-is-writable-and-is-swappable-in-n1550"> <div class="document" id="problem-with-is-writable-and-is-swappable-in-n1550">
@ -223,13 +32,13 @@ at <a class="reference" href="http://www.boost.org/LICENSE_1_0.txt">http://www.b
<div class="contents topic" id="table-of-contents"> <div class="contents topic" id="table-of-contents">
<p class="topic-title"><a name="table-of-contents">Table of Contents</a></p> <p class="topic-title"><a name="table-of-contents">Table of Contents</a></p>
<ul class="simple"> <ul class="simple">
<li><a class="reference" href="#introduction" id="id3" name="id3">Introduction</a></li> <li><a class="reference" href="#introduction" id="id1" name="id1">Introduction</a></li>
<li><a class="reference" href="#proposed-resolution" id="id4" name="id4">Proposed Resolution</a></li> <li><a class="reference" href="#proposed-resolution" id="id2" name="id2">Proposed Resolution</a></li>
<li><a class="reference" href="#rationale" id="id5" name="id5">Rationale</a></li> <li><a class="reference" href="#rationale" id="id3" name="id3">Rationale</a></li>
</ul> </ul>
</div> </div>
<div class="section" id="introduction"> <div class="section" id="introduction">
<h1><a class="toc-backref" href="#id3" name="introduction">Introduction</a></h1> <h1><a class="toc-backref" href="#id1" 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> <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 provide a mechanism for determining at compile time if an iterator
type is a model of the new Writable Iterator and Swappable Iterator type is a model of the new Writable Iterator and Swappable Iterator
@ -271,7 +80,7 @@ attempting to write into <tt class="literal"><span class="pre">B</span></tt> wil
<p>The same problem applies to <tt class="literal"><span class="pre">is_swappable</span></tt>.</p> <p>The same problem applies to <tt class="literal"><span class="pre">is_swappable</span></tt>.</p>
</div> </div>
<div class="section" id="proposed-resolution"> <div class="section" id="proposed-resolution">
<h1><a class="toc-backref" href="#id4" name="proposed-resolution">Proposed Resolution</a></h1> <h1><a class="toc-backref" href="#id2" name="proposed-resolution">Proposed Resolution</a></h1>
<ol class="arabic"> <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 <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 requirements in the Writable Iterator and Swappable Iterator concepts
@ -281,68 +90,14 @@ that require their models to support these traits.</p>
<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 <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 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> <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> otherwise. Also, <tt class="literal"><span class="pre">is_readable</span></tt> is required to satisfy
the requirements for the UnaryTypeTrait concept
(defined in the type traits proposal).</p>
<p>Remove the requirement for support of the <tt class="literal"><span class="pre">is_readable</span></tt> trait from <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> the Readable Iterator concept.</p>
</li> </li>
</ol> <li><p class="first">Remove the <tt class="literal"><span class="pre">iterator_tag</span></tt> class.</p>
<!-- 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> </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> <li><p class="first">Change the specification of <tt class="literal"><span class="pre">traversal_category</span></tt> to:</p>
<pre class="literal-block"> <pre class="literal-block">
traversal-category(Iterator) = traversal-category(Iterator) =
@ -366,7 +121,7 @@ traversal-category(Iterator) =
</ol> </ol>
</div> </div>
<div class="section" id="rationale"> <div class="section" id="rationale">
<h1><a class="toc-backref" href="#id5" name="rationale">Rationale</a></h1> <h1><a class="toc-backref" href="#id3" name="rationale">Rationale</a></h1>
<ol class="arabic simple"> <ol class="arabic simple">
<li>There are two reasons for removing <tt class="literal"><span class="pre">is_writable</span></tt> <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 and <tt class="literal"><span class="pre">is_swappable</span></tt>. The first is that we do not know of
@ -387,32 +142,24 @@ using only the iterator type itself and its <tt class="literal"><span class="pre
Therefore we remove the requirement for <tt class="literal"><span class="pre">is_readable</span></tt> from the 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 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> <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 <li>The purpose of the <tt class="literal"><span class="pre">iterator_tag</span></tt> class was to
bundle the traversal and access category tags
into the <tt class="literal"><span class="pre">iterator_category</span></tt> typedef.
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, <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 there is no reason for iterators to provide
information about the access capabilities of an iterator. information about the access capabilities of an iterator.
This new version provides only information about the traversal Thus there is no need for the <tt class="literal"><span class="pre">iterator_tag</span></tt>. The
capabilities and the old iterator category tag. Instead of accessing traversal tag can be directly used for the
the traversal category as a nested typedef <tt class="literal"><span class="pre">::traversal</span></tt>, <tt class="literal"><span class="pre">iterator_category</span></tt>. If a new iterator is intended to be backward
the <tt class="literal"><span class="pre">iterator_tag</span></tt> itself will be convertible to the traversal compatible with old iterator concepts, a tag type
tag. The <tt class="literal"><span class="pre">access_bits</span></tt> parameter is no longer needed for that is convertible to both one of the new traversal tags
specifying the access member (which is now gone). However, and also to an old iterator tag can be created and use
some access information is still needed so that we can for the <tt class="literal"><span class="pre">iterator_category</span></tt>.</li>
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 <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> direct result of the removal of <tt class="literal"><span class="pre">iterator_tag</span></tt>.</li>
</ol> </ol>
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