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166 lines
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166 lines
10 KiB
HTML
Executable File
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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">
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<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>
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<table class="field-list" frame="void" rules="none">
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<col class="field-name" />
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<col class="field-body" />
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<tr class="field"><th class="field-name">Author:</th><td class="field-body">David Abrahams and Jeremy Siek</td>
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</tr>
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<tr class="field"><th class="field-name">Contact:</th><td class="field-body"><a class="reference" href="mailto:dave@boost-consulting.com">dave@boost-consulting.com</a>, <a class="reference" href="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</a></td>
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</tr>
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<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>
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</tr>
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<tr class="field"><th class="field-name">date:</th><td class="field-body">$Date$</td>
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</tr>
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<tr class="field"><th class="field-name">Copyright:</th><td class="field-body">Copyright David Abrahams, Jeremy Siek 2003. Use, modification and
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distribution is subject to the Boost Software License,
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Version 1.0. (See accompanying file LICENSE_1_0.txt or copy
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at <a class="reference" href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</td>
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</tr>
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</tbody>
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</table>
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<div class="contents topic" id="table-of-contents">
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<p class="topic-title"><a name="table-of-contents">Table of Contents</a></p>
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<ul class="simple">
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<li><a class="reference" href="#introduction" id="id1" name="id1">Introduction</a></li>
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<li><a class="reference" href="#proposed-resolution" id="id2" name="id2">Proposed Resolution</a></li>
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<li><a class="reference" href="#rationale" id="id3" name="id3">Rationale</a></li>
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</ul>
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</div>
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<div class="section" id="introduction">
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<h1><a class="toc-backref" href="#id1" name="introduction">Introduction</a></h1>
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<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>
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provide a mechanism for determining at compile time if an iterator
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type is a model of the new Writable Iterator and Swappable Iterator
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concepts, analogous to <tt class="literal"><span class="pre">iterator_traits<X>::iterator_category</span></tt>
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for the old iterator concepts. For backward compatibility,
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<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
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iterators, but they also are intended to work for old
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iterators (iterators that meet the requirements for one of the
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iterator concepts in the current standard). In the case of old
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iterators, the writability and swapability is deduced based on the
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<tt class="literal"><span class="pre">iterator_category</span></tt> and also the <tt class="literal"><span class="pre">reference</span></tt> type. The
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specification for this deduction gives false positives for forward
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iterators that have non-assignable value types.</p>
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<p>To review, the part of the <tt class="literal"><span class="pre">is_writable</span></tt> trait definition which
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applies to old iterators is:</p>
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<pre class="literal-block">
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if (cat is convertible to output_iterator_tag)
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return true;
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else if (cat is convertible to forward_iterator_tag
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and iterator_traits<Iterator>::reference is a
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mutable reference)
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return true;
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else
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return false;
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</pre>
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<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
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assignment operator:</p>
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<pre class="literal-block">
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class B {
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public:
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...
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private:
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B& operator=(const B&);
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};
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</pre>
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<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&</span></tt>. In
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that case, <tt class="literal"><span class="pre">is_writable<It>::value</span></tt> will be true when in fact
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attempting to write into <tt class="literal"><span class="pre">B</span></tt> will cause an error.</p>
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<p>The same problem applies to <tt class="literal"><span class="pre">is_swappable</span></tt>.</p>
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</div>
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<div class="section" id="proposed-resolution">
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<h1><a class="toc-backref" href="#id2" name="proposed-resolution">Proposed Resolution</a></h1>
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<ol class="arabic">
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<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
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requirements in the Writable Iterator and Swappable Iterator concepts
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that require their models to support these traits.</p>
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</li>
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<li><p class="first">Change the <tt class="literal"><span class="pre">is_readable</span></tt> specification to be:
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<tt class="literal"><span class="pre">is_readable<X>::type</span></tt> is <tt class="literal"><span class="pre">true_type</span></tt> if the
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result type of <tt class="literal"><span class="pre">X::operator*</span></tt> is convertible to
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<tt class="literal"><span class="pre">iterator_traits<X>::value_type</span></tt> and is <tt class="literal"><span class="pre">false_type</span></tt>
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otherwise. Also, <tt class="literal"><span class="pre">is_readable</span></tt> is required to satisfy
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the requirements for the UnaryTypeTrait concept
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(defined in the type traits proposal).</p>
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<p>Remove the requirement for support of the <tt class="literal"><span class="pre">is_readable</span></tt> trait from
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the Readable Iterator concept.</p>
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</li>
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<li><p class="first">Remove the <tt class="literal"><span class="pre">iterator_tag</span></tt> class.</p>
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</li>
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<li><p class="first">Change the specification of <tt class="literal"><span class="pre">traversal_category</span></tt> to:</p>
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<pre class="literal-block">
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traversal-category(Iterator) =
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let cat = iterator_traits<Iterator>::iterator_category
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if (cat is convertible to incrementable_iterator_tag)
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return cat; // Iterator is a new iterator
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else if (cat is convertible to random_access_iterator_tag)
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return random_access_traversal_tag;
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else if (cat is convertible to bidirectional_iterator_tag)
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return bidirectional_traversal_tag;
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else if (cat is convertible to forward_iterator_tag)
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return forward_traversal_tag;
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else if (cat is convertible to input_iterator_tag)
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return single_pass_iterator_tag;
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else if (cat is convertible to output_iterator_tag)
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return incrementable_iterator_tag;
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else
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return null_category_tag;
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</pre>
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</li>
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</ol>
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</div>
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<div class="section" id="rationale">
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<h1><a class="toc-backref" href="#id3" name="rationale">Rationale</a></h1>
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<ol class="arabic simple">
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<li>There are two reasons for removing <tt class="literal"><span class="pre">is_writable</span></tt>
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and <tt class="literal"><span class="pre">is_swappable</span></tt>. The first is that we do not know of
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a way to fix the specification so that it gives the correct
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answer for all iterators. Second, there was only a weak
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motivation for having <tt class="literal"><span class="pre">is_writable</span></tt> and <tt class="literal"><span class="pre">is_swappable</span></tt>
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there in the first place. The main motivation was simply
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uniformity: we have tags for the old iterator categories
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so we should have tags for the new iterator categories.
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While having tags and the capability to dispatch based
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on the traversal categories is often used, we see
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less of a need for dispatching based on writability
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and swappability, since typically algorithms
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that need these capabilities have no alternative if
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they are not provided.</li>
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<li>We discovered that the <tt class="literal"><span class="pre">is_readable</span></tt> trait can be implemented
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using only the iterator type itself and its <tt class="literal"><span class="pre">value_type</span></tt>.
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Therefore we remove the requirement for <tt class="literal"><span class="pre">is_readable</span></tt> from the
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Readable Iterator concept, and change the definition of
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<tt class="literal"><span class="pre">is_readable</span></tt> so that it works for any iterator type.</li>
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<li>The purpose of the <tt class="literal"><span class="pre">iterator_tag</span></tt> class was to
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bundle the traversal and access category tags
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into the <tt class="literal"><span class="pre">iterator_category</span></tt> typedef.
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With <tt class="literal"><span class="pre">is_writable</span></tt> and <tt class="literal"><span class="pre">is_swappable</span></tt> gone, and
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<tt class="literal"><span class="pre">is_readable</span></tt> no longer in need of special hints,
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there is no reason for iterators to provide
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information about the access capabilities of an iterator.
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Thus there is no need for the <tt class="literal"><span class="pre">iterator_tag</span></tt>. The
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traversal tag can be directly used for the
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<tt class="literal"><span class="pre">iterator_category</span></tt>. If a new iterator is intended to be backward
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compatible with old iterator concepts, a tag type
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that is convertible to both one of the new traversal tags
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and also to an old iterator tag can be created and use
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for the <tt class="literal"><span class="pre">iterator_category</span></tt>.</li>
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<li>The changes to the specification of <tt class="literal"><span class="pre">traversal_category</span></tt> are a
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direct result of the removal of <tt class="literal"><span class="pre">iterator_tag</span></tt>.</li>
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</ol>
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</div>
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</div>
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