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<!DOCTYPE html>
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<html lang="en">
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<meta charset="utf-8">
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<title>Issue 4255: move_only_function constructor should recognize empty copyable_functions</title>
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<meta property="og:title" content="Issue 4255: move_only_function constructor should recognize empty copyable_functions">
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<meta property="og:description" content="C++ library issue. Status: New">
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#New">New</a> status.</em></p>
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<h3 id="4255"><a href="lwg-active.html#4255">4255</a>. move_only_function constructor should recognize empty copyable_functions</h3>
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<p><b>Section:</b> 22.10.17.4.3 <a href="https://wg21.link/func.wrap.move.ctor">[func.wrap.move.ctor]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Tomasz Kami&nacute;ski <b>Opened:</b> 2025-05-12 <b>Last modified:</b> 2025-05-12</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View other</b> <a href="lwg-index-open.html#func.wrap.move.ctor">active issues</a> in [func.wrap.move.ctor].</p>
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<p><b>View all other</b> <a href="lwg-index.html#func.wrap.move.ctor">issues</a> in [func.wrap.move.ctor].</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
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<p><b>Discussion:</b></p>
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<p>
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The standard currently requires that constructing <code>move_only_function</code>
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from empty <code>copyable_function</code>, creates an non-empty <code>move_only_function</code>,
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that contains an empty <code>copyable_function</code> as the target. For exampe:
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<code>
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std::copyable_function&lt;int(int)&gt; ce;
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std::move_only_function&lt;int(int)&gt; me(ce);
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</code>
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We require that invoking <code>me(1)</code> is undefined behavior (as it leads to call to the
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<code>ce(1)</code>), however it cannot be detected in the user code, as <code>me != nullptr</code>
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is true.
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</p>
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<p>
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We should require that the <code>move_only_function(F&amp;&amp; f)</code> constructor to create an
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empty object, if <code>f</code> is instantiation of of <code>copyable_function</code> and
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<code>f == nullptr</code> is true, i.e. f does not contain target object.
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</p>
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<p>
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This simplifies implementing avoidance of double wrapping per 22.10.17.1 <a href="https://wg21.link/func.wrap.general">[func.wrap.general]</a> p2,
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as transfering the target produces empty functor.
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</p>
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<p>
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The <code>copyable_function</code> cannot be constructed from <code>move_only_function</code>,
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as it requires functor to be copyable. Invkoing an empty <code>std::function</code> has well
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defined behavior (throws <code>bad_function_call</code>), and wrapping such object into
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other functors should reproduce that behavior
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</p>
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<p id="res-4255"><b>Proposed resolution:</b></p>
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<p>
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This wording is relative to <a href="https://wg21.link/N5008">N5008</a>.
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</p>
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<ol>
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<li><p>Modify 22.10.17.1 <a href="https://wg21.link/func.wrap.general">[func.wrap.general]</a> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class F&gt; move_only_function(F&amp;&amp; f);
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</pre>
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<blockquote>
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[&hellip;]
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<p>-8- <i>Postconditions:</i>: <code>*this</code> has no target object if any of the following hold:</p>
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<ul>
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<li><p>(8.1) <code>f</code> is a null function pointer value, <del>or</del></p></li>
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<li><p>(8.2) <code>f</code> is a null member pointer value, or</p></li>
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<li><p>(8.2) <code>remove_cvref_t&lt;F&gt;</code> is a specialization of the <code>move_only_function</code>
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<ins>or <code>copyable_function</code></ins> class template, and <code>f</code> has no target object.</p></li>
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</ul>
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</blockquote>
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</blockquote>
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</li>
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</ol>
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</body>
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</html>

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