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<!DOCTYPE html>
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<title>Issue 4283: std::trivially_relocate needs stronger preconditions on "nested" objects with dynamic lifetime</title>
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<meta property="og:title" content="Issue 4283: std::trivially_relocate needs stronger preconditions on &quot;nested&quot; objects with dynamic lifetime">
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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="4283"><a href="lwg-active.html#4283">4283</a>. <code class='backtick'>std::trivially_relocate</code> needs stronger preconditions on "nested" objects with dynamic lifetime</h3>
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<p><b>Section:</b> 20.2.6 <a href="https://wg21.link/obj.lifetime">[obj.lifetime]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Giuseppe D'Angelo <b>Opened:</b> 2025-06-23 <b>Last modified:</b> 2025-06-28</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#obj.lifetime">active issues</a> in [obj.lifetime].</p>
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<p><b>View all other</b> <a href="lwg-index.html#obj.lifetime">issues</a> in [obj.lifetime].</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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In 20.2.6 <a href="https://wg21.link/obj.lifetime">[obj.lifetime]</a> the <code class='backtick'>std::trivially_relocate</code> function
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is missing a precondition, that is, that any object alive in the range being
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relocated is itself trivially relocatable.
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<p/>
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We know the objects in the range are trivially relocatable, because
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there is a <i>Mandates</i>: element for this. The current draft has precise
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rules to determine whether a type is trivially relocatable or not; in
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general, subobjects are considered there (cf. 11.2 <a href="https://wg21.link/class.prop">[class.prop]</a>,
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"eligible for trivial relocation", which discusses base classes and non-static
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data members).
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<p/>
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However these rules do not take into account objects with dynamic
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lifetime whose storage is being provided by (sub)objects in the range.
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<p/>
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For instance, given a <code class='backtick'>wrapper</code> type like:
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</p>
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<blockquote><pre>
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// wraps a T
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template&lt;typename T&gt;
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struct wrapper {
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alignas(T) std::byte data[sizeof(T)];
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};
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</pre></blockquote>
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<p>
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then one can build a non-trivially relocatable object into <code class='backtick'>wrapper</code>
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objects:
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</p>
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<blockquote><pre>
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struct NTR { ~NTR() {} };
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static_assert(not std::is_trivially_relocatable_v&lt;NTR&gt;);
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using WS = wrapper&lt;NTR&gt;;
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static_assert(std::is_trivially_relocatable_v&lt;WS&gt;); // OK
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</pre></blockquote>
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<p>
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And now one can do this:
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</p>
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<blockquote><pre>
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WS* ws = /* &hellip; */; // create a wrapper
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new (&amp;ws-&gt;data) NTR(); // create a NTR object into it
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std::trivially_relocate(ws, ws+1, dest); // should be UB
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</pre></blockquote>
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<p>
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Attempting to trivially relocate <code class='backtick'>*ws</code> should result in undefined
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behavior because <code class='backtick'>NTR</code> isn't trivially relocatable. I don't believe that
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this fact is correctly captured by the preconditions of
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<code class='backtick'>std::trivially_relocate</code>.
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<p/>
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A similar issue is present for polymorphic types. In <a href="https://wg21.link/P2786" title=" Trivial Relocatability For C++26">P2786</a>'s
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design polymorphic types can be trivially relocatable (assuming all the other
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conditions hold). Given a trivially relocatable polymorphic type <code class='backtick'>P</code>,
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then this code:
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</p>
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<blockquote><pre>
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struct P { virtual void f(); };
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static_assert(std::is_trivially_relocatable_v&lt;P&gt;);
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using WP = wrapper&lt;P&gt;;
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WP* wp = /* &hellip; */; // create a wrapper
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new (&amp;wp-&gt;data) P(); // create a P object into it
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std::trivially_relocate(wp, wp+1, dest); // implementation defined
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</pre></blockquote>
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<p>
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is well-defined or UB, depending on the implementation. This is because
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on some implementations trivially relocating a polymorphic type requires
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patching its virtual table pointer; cf. the discussion in chapter 15.1
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of <a href="https://wg21.link/P2786R13">P2786R13</a>. However the "type erasure" done by
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<code>wrapper&lt;P&gt;</code> in the example (ultimately, it is just an array
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of bytes) does not allow implementations to do such patching, and the code
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is going to fail at runtime. Therefore this case also needs to be discussed by
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<code class='backtick'>std::trivially_relocate</code>'s specification.
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</p>
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<p id="res-4283"><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 20.2.6 <a href="https://wg21.link/obj.lifetime">[obj.lifetime]</a> as indicated:</p>
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<blockquote class="note">
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<p>
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[<i>Drafting note</i>: For the general part of the issue (all objects in the range must be of
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trivially relocatable type), we append another point at the end of the existing
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<i>Preconditions:</i> element of <code class='backtick'>trivially_relocate</code>.<br/>
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For the specifics of polymorphic types, we amend at the end of the description the existing
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<i>Remarks</i>: element]
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</p>
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</blockquote>
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<blockquote>
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<pre>
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template&lt;class T&gt;
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T* trivially_relocate(T* first, T* last, T* result);
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</pre>
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<blockquote>
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<p>
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-9- <i>Mandates</i>: [&hellip;]
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<p/>
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-10- <i>Preconditions</i>:
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</p>
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<ol style="list-style-type: none">
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<li><p>(10.1) &mdash; <code class='backtick'>[first, last)</code> is a valid range.</p></li>
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<li><p>(10.2) &mdash; <code class='backtick'>[result, result + (last - first))</code> denotes a region of storage that is a subset of the region
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reachable through <code class='backtick'>result</code> (6.8.4 <a href="https://wg21.link/basic.compound">[basic.compound]</a>) and suitably aligned for the type <code class='backtick'>T</code>.</p></li>
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<li><p>(10.3) &mdash; No element in the range <code class='backtick'>[first, last)</code> is a potentially-overlapping subobject.</p></li>
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<li><p><ins>(10.?) &mdash; All objects whose storage is being provided for (6.7.2 <a href="https://wg21.link/intro.object">[intro.object]</a>) by
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objects in the <code class='backtick'>[first, last)</code> range are of trivially relocatable type.</ins></p></li>
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</ol>
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<p>
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-11- <i>Postconditions</i>: [&hellip;]
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<p/>
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-12- <i>Returns</i>: <code class='backtick'>result + (last - first)</code>.
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<p/>
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-13- <i>Throws</i>: Nothing.
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<p/>
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-14- <i>Complexity</i>: Linear in the length of the source range.
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<p/>
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-15- <i>Remarks</i>: The destination region of storage is considered reused (6.7.4 <a href="https://wg21.link/basic.life">[basic.life]</a>).
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No constructors or destructors are invoked. <ins>If any polymorphic object (11.7.3 <a href="https://wg21.link/class.virtual">[class.virtual]</a>)
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exists in storage provided for (6.7.2 <a href="https://wg21.link/intro.object">[intro.object]</a>) by objects in the <code class='backtick'>[first, last)</code> range,
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it is implementation-defined whether the behavior is undefined.</ins>
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<p/>
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[<i>Note 2</i>: Overlapping ranges are supported. &mdash; <i>end note</i>]
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</p>
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