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  <title>Leftover properties of `this` in constructor
preconditions</title>
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<header id="title-block-header">
<h1 class="title" style="text-align:center">Leftover properties of
<code class="sourceCode cpp"><span class="kw">this</span></code> in
constructor preconditions</h1>
<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P3510R2</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2024-12-04</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project:</td>
    <td>Programming Language C++</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Audience:</td>
    <td>
      SG21<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Nathan Myers<br>&lt;<a href="mailto:ncm@cantrip.org" class="email">ncm@cantrip.org</a>&gt;<br>
      Gašper Ažman<br>&lt;<a href="mailto:gasper.azman@gmail.com" class="email">gasper.azman@gmail.com</a>&gt;<br>
    </td>
  </tr>
</table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#abstract" id="toc-abstract"><span class="toc-section-number">1</span> Abstract<span></span></a></li>
<li><a href="#motivation" id="toc-motivation"><span class="toc-section-number">2</span> Motivation<span></span></a></li>
<li><a href="#background" id="toc-background"><span class="toc-section-number">3</span> Background<span></span></a></li>
<li><a href="#analysis" id="toc-analysis"><span class="toc-section-number">4</span> Analysis<span></span></a>
<ul>
<li><a href="#difference-with-initialization-lists" id="toc-difference-with-initialization-lists"><span class="toc-section-number">4.1</span> Difference with initialization
lists<span></span></a></li>
</ul></li>
<li><a href="#proposal" id="toc-proposal"><span class="toc-section-number">5</span> Proposal<span></span></a></li>
<li><a href="#wording" id="toc-wording"><span class="toc-section-number">6</span> Wording<span></span></a></li>
<li><a href="#bibliography" id="toc-bibliography"><span class="toc-section-number">7</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" id="abstract"><span class="header-section-number">1</span> Abstract<a href="#abstract" class="self-link"></a></h1>
<p>In preconditions on constructors and postconditions on destructors,
all the non-static data members of the class are uninitialized, and
accessing them is undefined behaviour.</p>
<p>We therefore propose disallowing implicit access to non-static class
members to reduce the risk of such use, as a change to <span class="citation" data-cites="P2900R10">[<a href="https://wg21.link/p2900r10" role="doc-biblioref">P2900R10</a>]</span>.</p>
<h1 data-number="2" id="motivation"><span class="header-section-number">2</span> Motivation<a href="#motivation" class="self-link"></a></h1>
<p>We have a policy of not gratuitously introducing new kinds of UB in
contract contexts, as we seek to improve reliability and various kinds
of safety, not undermine them.</p>
<p>The following code is well-formed under <span class="citation" data-cites="P2900R10">[<a href="https://wg21.link/p2900r10" role="doc-biblioref">P2900R10</a>]</span>, but evokes UB:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="kw">class</span> X</span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a><span class="op">{</span></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a>   std<span class="op">::</span>string name;</span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a>   <span class="kw">explicit</span> X<span class="op">(</span><span class="kw">const</span> <span class="dt">char</span> <span class="op">*</span> n<span class="op">)</span></span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a>       pre<span class="op">(</span>name <span class="op">!=</span> <span class="kw">nullptr</span><span class="op">)</span>  <span class="co">// evaluated before</span></span>
<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a>     <span class="op">:</span> name<span class="op">{</span>n<span class="op">}</span>               <span class="co">// evaluated during</span></span>
<span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a>     <span class="op">{}</span></span>
<span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>
<p>This code might be recognized from <span class="citation" data-cites="P3172R0">[<a href="https://wg21.link/p3172r0" role="doc-biblioref">P3172R0</a>]</span>, from 2024-03-08, adopted at
the lightly-attended Tokyo meeting. This use of the uninitialized member
<code class="sourceCode cpp">name</code> in the uninitialized <code class="sourceCode cpp"><span class="op">*</span><span class="kw">this</span></code>
is language UB. While we may expect compilers to issue a stern warning
for such misuse, warnings are often not seen, and as often are not
understood. No one has suggested a reason to allow implicit mention of
non-static, uninitialized members in this context.</p>
<h1 data-number="3" id="background"><span class="header-section-number">3</span> Background<a href="#background" class="self-link"></a></h1>
<p>Permitting access to the about-to-be constructed object’s storage in
contract assertions on constructors is motivated by a desire to operate
on addresses of subobjects, particularly mixins, that may need to be
checked before initializing the object. Typically, the address of
interest is derived by casting; <code class="sourceCode cpp"><span class="kw">static_cast</span><span class="op">&lt;</span>Mixin<span class="op">&gt;(</span><span class="kw">this</span><span class="op">)</span></code>
provides a pointer value that may be offset from
<code class="sourceCode cpp"><span class="kw">this</span></code> by a
compile-time constant value. Giving
<code class="sourceCode cpp"><span class="kw">this</span></code> in this
context its more-natural <code class="sourceCode cpp"><span class="kw">const</span> <span class="dt">void</span><span class="op">*</span></code>
type would make this usage impossible in constexpr contexts: producing
the needed X pointer would require a <code class="sourceCode cpp"><span class="kw">reinterpret_cast</span><span class="op">&lt;</span><span class="kw">const</span> X<span class="op">*&gt;(</span><span class="kw">this</span><span class="op">)</span></code>,
an operation forbidden to
<code class="sourceCode cpp"><span class="kw">constexpr</span></code>
code.</p>
<p>Similar access to addresses of members by implicit naming is not
needed, and mention of such members is a source of unncecessary UB.</p>
<p><span class="citation" data-cites="P3172R0">[<a href="https://wg21.link/p3172r0" role="doc-biblioref">P3172R0</a>]</span> goes on to provide access to
such pointers in contract assertions on destructors, for more-or-less
analagous reasons, and with a similar (albeit perhaps less attractive)
hazard.</p>
<h1 data-number="4" id="analysis"><span class="header-section-number">4</span> Analysis<a href="#analysis" class="self-link"></a></h1>
<p>How did we get here? <span class="citation" data-cites="P3172R0">[<a href="https://wg21.link/p3172r0" role="doc-biblioref">P3172R0</a>]</span> notes the UB hazard, in its
section 2.3, and asserts that containing the problem is unimplementable.
It uses the example of passing
<code class="sourceCode cpp"><span class="kw">this</span></code> to a
function that may then improperly dereference the pointer, undetectably
to the compiler.</p>
<p>We do not find this example compelling; the UB in the called function
is the responsibility of that function, but our reponsibility is to
prevent a very obvious source of accidental UB in precondition
assertions themselves.</p>
<p>Mentioning
<code class="sourceCode cpp"><span class="kw">this</span></code> is very
rare in typical users’ code, and attracts attention, where mentioning
members by implicit reference, UB here, is extremely common, and does
not.</p>
<p>We note further that mentioning non-static member functions in this
context would also invite UB, as the implicit
<code class="sourceCode cpp"><span class="kw">this</span></code> passed
to such member functions is almost always used.</p>
<h2 data-number="4.1" id="difference-with-initialization-lists"><span class="header-section-number">4.1</span> Difference with initialization
lists<a href="#difference-with-initialization-lists" class="self-link"></a></h2>
<p>Initialization lists do allow implicit access to non-static class
members. It is up to the programmer to use only those data members that
have so far been constructed. This unfixable source of potential UB is
unfortunate, but there is anyway limited temptation to mention other
members in that context.</p>
<p>Contrast this to constructor preconditions: no data members have been
constructed at that point, so none may safely be used. Requiring an
explicit <code class="sourceCode cpp"><span class="kw">this</span><span class="op">-&gt;</span></code>,
if they must be mentioned, is not too onerous of a work-around in the
view of the authors, should the programmer really need an uninitialized
member’s address.</p>
<h1 data-number="5" id="proposal"><span class="header-section-number">5</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>The solution is simple: make implicit access to non-static class
members (both data and function) ill-formed in constructor preconditions
and destructor postconditions, while preserving access to
<code class="sourceCode cpp"><span class="kw">this</span></code>
itself.</p>
<h1 data-number="6" id="wording"><span class="header-section-number">6</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>In [over.call.func] Add a new paragraph after p3:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a>If an implied object argument `*this` relied upon in the predicate</span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a>of a precondition assertion of a constructor, or of a postcondition</span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a>assertion of a destructor, is the object being constructed or</span>
<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a>destroyed, respectively, and a non-static member function is</span>
<span id="cb2-5"><a href="#cb2-5" aria-hidden="true" tabindex="-1"></a>selected, the program is ill-formed.&quot;</span></code></pre></div>
<p>At the end of [expr.prim.id.general] p2, add</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a>If this transformation would be applied in a precondition of a</span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a>constructor of X, or a postcondition of a destructor of X, and</span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a>the implied `*this` refers to the object immediately constructed</span>
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a>or destroyed, respectively, the program is ill-formed.&quot;</span></code></pre></div>
<h1 data-number="7" id="bibliography"><span class="header-section-number">7</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references csl-bib-body hanging-indent" data-entry-spacing="1" role="doc-bibliography">
<div id="ref-P2900R10" class="csl-entry" role="doc-biblioentry">
[P2900R10] Joshua Berne, Timur Doumler, Andrzej Krzemieński. 2024-10-13.
Contracts for C++. <a href="https://wg21.link/p2900r10"><div class="csl-block">https://wg21.link/p2900r10</div></a>
</div>
<div id="ref-P3172R0" class="csl-entry" role="doc-biblioentry">
[P3172R0] Andrzej Krzemieński. 2024-03-08. Using `this` in constructor
preconditions. <a href="https://wg21.link/p3172r0"><div class="csl-block">https://wg21.link/p3172r0</div></a>
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