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    CWG Issue 764</TITLE>
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<P><EM>This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21
  Core Issues List revision 118b.
  See http://www.open-std.org/jtc1/sc22/wg21/ for the official
  list.</EM></P>
<P>2025-09-28</P>
<HR>
<A NAME="764"></A><H4>764.
  
Capturing unused variables in a lambda expression
</H4>
<B>Section: </B>7.5.6.3&#160; [<A href="https://wg21.link/expr.prim.lambda.capture">expr.prim.lambda.capture</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD2
 &#160;&#160;&#160;

 <B>Submitter: </B>Steve Adamczyk
 &#160;&#160;&#160;

 <B>Date: </B>6 February, 2009<BR>


<P>[Voted into the WP at the July, 2009 meeting as part of N2927.]</P>

<P>7.5.6 [<A href="https://wg21.link/expr.prim.lambda#5">expr.prim.lambda</A>] paragraph 5 says,</P>

<BLOCKQUOTE>

 The <I>compound-statement</I> of a lambda expression shall use
(6.3 [<A href="https://wg21.link/basic.def.odr">basic.def.odr</A>]) an automatic variable or reference from the
context where the lambda expression appears only if the name of the
variable or reference is a member of the effective capture set...

</BLOCKQUOTE>

<P>The reference to 6.3 [<A href="https://wg21.link/basic.def.odr">basic.def.odr</A>] makes clear that the
technical meaning of &#8220;use&#8221; is in view here, and that the
names of variables can appear without being captured if they are
constants used as values or if they are unevaluated operands.</P>

<P>There appears to be a disconnect with the preceding paragraph,
however, in the description of which variables are implicitly
captured by a <I>capture-default</I>:</P>

<BLOCKQUOTE>

for each name <TT>v</TT> that appears in the lambda expression and denotes a
local variable or reference with automatic storage duration in the
context where the lambda expression appears and that does not appear
in the <I>capture-list</I> or as a parameter name in the
<I>lambda-parameter-declaration-list</I>...

</BLOCKQUOTE>

<P>It would be more consistent if only variables that were required by
paragraph 5 to be captured were implicitly captured, i.e., if
&#8220;that appears in the lambda expression&#8221; were replaced by
&#8220;that is used (6.3 [<A href="https://wg21.link/basic.def.odr">basic.def.odr</A>]) in the
<I>compound-statement</I> of the lambda expression.&#8221;  For
example,</P>

<PRE>
    struct A {
      A();
      A(const A&amp;);
      ~A();
    };
    void f() {
      A a;
      int i = [=]() { return sizeof a; }();
    }
</PRE>

<P>Here, <TT>a</TT> will be captured (and copied), even though it is
not &#8220;used&#8221; in the lambda expression.</P>

<P><B>Proposed resolution (July, 2009)</B></P>

<P>See document PL22.16/09-0117 = WG21 N2927.</P>

<BR><BR>
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