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    CWG Issue 1055</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="1055"></A><H4>1055.
  
Permissible uses of <TT>void</TT>
</H4>
<B>Section: </B>6.9.2&#160; [<A href="https://wg21.link/basic.fundamental">basic.fundamental</A>]
 &#160;&#160;&#160;

 <B>Status: </B>C++11
 &#160;&#160;&#160;

 <B>Submitter: </B>Nikolay Ivchenkov
 &#160;&#160;&#160;

 <B>Date: </B>2010-03-17<BR>


<P>[Voted into the WP at the March, 2011 meeting as part of paper N3262.]</P>

<P>According to 6.9.2 [<A href="https://wg21.link/basic.fundamental#9">basic.fundamental</A>] paragraph 9,</P>

<BLOCKQUOTE>

Any expression can be explicitly converted to type <I>cv</I>
<TT>void</TT> (7.6.3 [<A href="https://wg21.link/expr.cast">expr.cast</A>]).  An expression of type
<TT>void</TT> shall be used only as an expression statement
(8.3 [<A href="https://wg21.link/stmt.expr">stmt.expr</A>]), as an operand of a comma expression
(7.6.20 [<A href="https://wg21.link/expr.comma">expr.comma</A>]), as a second or third operand of
<TT>?:</TT> (7.6.16 [<A href="https://wg21.link/expr.cond">expr.cond</A>]), as the operand of
<TT>typeid</TT>, or as the expression in a return statement
(8.8.4 [<A href="https://wg21.link/stmt.return">stmt.return</A>]) for a function with the return type
<TT>void</TT>.

</BLOCKQUOTE>

<P>First, this is self-contradictory: if &#8220;any expression&#8221;
can be converted to <TT>void</TT>, why is such a conversion not listed
among the acceptable uses of an expression of type <TT>void</TT>?</P>

<P>Second, presumably an expression of type <TT>void</TT> can be used
as an operand of <TT>decltype</TT>, but this use is not listed.</P>

<P>Finally, there are several places in the Standard that speak of
expressions having a cv-qualified <TT>void</TT> type
(7.6.16 [<A href="https://wg21.link/expr.cond#2">expr.cond</A>] paragraph 2, 8.8.4 [<A href="https://wg21.link/stmt.return#3">stmt.return</A>] paragraph 3)
.  However, an expression of type <TT>void</TT> is a
non-class prvalue, and there are no cv-qualified non-class prvalues
(7.2.1 [<A href="https://wg21.link/basic.lval#4">basic.lval</A>] paragraph 4).</P>

<P><B>Proposed resolution (February, 2011) [SUPERSEDED]:</B></P>

<OL>
<LI><P>Change 6.9.2 [<A href="https://wg21.link/basic.fundamental#9">basic.fundamental</A>] paragraph 9 as follows:</P></LI>

<BLOCKQUOTE>

...Any expression can be explicitly converted to type <I>cv</I>
<TT>void</TT> (7.6.3 [<A href="https://wg21.link/expr.cast">expr.cast</A>]). An expression of type
<TT>void</TT> shall be used only as an expression statement
(8.3 [<A href="https://wg21.link/stmt.expr">stmt.expr</A>]), as an operand of a comma expression
(7.6.20 [<A href="https://wg21.link/expr.comma">expr.comma</A>]), as a second or third operand of
<TT>?:</TT> (7.6.16 [<A href="https://wg21.link/expr.cond">expr.cond</A>]), as the operand of
<TT>typeid</TT> <INS>or <TT>decltype</TT></INS>, <DEL>or</DEL> as the
expression in a return statement (8.8.4 [<A href="https://wg21.link/stmt.return">stmt.return</A>]) for a
function with the return type <TT>void</TT><INS>, or as the operand
of an explicit conversion to type <I>cv</I> <TT>void</TT></INS>.

</BLOCKQUOTE>

<LI><P>Change 7.6.16 [<A href="https://wg21.link/expr.cond#2">expr.cond</A>] paragraph 2 as follows:</P></LI>

<BLOCKQUOTE>

If either the second or the third operand has type <DEL>(possibly
cv-qualified)</DEL> <TT>void</TT>, then...

</BLOCKQUOTE>

<LI><P>Change 8.8.4 [<A href="https://wg21.link/stmt.return#3">stmt.return</A>] paragraph 3 as follows:</P></LI>

<BLOCKQUOTE>

A return statement with an expression of type
<DEL>&#8220;<I>cv</I></DEL> <TT>void</TT><DEL>&#8221;</DEL> can be
used only in functions with a return type of <I>cv</I> <TT>void</TT>;
the expression is evaluated just before the function returns to its
caller.

</BLOCKQUOTE>

</OL>

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