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    CWG Issue 891</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="891"></A><H4>891.
  
<TT>const_cast</TT> to rvalue reference from objectless rvalue
</H4>
<B>Section: </B>7.6.1.11&#160; [<A href="https://wg21.link/expr.const.cast">expr.const.cast</A>]
 &#160;&#160;&#160;

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

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

 <B>Date: </B>8 May, 2009<BR>


<P>[Voted into WP at March, 2010 meeting.]</P>

<P>7.6.1.11 [<A href="https://wg21.link/expr.const.cast#4">expr.const.cast</A>] paragraph 4 says,</P>

<BLOCKQUOTE>

...Similarly, for two effective object types <TT>T1</TT> and
<TT>T2</TT>, an expression of type <TT>T1</TT> can be explicitly
converted to an rvalue of type <TT>T2</TT> using the cast
<TT>const_cast&lt;T2&amp;&amp;&gt;</TT> if a pointer to <TT>T1</TT> can be
explicitly converted to the type &#8220;pointer to <TT>T2</TT>&#8221;
using a <TT>const_cast</TT>. The result of a reference
<TT>const_cast</TT> refers to the original object.

</BLOCKQUOTE>

<P>However, in some rvalue-reference <TT>const_cast</TT>s there is
no &#8220;original object,&#8221; e.g.,</P>

<PRE>
    const_cast&lt;int&amp;&amp;&gt;(2)
</PRE>

<P><B>Notes from the July, 2009 meeting:</B></P>

<P>The coresponding cast to an lvalue reference to const is ill-formed:
in such cases, the operand must be an lvalue.  The consensus of the
CWG was that a cast to an rvalue reference should only accept an rvalue
that is an rvalue reference (i.e., an object).</P>

<P><B>Proposed resolution (February, 2010):</B></P>

<P>Change 7.6.1.11 [<A href="https://wg21.link/expr.const.cast#4">expr.const.cast</A>] paragraph 4 as follows:</P>

<BLOCKQUOTE>

An lvalue of type <TT>T1</TT> can be explicitly converted to an
lvalue of type <TT>T2</TT> using the cast
<TT>const_cast&lt;T2&amp;&gt;</TT> (where <TT>T1</TT> and
<TT>T2</TT> are object types) if a pointer to <TT>T1</TT> can be
explicitly converted to the type &#8220;pointer to
<TT>T2</TT>&#8221; using a <TT>const_cast</TT>.  Similarly, for
two object types <TT>T1</TT> and <TT>T2</TT>, an
<DEL>expression</DEL> <INS>lvalue</INS> of type <TT>T1</TT>
<INS>or, if <TT>T1</TT> is a class type, an expression of type
<TT>T1</TT>,</INS> can be explicitly converted to an rvalue of
type <TT>T2</TT> using the cast
<TT>const_cast&lt;T2&amp;&amp;&gt;</TT> if a pointer to
<TT>T1</TT> can be explicitly converted to the type
&#8220;pointer to <TT>T2</TT>&#8221; using a <TT>const_cast</TT>.
The result of a reference <TT>const_cast</TT> refers to the
original object.

</BLOCKQUOTE>

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