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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="1556"></A><H4>1556.
  
Constructors and explicit conversion functions in direct initialization
</H4>
<B>Section: </B>12.2.2.5&#160; [<A href="https://wg21.link/over.match.copy">over.match.copy</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD3
 &#160;&#160;&#160;

 <B>Submitter: </B>Edward Catmur
 &#160;&#160;&#160;

 <B>Date: </B>2012-09-18<BR>


<P>[Moved to DR at the April, 2013 meeting.]</P>



<P>
<A HREF="899.html">Issue 899</A> added wording to the second
bullet of 12.2.2.5 [<A href="https://wg21.link/over.match.copy#1">over.match.copy</A>] paragraph 1 permitting use of
<TT>explicit</TT> conversion functions when calling a copy constructor
in a direct-initialization context.  <A HREF="1087.html">Issue 1087</A> addressed the problem in the earlier resolution that the
phrase &#8220;copy constructor&#8221; did not include move
constructors and template constructors.  However, the term &#8220;copy
constructor&#8221; implicitly (and correctly) restricted the
constructor parameter to be the constructor's class, i.e., the class
of the object being directly initialized.  The new phrasing, &#8220;a
constructor that takes a reference to possibly cv-qualified <TT>T</TT>
as its first argument,&#8221; incorrectly assumed that <TT>T</TT>
referred to the type of the object being initialized; however, that is
not the case, and a converting constructor from <TT>T</TT> to the type
of the object being initialized is inadvertently permitted.  The
wording needs a further tweak to restore the intended context.</P>

<P><B>Proposed resolution (October, 2012):</B></P>

<P>Change the second bullet of 12.2.2.5 [<A href="https://wg21.link/over.match.copy#1">over.match.copy</A>] paragraph 1
as follows:</P>

<BLOCKQUOTE>

<P>...Assuming that &#8220;<I>cv1</I> <TT>T</TT>&#8221; is the type of the
object being initialized, with <TT>T</TT> a class type, the candidate
functions are selected as follows:</P>

<UL>
<LI><P>The converting constructors (11.4.8.2 [<A href="https://wg21.link/class.conv.ctor">class.conv.ctor</A>])
of <TT>T</TT> are candidate functions.</P></LI>

<LI><P>When the type of the initializer expression is a class type
&#8220;<I>cv</I> <TT>S</TT>&#8221;, the non-explicit conversion
functions of <TT>S</TT> and its base classes are considered. When
initializing a temporary to be bound to the first parameter of a
constructor that takes a reference to possibly cv-qualified <TT>T</TT>
as its first argument, called with a single argument in the context of
direct-initialization <INS>of an object of type &#8220;<I>cv2</I>
<TT>T</TT>&#8221;</INS>, explicit conversion functions are also
considered.  Those that are not hidden within <TT>S</TT> and yield a
type whose cv-unqualified version is the same type as <TT>T</TT> or is
a derived class thereof are candidate functions. Conversion functions
that return &#8220;reference to <TT>X</TT>&#8221; return lvalues or
xvalues, depending on the type of reference, of type <TT>X</TT> and
are therefore considered to yield <TT>X</TT> for this process of
selecting candidate functions.</P></LI>



</UL>

</BLOCKQUOTE>

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