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    CWG Issue 1174</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="1174"></A><H4>1174.
  
When is a pure virtual function &#8220;used?&#8221;
</H4>
<B>Section: </B>6.3&#160; [<A href="https://wg21.link/basic.def.odr">basic.def.odr</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>Daniel Kr&#252;gler
 &#160;&#160;&#160;

 <B>Date: </B>2010-08-07<BR>


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



<P>According to 6.3 [<A href="https://wg21.link/basic.def.odr#2">basic.def.odr</A>] paragraph 2,</P>

<BLOCKQUOTE>

A variable or non-overloaded function whose name appears as a
potentially-evaluated expression is <I>used</I>... A virtual member
function is used if it is not pure.

</BLOCKQUOTE>

<P>However, that does not adequately address when a pure virtual
function is used or not used.  For example,</P>

<PRE>
    struct S {
      virtual void pure1() = 0;
      virtual void pure2() = 0;
    };
    void f(S* p) {
      p-&gt;pure1();
      p-&gt;S::pure2();
    };
</PRE>

<P>Both <TT>pure1</TT> and <TT>pure2</TT> satisfy the criterion that
their name appears in a potentially-evaluated expression, but
<TT>pure1</TT> should not be considered &#8220;used&#8221; (which
would require that it be defined); <TT>pure2</TT> <I>is</I>
&#8220;used&#8221; and must be defined.</P>

<P><B>Proposed resolution (November, 2010) [SUPERSEDED]:</B></P>

<P>Change 6.3 [<A href="https://wg21.link/basic.def.odr#2">basic.def.odr</A>] paragraph 2 as follows:</P>

<BLOCKQUOTE>

...A variable <DEL>or non-overloaded function</DEL> whose name appears
as a potentially-evaluated expression is odr-used unless it is an
object that satisfies the requirements for appearing in a constant
expression (7.7 [<A href="https://wg21.link/expr.const">expr.const</A>]) and the lvalue-to-rvalue
conversion (7.3.2 [<A href="https://wg21.link/conv.lval">conv.lval</A>]) is immediately applied...  A
virtual member function is odr-used if it is not pure.  A
<INS>non-overloaded function whose name appears as a
potentially-evaluated expression or a</INS> member of a set of
candidate functions <DEL>is odr-used</DEL> if <DEL>it is</DEL>
selected by overload resolution when referred to from a
potentially-evaluated expression<INS>, are odr-used, unless the
function is pure and its name is not explicitly
qualified</INS>. [<I>Note:</I>...

</BLOCKQUOTE>

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