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    CWG Issue 1306</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="1306"></A><H4>1306.
  
Modifying an object within a <TT>const</TT> member function
</H4>
<B>Section: </B>_N4868_.11.4.3.2&#160; [<A href="https://wg21.link/class.this">class.this</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>James Kanze
 &#160;&#160;&#160;

 <B>Date: </B>2011-04-26<BR>


<P>[Voted into the WP at the February, 2012 meeting;
moved to DR at the October, 2012 meeting.]</P>

<P>According to _N4868_.11.4.3.2 [<A href="https://wg21.link/class.this#2">class.this</A>] paragraph 2,</P>

<BLOCKQUOTE>

In a <TT>const</TT> member function, the object for which the function
is called is accessed through a <TT>const</TT> access path; therefore,
a <TT>const</TT> member function shall not modify the object and its
non-static data members.

</BLOCKQUOTE>

<P>This is clearly overstating the case: <TT>mutable</TT> members can
be modified, a <TT>const_cast</TT> can be used, and class member
access expressions not involving <TT>this</TT> can also allow the
object to be modified.  The effect of the cv-qualification of a member
function on the type of <TT>*this</TT> is clear from the preceding
paragraph; this statement appears both unnecessary and incorrect.</P>

<P><B>Proposed resolution (August, 2011):</B></P>

<P>Merge _N4868_.11.4.3.2 [<A href="https://wg21.link/class.this">class.this</A>] paragraphs 1 and 2 and change
the text as follows:</P>

<BLOCKQUOTE>

In the body of a non-static (11.4.2 [<A href="https://wg21.link/class.mfct">class.mfct</A>]) member
function, the keyword <TT>this</TT> is a prvalue expression whose
value is the address of the object for which the function is called.
The type of <TT>this</TT> in a member function of a class <TT>X</TT>
is <TT>X*</TT>.  If the member function is declared <TT>const</TT>,
the type of <TT>this</TT> is <TT>const X*</TT>, if the member function
is declared <TT>volatile</TT>, the type of <TT>this</TT> is
<TT>volatile X*</TT>, and if the member function is declared <TT>const
volatile</TT>, the type of <TT>this</TT> is <TT>const volatile
X*</TT>. <DEL>In</DEL> <INS>[<I>Note:</I> thus in</INS> a
<TT>const</TT> member function, the object for which the function is
called is accessed through a <TT>const</TT> access path<DEL>; therefore, a
<TT>const</TT> member function shall not modify the object and its
non-static data members</DEL>. <INS>&#8212;<I>end note</I>]</INS>
[<I>Example:</I>...

</BLOCKQUOTE>

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