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    CWG Issue 883</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="883"></A><H4>883.
  
<TT>std::memcpy</TT> vs <TT>std::memmove</TT>
</H4>
<B>Section: </B>6.9&#160; [<A href="https://wg21.link/basic.types">basic.types</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>Lawrence Crowl
 &#160;&#160;&#160;

 <B>Date: </B>29 April, 2009<BR>


<P>[Voted into WP at October, 2009 meeting.]</P>

<P>The <TT>std::memcpy</TT> library function is singled out for
special treatment in 6.9 [<A href="https://wg21.link/basic.types#3">basic.types</A>] paragraph 3:</P>

<BLOCKQUOTE>

For any trivially copyable type <TT>T</TT>, if two pointers to
<TT>T</TT> point to distinct <TT>T</TT> objects <TT>obj1</TT> and
<TT>obj2</TT>, where neither <TT>obj1</TT> nor <TT>obj2</TT> is a
base-class subobject, if the value of <TT>obj1</TT> is copied into
<TT>obj2</TT>, using the <TT>std::memcpy</TT> library function,
<TT>obj2</TT> shall subsequently hold the same value as <TT>obj1</TT>.

</BLOCKQUOTE>

<P>This specification should not be restricted to <TT>std::memcpy</TT>
but should apply to any bytewise copying, including
<TT>std::memmove</TT> (as is done in the footnote in the preceding
paragraph, for example).</P>

<P><B>Proposed resolution (July, 2009):</B></P>

<P>Change 6.9 [<A href="https://wg21.link/basic.types#3">basic.types</A>] paragraph 3 as follows:</P>

<BLOCKQUOTE>

For any trivially copyable type <TT>T</TT>, if two pointers to
<TT>T</TT> point to distinct <TT>T</TT> objects <TT>obj1</TT> and
<TT>obj2</TT>, where neither <TT>obj1</TT> nor <TT>obj2</TT> is a
base-class subobject, if the <DEL>value of</DEL> <INS>underlying bytes
(6.8.1 [<A href="https://wg21.link/intro.memory">intro.memory</A>]) making up</INS> <TT>obj1</TT> <DEL>is</DEL>
<INS>are</INS> copied into <TT>obj2</TT><DEL>, using the
<TT>std::memcpy</TT> library function</DEL> <INS>[<I>Footnote</I>: By
using, for example, the library functions (16.4.2.3 [<A href="https://wg21.link/headers">headers</A>]) <TT>std::memcpy</TT> or <TT>std::memmove</TT>.
&#8212;<I>end footnote</I>]</INS>, <TT>obj2</TT> shall subsequently
hold the same value as <TT>obj1</TT>. [<I>Example:</I>...

</BLOCKQUOTE>

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