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From: "tudorb at fb dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/57049] std::swap does self move assignment, which is illegal
Date: Tue, 23 Apr 2013 15:54:00 -0000	[thread overview]
Message-ID: <bug-57049-4-yQTcH9LyI4@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-57049-4@http.gcc.gnu.org/bugzilla/>


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57049

--- Comment #1 from Tudor Bosman <tudorb at fb dot com> 2013-04-23 15:54:18 UTC ---
Actually, I'll take this back.  I don't believe this is a bug.

17.6.4.9 constraints arguments passed to STL functions.  So if there is a
library function that takes a rvalue argument (such as
string::operator=(string&& other)), then the library is free to assume that the
argument doesn't alias (so &other != this).  So the assertion in basic_fbstring
is correct.

The default implementation of swap() does self-move assignment.  That would be
illegal if the types were MoveAssignable STL types (because then they'd call
T::operator=(T&&) which would be illegal according to 17.6.4.9) BUT ALL STL
TYPES HAVE SPECIALIZED IMPLEMENTATIONS OF SWAP which don't do self move
assignment.

So the default swap() will do self-move-assignment on user types, but there's
no language in the standard that bans self-move-assignment there.


  reply	other threads:[~2013-04-23 15:54 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-04-23 15:46 [Bug libstdc++/57049] New: " tudorb at fb dot com
2013-04-23 15:54 ` tudorb at fb dot com [this message]
2013-04-25  9:55 ` [Bug libstdc++/57049] " daniel.kruegler at googlemail dot com
2013-06-11  9:00 ` paolo.carlini at oracle dot com

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