[Bug c++/40682] New: Require direct binding of short-lived references to rvalues

[Bug c++/40682] New: Require direct binding of short-lived references to rvalues

[dragan at plusplus dot co dot yu]

In a recent discussion on comp.std.c++
"rvalue references returned from a function",
as pointed by Niels Dekker and Micael Dark,
a defect report:

  http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#391

was accepted into C++0x WP. This has a significant impact on GCC.

The current behavior also creates a problem with the following:

  struct R {
    R(const R &);
  };

  R && func();

  int main() {
    R const & r1 = func();
    R &&      r2 = func();
  }

Both r1 and r2 will be bound to a temporary which will be
created by copying (or if available - moving) the returned
rvalue. This is invalid according to the mentioned changes.


-- 
           Summary: Require direct binding of short-lived references to
                    rvalues
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dragan at plusplus dot co dot yu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40682

[Bug c++/40682] [C++0x] Require direct binding of short-lived references to rvalues

[paolo dot carlini at oracle dot com]

------- Comment #1 from paolo dot carlini at oracle dot com  2009-07-08 11:41 -------
Note, in general work on C++0x features doesn't really belong to Bugzilla,
unless existing code crashes on new testcases, things like that. You understand
that in general the situation would otherwise quickly explode. About this
specific issue, I suspect we even have something already open. Maybe Jon can
help...


-- 

paolo dot carlini at oracle dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jwakely dot gcc at gmail dot
                   |                            |com
            Summary|Require direct binding of   |[C++0x] Require direct
                   |short-lived references to   |binding of short-lived
                   |rvalues                     |references to rvalues


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40682

[Bug c++/40682] [C++0x] Require direct binding of short-lived references to rvalues

[dragan at plusplus dot co dot yu]

------- Comment #2 from dragan at plusplus dot co dot yu  2009-07-08 12:38 -------
Although this is a feature request in the context that the old behavior
was correctly implemented and it will be different in C++0x, it still presents
a bug in the current C++0x implementation. It creates copies of
objects that are returned by a rvalue reference, which would be wrong.
I wasn't _really_ trying to request a new functionality, although
I understand your concern.

However, IMHO in this particular case we are talking about one of
the most fundamental features (binding a reference). This change
could also expose possible bugs in the (user and library) code that
relies on creation of a temporary, which even in C++03 is not
considered a portable code. Don't tell me you'd rather deal with
the issue after all the other C++0x stuff gets implemented...

Anyway, my motive was to help the development of GCC. Personally,
I didn't and will not rely on either of the two behaviors, but
will try to write code that works in both situations.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40682

[Bug c++/40682] [C++0x] Require direct binding of short-lived references to rvalues

[paolo dot carlini at oracle dot com]

------- Comment #3 from paolo dot carlini at oracle dot com  2009-07-08 12:47 -------
To be clear, I'm not telling you anything specific about the development
process. Actually, that's exactly the point, this is ongoing development of
experimental features, no guarantees, no guarantees of perfect adherence to the
last public draft, etc.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40682

[Bug c++/40682] [C++0x] Require direct binding of short-lived references to rvalues

[jason at gcc dot gnu dot org]

------- Comment #4 from jason at gcc dot gnu dot org  2009-10-07 19:45 -------
Neither 4.4.1 nor 4.5 try to call the copy constructor in your example; I
believe I fixed this issue in 4.4.


-- 

jason at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40682