[Bug c++/50056] New: Binding a temporary object to a reference

[Bug c++/50056] New: Binding a temporary object to a reference

[wolfgang.roehrl@gi-de.com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50056

             Bug #: 50056
           Summary: Binding a temporary object to a reference
    Classification: Unclassified
           Product: gcc
           Version: 4.5.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: wolfgang.roehrl@gi-de.com


Hi all,

I would like to post a bug report for the GNU C/C++ compiler
version 4.5.2 (powerpc-rtems4.11).

We use the compiler to generate code for a PowerPC processor.

The compiler is invoked with the following options:

ccppc -c -x c++ -ansi -Wall -Werror -g -mcpu=8540 -fverbose-asm -mbig
      -mfloat-gprs=double -mspe -mabi=spe -meabi -msdata -fno-common
      -mmultiple -mno-string -misel -mstrict-align -fgcse-sm
      -fno-rename-registers -fno-section-anchors -G 8 -Os
      -fno-exceptions -fno-rtti
      -I<some include paths>
      -D<some #define's>
      X.CPP -oX.O

// file X.CPP

struct S
{
    S();
    ~S();
    void func () const;
};

void test (void)
{
    const S& s = static_cast<const S&>(S());

    s.func ();
}


The code compiles fine but function test() is translated to the following
pseudo code:
  call S::S()
  call S::~S()    --> now we have a dangling reference s without an object
  call S::func()  --> calling an element function without an object


Kind regards
W. Roehrl

[Bug c++/50056] Binding a temporary object to a reference

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50056

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-08-12 12:46:42 UTC ---
Are you sure G++ isn't correct?
The cast creates a temporary reference, which binds to the temporary object,
then initalizes 's' with the temporary reference. That means the temporary
object's lifetime is extended to the same lifetime as the reference, but the
reference is temporary so is destroyed at the end of the full expression.

call S::S()
bind to temporary reference
initialize s from temporary reference
temporary reference goes out of scope
call S::~S()
call S::func()
s goes out of scope

if you do this:

const S& s = S();

then you get the behaviour I assume you are expecting:

call S::S()
initialize s temporary object
call S::func()
s goes out of scope
call S::~S()

[Bug c++/50056] Binding a temporary object to a reference

[wolfgang.roehrl@gi-de.com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50056

--- Comment #2 from Wolfgang Roehrl <wolfgang.roehrl@gi-de.com> 2011-08-12 14:51:25 UTC ---
Hi Jonathan,

I would analyze the crucial line of the code as follows:

The cast creates a temporary reference, which binds to the temporary 
object,
and then initalizes 's' with the temporary reference. The latter means:

A) 's' is initialized with a temporary S-object which appears as an 
lvalue:
   Cf. 5/6: "If an expression initially has the type "reference to T" 
(8.3.2,
   8.5.3), the type is adjusted to "T" prior to any further analysis, the
   expression designates the object or function denoted by the reference,
   and the expression is an lvalue."

B) 's' is directly bound to the temporary S-object:
   Cf. 8.5.3/5, Bullet 1: "If the initializer expression
   - is an lvalue (but is not a bit-field), and "cv1 T1" is reference-
     compatible with "cv2 T2," ... then the reference is bound directly
     to the initializer expression lvalue."

C) 12.2/5 says that the lifetime of a temporary which is bound to a 
reference
   is extended to the lifetime fo the reference.

I'm not sure if this analysis is really correct but it seems reasonable to 
me.
Otherwise we would have a reference without an object.

Best regards,
W. Roehrl







"redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> 
12.08.2011 14:47

An
wolfgang.roehrl@gi-de.com
Kopie

Thema
[Bug c++/50056] Binding a temporary object to a reference






http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50056

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-08-12 
12:46:42 UTC ---
Are you sure G++ isn't correct?
The cast creates a temporary reference, which binds to the temporary 
object,
then initalizes 's' with the temporary reference. That means the temporary
object's lifetime is extended to the same lifetime as the reference, but 
the
reference is temporary so is destroyed at the end of the full expression.

call S::S()
bind to temporary reference
initialize s from temporary reference
temporary reference goes out of scope
call S::~S()
call S::func()
s goes out of scope

if you do this:

const S& s = S();

then you get the behaviour I assume you are expecting:

call S::S()
initialize s temporary object
call S::func()
s goes out of scope
call S::~S()

[Bug c++/50056] Binding a temporary object to a reference

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50056

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |jason at gcc dot gnu.org
         Resolution|                            |INVALID

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> 2011-08-16 23:43:19 UTC ---
clang 2.8 and EDG 4.3 behave the same way as G++, so I don't think this is a
bug in GCC; feel free to raise it as a language issue, though.

This section could use to be reformulated in terms of class prvalues.  S() is a
class prvalue, but static_cast<const S&>(S()) is an lvalue.