[Bug c++/57155] New: casting to const reference error

[Bug c++/57155] New: casting to const reference error

[rockeet at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57155

             Bug #: 57155
           Summary: casting to const reference error
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: rockeet@gmail.com


#include <assert.h>
int main() {
    const char* buf = "12345678";
    const int& cref = (const int&)buf[0];
    const int&  ref = (      int&)buf[0];
    assert(cref == ref);
    return 0;
}

// Above assertion failed on g++4.1.2, g++4.4.6, g++4.7.1 and g++4.7.2
// These are all my tested g++ version
// cref just got first byte of buf

[Bug c++/57155] casting to const reference error

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57155

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|major                       |normal

[Bug c++/57155] casting to const reference error

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57155

Jonathan Wakely <redi at gcc dot gnu.org> changed:

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

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2013-05-03 09:08:03 UTC ---
    const int& cref = (const int&)buf[0];

This creates a temporary of type int, initialized with buf[0], and binds a
reference to it, the same as:

    const int& cref = int(buf[0]);

So it's correct that it has the same value as buf[0].

[Bug c++/57155] casting to const reference error

[rockeet at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57155

--- Comment #2 from rockeet <rockeet at gmail dot com> 2013-05-03 10:35:53 UTC ---
In this case, C++ shouldn't create a temporal object and bound the const ref to
it. The C++ standard (C++11) says:

5.2.1
Subscripting
[expr.sub]
1
A postfix expression followed by an expression in square brackets is a postfix
expression. One of the expres-
sions shall have the type “pointer to T” and the other shall have unscoped
enumeration or integral type.
The result is an lvalue of type “T.” The type “T” shall be a completely-defined
object type.61The expression
E1[E2] is identical (by definition) to *((E1)+(E2)) [Note: see 5.3 and 5.7 for
details of * and + and 8.3.4
for details of arrays. —end note ]

5.3.1
Unary operators
[expr.unary.op]
1
The unary * operator performs indirection: the expression to which it is
applied shall be a pointer to an
object type, or a pointer to a function type and the result is an lvalue
referring to the object or function
to which the expression points. If the type of the expression is “pointer to
T,” the type of the result is “T.”
[Note: a pointer to an incomplete type (other than cv void) can be
dereferenced. The lvalue thus obtained
can be used in limited ways (to initialize a reference, for example); this
lvalue must not be converted to a
prvalue, see 4.1. —end note ]

5.4
Explicit type conversion (cast notation)
[expr.cast]
1
The result of the expression (T) cast-expression is of type T. The result is an
lvalue if T is an lvalue reference
type or an rvalue reference to function type and an xvalue if T is an rvalue
reference to object type; otherwise
the result is a prvalue. [Note: if T is a non-class type that is cv-qualified,
the cv-qualifiers are ignored when
determining the type of the resulting prvalue; see 3.10. —end note ]
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idT577

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mattyclarkson at gmail dot
                   |                            |com

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> 2013-05-03 10:36:00 UTC ---
*** Bug 57158 has been marked as a duplicate of this bug. ***

[Bug c++/57155] casting to const reference error

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57155

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> 2013-05-03 10:51:10 UTC ---
Keep reading 5.4, you didn't get to the bit about type conversions.