[Bug c++/53762] New: pointer-casts don't always check null-pointers

[Bug c++/53762] New: pointer-casts don't always check null-pointers

[imzhuli at vip dot qq.com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53762

             Bug #: 53762
           Summary: pointer-casts don't always check null-pointers
    Classification: Unclassified
           Product: gcc
           Version: 4.2.1
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: imzhuli@vip.qq.com


Created attachment 27695
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27695
examples of lacking null-pointer check

under these situations, null-pointers are not converted to null-pointers as the
standard says;

1. with 'this' pointer:
class A
{
  public:
    int asomething; // important: if there's nothing in class A, the functions
seems right;
};
class B
: public A
{
  public:
    virtual ~B() {}
    void* thisoffset() {
      return static_cast<A*>(this);
    }
    void* offset() {
      B * pb = this;
      return static_cast<A*>(pb);
    }
};

the two 'offset' function returns a 'offset of object of type A in objec of
type B ', and a null-pointer. i think null-point should be right.

2. with reference from DERIVED class to BASE class:
// class A & B defined as the above
A* pa = NULL; B*pb = NULL;
pa = &static_cast<A&>(*pb);
^^^^^^^^^^^^^^^^^^^^^^^^^^  will not perform null-pointer check;

A* pa = NULL; B*pb = NULL;
pa = static_cast<A*>(pb);
^^^^^^^^^^^^^^^^^^^^^^^^^  will perform null-pointer check;

[Bug c++/53762] pointer-casts don't always check null-pointers

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53762

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

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

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-06-24 16:26:17 UTC ---
Your program dereferences null pointers, that's undefined behaviour.

[Bug c++/53762] pointer-casts don't always check null-pointers

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53762

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-06-24 16:30:54 UTC ---
These are allowed:

  pa = static_cast<A*>(pb);
  pb = static_cast<B*>(pa);

All the other casts are not because they involve dereferencing a null pointer
which is not allowed by [expr.unary.op]/1

Also, GCC  4.2 is not supported or maintained so there's no point reporting
bugs against it, you should check a newer release before reporting bugs.

[Bug c++/53762] pointer-casts don't always check null-pointers

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53762

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

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

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-06-24 18:09:39 UTC ---
Invalid as mentioned as:
pa = &static_cast<A&>(*pb);
                      ^^^
Deferences a null pointer at the caret.