Re: The differences between the object and the pointer points to
object
On Dec 10, 5:49 pm, James Kanze <james.ka...@gmail.com> wrote:
On Dec 10, 7:11 am, Bruclee <kailang.d...@gmail.com> wrote:
On Dec 9, 10:26 pm, James Kanze <james.ka...@gmail.com> wrote:
On Dec 9, 2:17 pm, kailang.d...@gmail.com wrote:
I have a program as follows.
class ClassA
{
public:
int buff[1];
virtual void test(void) {cout<<"ClassA::test()"<<en=
dl;}
};
void entry(void)
{
cout<<"Hey,i am here!"<<endl;
}
ClassA Obj1,Obj2,*pObj;
int main()
{
pObj=&Obj2;
//Test group 1
Obj2.test();
pObj->test();
//Obj1.buff[1] covers the pvftable field of Obj2
int vtab=(int)(entry);
I'm not too sure what you thing you're doing in the above.
Obj1.buff[1]=(int)&vtab;
Nor here (but this is clearly undefined behavior---anything
can happen).
I intended to overwrite the pvftable entry of Obj2 by let the
buff array be out of bound,because Obj2 comes after Obj1 in
the memory.Finally,the vpftable of Obj2 will be covered by the
address of entry function.
I understand that. I was being slightly ironic---just to insist
on the fact that this really doesn't have any meaning. (For
starters, you're not guaranteed that Obj1 and Obj2 are adjacent,
or in that order, in memory.)
What you're doing depend very, very much on internal details of
the compiler. I think you knew that at least partially, but I
wasn't sure just how much, since you seemed surprised at the
output.
[...]
I noticed some difference between the different compiler.Under
windows,Visual C++6.0 places pvftable right
the begining of the object while C++ GNU place the pvftable at the end
of
the object and Obj1 and Obj2 are adjacent in memory under windows.
Without considering portability,I want to find if we can do some
crack(conduct the program to invoke the function designated by
ourselves
instead of the virtual function issued by program or manufacturer) by
overwrite the pvftable of the object under MS VC6.0 since we use a lot
of
virtual functions provided by class library or manufacturer through
inheritance.
Since the simple idea above, i do the test to check the safty of class
with
virtual functions.
Thank you for your enthusiasm help!
-Bruce
Turn up the optimization level, and "Hey, I am here" might
disappear completely.
I tried to change the optimization level to Minimize Size or
Maximize Speed.But "Hey, I am here" still appear.Is my
optimization level setting error or other reasons?
I said "might". It depends on the optimizer, but it's entirely
within the capabilities of modern optimizers to determine that
your pointer never points to anything but a ClassA, and so not
generate the code to determine the type dynamcally.
--
James Kanze (GABI Software) email:james.ka...@gma=
il.com
Conseils en informatique orient=E9e objet/
Beratung in objektorientierter Dat=
enverarbeitung
9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, France, +33 (0)1 30 23 00 34- =
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