Re: How does this work?
"Stuart Redmann" <DerTopper@web.de> wrote in message
news:ed93nj$ftl$1@news.dtag.de...
kamlesh.ajmere@gmail.com wrote:
can anyone tell me how does this work without allocating memory to
object,
#include "stdafx.h"
#include <iostream>
using namespace std;
class Test
{
public:
int getNumber();
};
int Test::getNumber()
{
return 10;
}
int _tmain(int argc, _TCHAR* argv[])
{
Test *p = NULL;
int n = p->getNumber();
cout << n;
return 0;
}
That's an easy case: Since you dereference a null pointer you get
undefined behaviour. Your compiler is free to do whatever it wants. It
may throw an exception, crash your application or even format your hard
disk. Since you are using VC, you will get the number you didn't expect
to get. Since the compiler doesn't need to examine the 'this' pointer of
the variable 'p', no invalid memory access will appear (as Mihajlo
already stated, the compiler doesn't need to access the virtual method
table, since getNumber is not virtual, and getNumber itself does not
access any members of the class).
Regards,
Stuart
But this is (or should be) a run-time issue, not a compile-time issue. The
compiler should not know or care that the pointer is NULL.
Dereferencing of a NULL pointer _invokes_ undefined behavior, but the
statements/syntax are perfectly valid C/C++, and the compiler should not be
playing a role in which undefined behavior is invoked.