null references

[sanzilla]

hi all ,

Using the visual C++ 6.0 I have wrote a simple console program. This is the code.

#include <iostream>
using namespace std ;

class Employee  
{

	int a   , int b  ;
public:
	Employee ()
	{
		a = 10 ;
		b = 100 ;
	}

	void print ( )
	{
		cout << a <<"  " << b << endl ;                // Line [B] 
		cout << " the print fucntion is called " << endl ;
	}

};

Employee * getEmployee ()
{
	return NULL ;
}


int main ()
{
	Employee &anEmployee = * getEmployee () ;  // The line [A]
	anEmployee.print() ;
	return 0;
}

Dear all , my problem is this program compiles well with the line [A] , but there is a run time error when the accessing the int a and int b on the
class in the Line [B]. So using a NULL reference I able to access it's non
static method print() , but unable to access to the variables . Can anyone explain why ?

so that means the object code and data are putting in different places by the compiler ? I need to read more about how the compiler is generating the code for
these source lines . Anybody have a idea ?

[jsmith]

See http://www.cplusplus.com/forum/general/4329/

The answer to your question is the same as that one.

[sanzilla]

Although the prototype for memberFunction is
void memberFunction( int arg_param )

what really happens beneath the scenes is the compiler passes a second parameter to this function, that parameter being a pointer to the instance of A that you are calling the member function on. (The so-called "this" pointer, essentially). Because memberFunction doesn't actually look at any data members of A, the pointer is not derefenced; it is simply unused. This is why the code above does not crash. This also explains why it does crash if you attempt to print m_memberVariableX, because now you are attempting to dereference the "this" pointer to access the member variable.

You should find that another way to crash this program is to make memberFunction (as it is now) a virtual function. Even though it still doesn't access any data members, it still crashes.

yes I dig that thread to the death , Thanks a lot jsmith. he has explained this very great , but that is a logical explanation . I need now physical explanation ,

How this code will be physically implemented ? what causes that error to happen . I know why that error happens , thus because he accessed a memory location that not belongs to the process or thread .

This is the exception informations that I have got.
Exception Information
code: 0x00000005 Flags : 0x00000000
Address : 0x000000000000401625

well using this information , I go to the debug this program , and I got the
breakpoint at that address .

18:           cout << a <<"  " << b << endl ;
0040161D   push        offset @ILT+205(std::endl) (004010d2)
00401622   mov         eax,dword ptr [ebp-4]
00401625   mov         ecx,dword ptr [eax+4]
00401628   push        ecx
00401629   push        offset string "  " (0046c044)
0040162E   mov         edx,dword ptr [ebp-4]
00401631   mov         eax,dword ptr [edx]
00401633   push        eax
00401634   mov         ecx,offset std::cout (004777e0)
00401639   call        @ILT+260(std::basic_ostream<char,std::char_traits<char> >::operator<<) (00401109)
0040163E   push        eax
0040163F   call        @ILT+650(std::operator<<) (0040128f)
00401644   add         esp,8
00401647   mov         ecx,eax
00401649   call        @ILT+260(std::basic_ostream<char,std::char_traits<char> >::operator<<) (00401109)
0040164E   mov         ecx,eax
00401650   call        @ILT+485(std::basic_ostream<char,std::char_traits<cha

Oky here as I underestood , the object address is passing as a parameter to the function print it is @ [ebp-4] ( just getting that out of the stack) , so it put to the eax , and is 0 ( because I pass NULL no ) , and the next instruction is
ecx,dword ptr [eax+4]
is that refering to the b ? am I correct , then the instructions , .....
0040162E mov edx,dword ptr [ebp-4]
00401631 mov eax,dword ptr [edx]

are accessing the variable a .....

wow man , this is great ! So there is a way and a well definided procedure that compiler generates the object code . I need to know more . Pls help .
some hyperlinks to read is greater .

thankx man.

[helios]

References are usually implemented as obscured pointers.
This thread deals with pointers vs. references extensively: http://www.cplusplus.com/forum/beginner/3958/
I wrote a hack that demonstrates references in VC++ are pointers (Sep 18, 2008 at 7:39pm): http://www.cplusplus.com/forum/beginner/3958/page2.html

[sanzilla]

coolz !
you ppl are really great guys man .
I going to love C++ because of y guys .