Why and How it is working.

[sudhakarm]

Hi friends

Please read the following code.

  #include <iostream.h>
  #include <conio.h>

  class alpha
  {
  public :

	alpha()
	{
	  cout << endl << "Constructor called ...";
	}

	void showalpha()
	{
	  cout << endl << "This is alpha class ...";
	}

  };

  void main()
  {
  clrscr();

  alpha *a = new alpha();
  alpha *b = NULL;

  a->showalpha();
  b->showalpha();

  cout << endl << "Value in a = " << a;
  cout << endl << "Value in b = " << b;

  getch();
  }

Output of the above program is :

Constructor called ....
This is alpha class ...
This is alpha class ...

Value in a = 0x90500004
Value in b = 0x0

Actually *b contains NULL value then how it's calling showalpha() and how it's working fine.

Can any body pls explain to me.

[marten_range]

It works since the code in showalpha doesn't try to access any member variables.

Hope this help.

[Graham]

However, the program is incorrect and this behaviour should not be relied upon.

[Axter]

FYI:
iostream.h is not part of the C++ standard, and should not be used in any new code.
Use extensionless version <iostream> which is part of the standard.

[gstercken]

To add to what the others said:

• It works because the function's this-pointer is never dereferenced - as marten_range said, as long as you don't access a member variable or call a member function which does so, that kind of call will most probably work due to the way function calls are implemented in most compilers.
• However, as Graham said, calling a member function through a NULL (or otherwise invalid) pointer is considered undefined by the standard. You might encounter a compiler where that code fails.
• As a sidenote: MFC relies on the feature you described. For example, the CWnd member function GetSafeHwnd() tests if 'this' is NULL, and returns 0 in that case. That's a cute little hack which saves you some typing when it comes to testing whether a window pointer is valid, but again, it is actually non-standard C++.
• void main() is also wrong - by the standard, main() must return int.