🚀 go-pugleaf

In article <1992Dec24.202339.26224@microsoft.com> jimad@microsoft.com (Jim Adcock) writes:
|In article <85744@ut-emx.uucp> jamshid@ut-emx.uucp (Jamshid Afshar) writes:
||[...] Programmers will continue to do things like
||	class Register {
||	   Set<Register*> alive;
  	  ^woops, I forgot 'static'
||	protected:
||	   Register() { alive += this; }
||	   virtual ~Register() { alive -= this; }
||	   static void foo();  // do something to all registered objects
||	};

|Given that you do not specify your implementation, such as telling us
|what pointer comparisons you use or don't use, its hard to say
|whether your example does or doesn't work under all current C++
|implementations.

I forgot the keyword 'static' in my example, but I thought the
intentions were obvious.  The problem with the above idiom is that
Register() could add the same 'Register*' twice (the second add being
ignored) when constructing a Register-derived object that contains
another Register-derived object as its first data member.

All current and probably future C++ compilers use a hidden vtbl
pointer to implement virtual functions so you probably won't encounter
any problems with the above idiom, but it is "wrong" unless rules are
changed.

|Below find an example that frequently catches programmers off guard.
|Again, C++ pointers DO NOT model any notion of object identity. [...]
|class A { char bsPadding; };
|class B : public A {};
|class C : public A {};
|class D : public B, public C {};
	A* pa1 = (B*)&d;    A* pa2 = (C*)&d;
	if (pa1==pa2)
|		cout << "pointers match\n";
|	else 
|		cout << "pointers don't match -- even though of the same"
|			" type and 'to the same object'!\n";

I know this and I don't have a problem with it.  There are two
different A objects (they would each have separate data members) so I
don't think they are pointers "to the same object".

Jamshid Afshar
jamshid@emx.utexas.edu