🚀 go-pugleaf

In article ft6@galaxy.ucr.edu, thp@cs.ucr.edu (Tom Payne) writes:
>....  It seems that to accomodate
>certain hardware-based validity checkers for pointers, the standard
>has precluded the possibility of portable software-based validity
>checking.  IMHO an unfortunate decision.

Even if the value of a pointer after its referent was deleted were 
required to be valid, it is still not possible to write a portable pointer
validity checker for C or C++. Example, assuming that it is always safe
to read the value of any pointer:

	void foo(int* p) {
		// is p valid?
	}
How are you going to determine portably whether p is valid in foo? 

I might call foo like this, for example:
	int f2() {
		struct two { int a, b; } ray[10];
		foo(&ray[5].b);
	}
The value of p in foo is valid, even though it does not correspond to any
heap object or the start of any entire object. Furthermore, suppose in foo
I save the value of p in a static variable and use it again in another
invocation of foo. Is it still valid? If the current address of p
is the same as the saved value from a previous invocation, does it refer
to the same object?

These questions can be answered in system-specific ways, but there is
no portable method. A saved address might or might not still be valid.
Addresses that compare equal might refer to the same or to different
objects. 
---
Steve Clamage, stephen.clamage@eng.sun.com
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