🚀 go-pugleaf

Steve Clamage (clamage@Eng.Sun.COM) wrote:
: 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? 

By passing it to a checking routine, of course. ;-)  Given the curent
standard, a call to such a routine can elicit undefined behavior in an
otherwise portable program:

   void bar(int*& p) {
      checkOut(p);  // p is passed by value to checkOut.
      p = 0;
   } 

I don't mean to imply that one can portably detect *all* invalid
pointers, but with help from the compiler software-based solutions
can give coverage that is as complete as those based on hardware.
In either case, it is not particularly helpful that the behavior
becomes undefined the moment an invalid value is detected.
    
Tom Payne (thp@cs.ucr.edu)
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