kst@aonix.com (Keith Thompson) writes:

> In <dewar.852652911@merv> dewar@merv.cs.nyu.edu (Robert Dewar) writes:
> > Actually, when I talked to Alex, he indicated that there were critical
> > features in Ada 95 that were missing in C and C++ for this kind of
> > work. In particular, he noted that C lacks a general address comparison
> > operation, useful for hashing data structures, whereas Ada does have such
> > an operation (in System.Storage_Elements on type Integer_Address). It is
> > a bit ironic to identify this particular feature as crucial (since most
> > people mistakenly think that C has such a feature and Ada doesn't!)
> 
> Well, sort of.
> 
> It's true that C pointer comparison is undefined unless the pointers being
> compared point into the same object (or one past the end of an array).
> However, any real-world C implementation isn't going to go out of its
> way to make such comparisons invalid.  If pointers "look like" integers
> at the hardware level, pointer comparison is almost certainly going to be
> implemented as integer comparison.  It may be signed or unsigned, but it's
> almost certainly going to be consistent within a given implementation.

Since this article is posted to several different groups, including 
comp.lang.c++, I'd like to point out that the situation in C and in 
C++ is somewhat different.

Keith's description is correct in the case of C.  In C++, though, it's
only part of the story.  Pointer comparison in C++ using the <
operator does work just the way that he says; C++ didn't change the
meaning of the < operator.  However, C++ includes another way of
comparing the magnitude of two values: the standard library function
object less<T>.  

Operator< isn't necessarily a total ordering on pointers, but less<T>
is.  To quote the draft standard (section 20.3.3): "For templates
greater, less, greater_equal, and less_equal, the specializations for
any pointer type yield a total order, even if the built�in operators
<, >, <=, >= do not."