🚀 go-pugleaf

> ==========John Max Skaller, 11/10/95==========
> 
> 
> martelli@cadlab.it (Alex Martelli) wrote:
> >
> >parker@WPI.EDU (Chris Parker) writes:
> >     ...
> >If that subclass is destroyed through a pointer to an ancestor whose
> >dtor isn't virtual, undefined behaviour (violation of a "shall").
> 
> 
> No no! Undefined behaviour is NOT a violation.
> 
> Conformance applies to COMPILERS. When behaviour
> is not defined, the compiler can produce code that
> does anything, because there is no requirement
> to satisfy.
> 
> So it's exactly the opposite of what you said:
> where behaviour is undefined is the one place
> it is IMPOSSIBLE for a "shall" to be violated,
> because there isn't one :-)
> 

Actually, "undefined behavior" is a precisely
defined term in the WP, and not just a vague
common term.  The meanings are constrained (1.3)
by text that starts "Permissible  undefined  behavior
ranges  from" and then give three categories
of behavior that are permissible.  The compiler
can ignore it and let the chips fall where they may.
The compiler can establish wierd responses
by making it part of the "environment" by
explicitly documenting that (a warning label
if you will).  The compiler can terminate
translation or execution with a warning message.

In general, the application of this term to
situations by the committee is based on the
situation being potentially too hard to diagnose
in some of its manifestations.

There is a commonsense "shall" where these
situations are concerned.  The programmer
shall avoid these situations like the plague
because the compiler will invariably do the first
of the three permissible behaviors and not help
detect it (Murphy's Law is they do not detect
your case, whatever it is).

Good Day
Dick
Dick.Menninger@DaytonOH.ATTGIS.COM


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