Re: Language pitfalls (was Re: FORTRAN bug)

[dale.cts.com!jhb@nosc.mil (John Bollenbacher)]

Harry Koehnemann (koehnema@enuxha.eas.asu.edu) wrote:
: In article <EACHUS.92Dec15202249@oddjob.mitre.org> eachus@oddjob.mitre.org (R
obert I. Eachus) writes:
: >In article <1992Dec15.203558.18211@inmet.camb.inmet.com> stt@spock.camb.inme
t.com (Tucker Taft) writes:
: >
: >   Oh boy.  That is a nasty one.  This argues for a "friendly" 
: >   Ada compiler giving a warning about any use of "null;" other
: >   than the idiomatic ones like "when others => null;" or "begin null; end;"
: >   (especially in a function that returns an access type ;-).
: >
: >    Actually, there is an Ada rule which normally catches this, and
: >which Robert Dewar and I have argued should be removed in Ada 9X.  (A
: >function must contain a return statement RM 6.5(1).)  If it belongs on
: >the top ten list, then the rule should stay.
: 
: The "null;"/"return null;" was a problem with the base case in a
: recursive function (null is overloaded, and even worse it's an
: expression *and* a statement - isn't that something we hate about
: C :).  This implies there will be additional returns somewhere in
: the routine and it will not be caught by the compiler.  The real
: problem is that there exists an execution path that can lead out
: of the function without encountering a return.  

Seems to me the real problem is that there is a potential path out if the
function which does not raise an exception but has the 'wrong' value, e.g.
    if mumble = null then
      null;
    end if;
    ...
    return new mumble_ptr;
Thanks for pointing out this gotcha. I'd never seen it before.

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