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From: philip@pescadero.Stanford.EDU (Philip Machanick)
Newsgroups: comp.lang.c++
Subject: Re: Please help me...
Keywords: Tracer, Help, Destruct
Message-ID: <1991Jan10.223143.5752@Neon.Stanford.EDU>
Date: 10 Jan 91 22:31:43 GMT
References: <7748@castle.ed.ac.uk> <7749@castle.ed.ac.uk>
Sender: news@Neon.Stanford.EDU (USENET News System)
Reply-To: philip@pescadero.stanford.edu
Distribution: comp
Organization: Computer Science Department, Stanford University
Lines: 33

In article <7749@castle.ed.ac.uk>, eanv20@castle.ed.ac.uk (John Woods) writes:
|> Ahah. Could it be that only _named_ local objects may not be destructed before
|> they exit their scope? Must read my ARM more carefully. Sorry for wasting
|> everybody's bandwidth and time. However, this would render the example 
|> function void f() ARM 3.5 (p22) incorrect. Errata list, anyone?

I tried your example on cfront 2.0, with similar results. Replacing
  Tracer("foo");
by Tracer a ("foo");  // the name a is not used anywhere else
also seemed to give the "expected" behaviour.

I've seen a claim that this technique may also be used to implement a lock:
  { Lock (lock_id);	// Lock is a constructor: makes an object which
                        // locks the identified lock and remembers its identity
     // critical region
  }  // destructor sees that unlock happens at close of scope

If the behaviour you report here is typical, this is clearly not a safe
alternative to

  lock(lock_id);
     // critical region
  unlock(lock_id);

Any clarification of this issue would be useful.

(One advantage of introducing a name for the Tracer or Lock object is you
no longer get "warning: result of constructor call expression not used"
messages; a disadvantage is it obscures the fact that the constructor is
used purely for its side-effect.)
-- 
Philip Machanick
philip@pescadero.stanford.edu