Path: utzoo!attcan!uunet!pdn!oz!alan
From: alan@oz.nm.paradyne.com (Alan Lovejoy)
Newsgroups: comp.lang.c++
Subject: Re: Combining event driven and object orient programming
Message-ID: <6660@pdn.paradyne.com>
Date: 13 Oct 89 21:48:01 GMT
References: <13928@well.UUCP< <13929@well.UUCP>
Sender: usenet@pdn.paradyne.com
Reply-To: alan@oz.paradyne.com (Alan Lovejoy)
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Organization: AT&T Paradyne, Largo, Florida
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In article <13929@well.UUCP< nagle@well.UUCP (John Nagle) writes:
<
<       What you really need is serious concurrency support.  Here's why.
<
<       In principle, if one has cheap multitasking (called, variously,
<"threads", "lightweight processes", or "weak fork"), an elegant
<solution is for events which will require processing beyond the
<current event to fork off a new task to handle them, requesting that
<related events be forwarded to that task.  The task can have lots of
<context, and lots of "auto" storage-class (in the C sense) objects.
<
<       It helps if Ada-like exception handling is available.  This
<provides a clean way to bail out with proper unwinding when necessary.
<The combination of multitasking, destructors, and exception processing
<makes cleanup almost painless.
<
<       Also essential is automatic multitasking interlocks, or "monitors".
<The idea is that if you want to access data shared with another task,
<you must call an access function of a class that is declared as allowing
<object sharing across task boundaries.  When one task has control inside
<a function of such a class, other tasks are blocked if they call a 
<function of the class.  This protects shared data automatically.
<
<       Now how do we get multithread C++?

Gee, this reminds me of something...  Perhaps some other programming language
I used at one time... Oh! I know, it's like Modula-2!  Or is it?  Let's see:
Lightweight processes, check.  Monitors, check.  Exception handling...well,
Modula-2 doesn't have exception handling.  Oh well.  It doesn't have classes,
constructors or destructors either.  I guess you can't have everything.

My impression is that Modula-2's threads are seldom used, for some reason.
Whether this is due to a problem with Modula-2, with the M2 implementation 
of threads/monitors, with the Modula-2 user community, or simply indicative
of the usefulness of threads, I couldn't really say.

Of course, exception handling can be kludged-up using setjmp/longjmp.  Monitors
and threads require compiler help, or at least extensive non-portable work
with asm().  P&V operations can be used to establish critical sections, 
instead of the more elegant "monitor" abstraction.  

All things considered, it would seem best to put support for threads/monitors 
in the compiler--provided threads/monitors would actually be useful to a 
significant community of programmers.  Whether this is the case is not clear, 
at least to me.  



____"Congress shall have the power to prohibit speech offensive to Congress"____
Alan Lovejoy; alan@pdn; 813-530-2211; AT&T Paradyne: 8550 Ulmerton, Largo, FL.
Disclaimer: I do not speak for AT&T Paradyne.  They do not speak for me. 
Motto: If nanomachines will be able to reconstruct you, YOU AREN'T DEAD YET.