Re: [boost] Boost.Threads - Questions on Usage

From: Jeremy Siek (jsiek_at_[hidden])
Date: 2001-07-17 11:55:06

On Tue, 17 Jul 2001 williamkempf_at_[hidden] wrote:
> >
> >class X {
> >   mutex* mtx;
> >   X() : mtx(new mutex()) { }
> >   ~X() { delete mtx; }
> >};
> >
> >which is a pain.
>
> And simply wrong ;).  A mutex *IS* a noncopyable concept.  If you
> remember, we discussed this early on in the design.  Allowing a mutex
> to be copyable just leads to incorrect code, similar to what you have
> with your work around above.  Why incorrect?  Because the
> implementation you have above results in two distinct sets of data
> being protected by a single mutex.  If CopyA locks the mutex then
> CopyB is locked as well.  This can lead to hard to diagnose deadlocks.

Actually, in the example above I had left out the copy constructor for
brevity, I did not mean to imply that the generated constructor would be
used.

> However, just because the mutex isn't copyable doesn't mean that an
> object that contains a mutex must be noncopyable.  It just means you
> can't rely on the compiler supplied copy c-tor and assignment
> operator.  This turns out to be a good thing any way, since such
> operations should lock both mutexes before doing the copy, which is
> something the compiler supplied versions aren't going to do any way.

So you have to write a copy constructor like this:

X(const X& x) : mtx(new mutex()), ... { }

Which is also a pain.

> >So my question is this: is there any way we can get mutex to be
> copyable?
>
> No.  At least not while maintaining correct MT behavior.  I'll add
> this one to the FAQ as well, along with appropriate code examples
> addressing both questions.

So exactly what MT behaviour are you refering to?

Perhaps there is a way to work around the bad behavoir while keeping
copyable.

Cheers,
Jeremy

P.S. The reason I'm pushing on this is that the usage pattern I'm looking
at is a very common case, perhaps the most common case following using a
mutex in a simple scope.

----------------------------------------------------------------------
 Jeremy Siek                        www: http://www.lsc.nd.edu/~jsiek/
 Ph.D. Candidate, IU B'ton          email: jsiek_at_[hidden]
 Summer Manager, AT&T Research      phone: [phone]
----------------------------------------------------------------------