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From: Michael Glassford (glassfordm_at_[hidden])
Date: 2004-07-07 11:30:41

Howard Hinnant wrote:
> On Jul 7, 2004, at 9:19 AM, Michael Glassford wrote:
>> There are still the constructor issues to deal with in such a lock
>> taxonomy, however. The main one: how do you define destructors that
>> are consistent within a lock concept and also consistent between
>> concepts. E.g., on the one hand it seems that a one-parameter
>> constructor should do the same thing in all lock types--so it would
>> have to block; on the other hand, it seems that TryLock constructors
>> should not block unless instructed to do otherwise.
> Agreed, and I think the scoped_try_lock got it wrong (i.e.
> scoped_try_lock(mut) should block).

I've come to agree with you on this, and hope to fix it one way or
another (see other replies in this discussion).

> This somewhat irritates me as I
> have coded and shipped Metrowerks::scoped_try_lock with the wrong
> semantics (doesn't block). :-\

> I'm finding more and more uses for generic lock code.

I'm glad to know this; I wondered at first whether generic uses of locks
would be at all common or important.

> The generic
> helper I showed yesterday was one. Another is:
> template <class TryLock1, class TryLock2>
> class lock_both
> {
> public:
> lock_both(TryLock1& m1, TryLock2& m2, bool lock_it = true);
> ~lock_both();
> void lock();
> bool try_lock();
> void unlock();
> bool locked() const;
> operator int bool_type::* () const;
> private:
> lock_both(const lock_both&);
> lock_both& operator=(const lock_both&);
> };
> which allows you to lock two locks without fear of deadlock. So
> consistent syntax/semantics across lock types is a real win.

Agreed. I always like as much consistency as possible, which is why I
started the whole lock constructor discussion.


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