Le 31/05/2012 20:17, Andrei Alexandrescu a écrit :
On 5/31/12 5:19 AM, deadalnix wrote:
The solution consisting in passing a delegate as parameter or as
template is superior, because it is now clear who is in charge of the
synchronization, reducing greatly chances of deadlock.
It can also be a lot clunkier for certain abstractions. Say I want a
ProducerConsumerQueue. It's much more convenient to simply make it a
synchronized class with the classic primitives, instead of primitives
that accept delegates etc.
Nevertheless I think there's merit in this idea. One thing to point out
is that the idiom can easily be done today with a regular class holding
a synchronized class private member.
So we got everything we need.
Andrei
I was thinking about that. Here is what I ended up to think is the best
solution :
synchronized classes exists. By default, they can't be use as parameter
for synchronized(something) .
synchronized(something) will be valid is something provide
opSynchronized(scope delegate void()) or something similar. Think
opApply here. The synchronized statement is rewritten in a call to that
delegate.
Here are the benefit of such an approach :
1/ Lock and unlock are not exposed. You can only use them by pair.
2/ You cannot lock on any object, so you avoid most liquid locks and
don't waste memory.
3/ synchronized classes ensure that a class can be shared and it
internal are protected from concurrent access.
4/ It is not possible possible by default to lock on synchronized
classes's instances. It grant better control over the lock and it is now
clear which piece of code is responsible of it.
5/ The design allow the programmer to grant the permission to lock on
synchronized classes's instances if he/she want to.
6/ It is now possible to synchronize on a broader range of user defined
stuffs.
The main drawback is the same as opApply : return (and break/continue
but it is less relevant for opSynchronized). Solution to this problem
have been proposed in the past using compiler and stack magic.
It open door for stuff like :
ReadWriteLock rw;
synchronized(rw.read) {
}
synchronized(rw.write) {
}
And many types of lock : spin lock, interprocesses locks, semaphores, .
. . And all can be used with the synchronized syntax, and without
exposing locking and unlocking primitives.
What do people think ?
