Subj : Re: Improving read-write lock
To   : comp.programming.threads
From : Alexander Terekhov
Date : Thu Feb 24 2005 03:00 pm


David Holmes wrote:
> 
> "Joe Seigh" <jseigh_01@xemaps.com> wrote in message
> news:opsmd8w7vwqm36vk@grunion...
> > IIRC, Java exceptions don't unlock monitor locks.  Of course Java doesn't

Java's "synchronized" thing is kinda RAII for monitor locks.

> > use dtors for scoping since it has GC.

Java's silly approach for scoping and deterministic cleanup apart from 
monitor locks is try/finally. C# does sorta improve it a bit with 
"using" and IDisposable.Dispose().

----
A using statement of the form 
using (R r1 = new R()) {   
  r1.F();   
}   

is precisely equivalent to 

r1 = new R();   
try {   
  r1.F();   
}  
finally {   
  if (r1 != null) ((IDisposable)r1).Dispose();   
}   

A resource-acquisition may acquire multiple 
resources of a given type. This is equivalent 
to nested using statements. A using statement 
of the form 

using (R r1 = new R(), r2 = new R()) {   
  r1.F();   
  r2.F();   
}   

is precisely equivalent to: 

using (R r1 = new R())   
  using (R r2 = new R()) {   
    r1.F();   
    r2.F();   
  }   

which is, by expansion, precisely equivalent to: 

R r1 = new R();   
try {  33 
  R r2 = new R();   
  try {   
    r1.F();   
    r2.F();   
  }   
  finally {   
    if (r2 != null) ((IDisposable)r2).Dispose();   
  }   
}   
finally {   
  if (r1 != null) ((IDisposable)r1).Dispose();   
}   
----

> >
> 
> Java monitor locks are automatically unlocked when an exception propagates
> out of a synchronized block or method. I think the C++ folk popularised this
> peculiar notion with RAII for locking. :(

Brain-dead current semantics of exceptions specs aside for a moment, 
C++ doesn't require unwinding for unexpected exceptions and sensible 
implementations do not unwind; they just call terminate() at throw 
point. So C++ RAII (for locking and other stuff) isn't that bad. It's 
certainly better in this respect than brain-dead Java/C#'s "finally"
that "catch" everything.

regards,
alexander.

.