On 2011-10-19 20:36:37 +0000, Andrew Wiley <wiley.andrew.j@gmail.com> said:

On Mon, Oct 17, 2011 at 3:43 PM, Michel Fortin <michel.fortin@michelf.com>wrote:

On 2011-10-17 20:33:59 +0000, Andrew Wiley <wiley.andrew.j@gmail.com>
said:

Okay, I realize there have been some discussions about this, but I have a
few questions about shared delegates because right now they are definitely
broken, but I'm not sure how.
Take this code example:

synchronized class Thing {
void doSomeWork(void delegate() work) {
work();
}
void work() {}
}

void main() {
auto th = new Thing();
th.doSomeWork(&th.work);
}

This doesn't compile because the type of "&th.work" is "void delegate()
shared", which cannot be cast implicitly to "void delegate()".
My first question would be whether that type is correct. It's true that
the
data pointer of the delegate points to a shared object, but given that the
function locks it, does that really matter in this case? I guess I'm just
not clear on the exact meaning of "shared" in general, but it seems like
whether the data is shared or not is irrelevant when the delegate points
to
a public member of a synchronized class. If it was a delegate pointing to
a
private/protected member (which should be illegal in this case), that
would
not be true.
If that type is correct, the problem is that "void delegate() shared"
doesn't parse as a type (there is a workaround because you can create
variables of this type through alias and typeof).

What, exactly, is wrong here?

I think what's wrong is that a shared delegate should implicitly convert to
a non-shared one. The delegate is shared since it can be called safely from
any thread, and making it non-shared only prevent you from propagating it to
more thread so it's not harmful in any way.

Actually, I've been thinking about this some more, and I think that the
delegate should only implicitly convert if the argument types are safe to
share across threads as well.

I disagree.

If I had a class that looked like this:
synchronized class Thing2 {
      void doSomeWork(int i) {}
      void doSomeOtherWork(Thing2 t) {}
      void work() {}
}

The actual argument type in doSomeOtherWork is required to be shared(Thing2)
(which isn't a problem here because Thing2 is a synchronized class, but you
see the point).

Is it? Whether the argument was shared or not, the thread in which the function code runs can only access thread-local data from that thread, including the arguments and global variables. It won't be able to send  references to non-shared data to other threads just because its context pointer is shared (or synchronized).