Shared Delegates

Michel Fortin michel.fortin at
Mon Oct 17 13:43:52 PDT 2011

On 2011-10-17 20:33:59 +0000, Andrew Wiley <wiley.andrew.j at> 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(&;
> }
> This doesn't compile because the type of "&" 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.

Michel Fortin
michel.fortin at

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