DIP44: scope(class) and scope(struct)

[deadalnix]

On Tuesday, 27 August 2013 at 16:57:36 UTC, H. S. Teoh wrote:
> On Tue, Aug 27, 2013 at 05:46:40PM +0200, deadalnix wrote:
>> On Tuesday, 27 August 2013 at 14:26:53 UTC, H. S. Teoh wrote:
>> >What if move(r2) throws? Then res1 won't get cleaned up, 
>> >because r1
>> >has already been nulled by the move.
>> >
>> 
>> I don't think move can throw.
>
> Well, it's not marked nothrow, so I wouldn't count on that.
>
> Also, the fact that move() uses memcpy is a bit worrying; Adam 
> Ruppe &
> myself ran into a nasty bug involving closures over struct 
> members when
> the struct may get moved upon return from a function. For 
> example:
>
> 	struct S {
> 		int id;
> 		void delegate()[] cleanups;
>
> 		this() {
> 			id = acquireResource();
> 			cleanups ~= {
> 				// N.B.: closure over this.id
> 				releaseResource(id);
> 			};
> 		}
> 	}
>
> 	S makeS() {
> 		// Problem: S.cleanups[0] is a closure over the struct
> 		// instance on this function's stack, but once S is
> 		// returned, it gets memcpy'd into the caller's stack
> 		// frame. This invalidates the delegate's context
> 		// pointer.
> 		return S(1);
> 	}
>
> 	void main() {
> 		auto s = makeS();
> 		// Problem: s.cleanups[0] now has an invalid context
> 		// pointer. If the stack is reused after this point, the
> 		// dtor of s will get a garbage value for s.id.
> 	}
>
> Using move() to move a resource from a local variable into a 
> member
> looks like it might be vulnerable to this bug as well -- if the 
> resource
> has closures over member variables it might trigger this 
> problem.
>
>
> T

Funny, I ran into this twice this week XD

struct are movable by definition, so the compiler should reject 
this unless the delegate is scope.