@nogc exceptions?

[Nicholas Wilson]

On Sunday, 19 May 2019 at 04:24:39 UTC, Manu wrote:
> What's the story with @nogc exceptions?
> There was DIP1008 or whatever, I didn't follow it because I 
> never use
> exceptions... but I kinda need it.

It used to not work, but it does now.

> IIRC, the idea was the throw statement

Specifically it works with `throw new [My]Exception(args);` and 
only that form (since otherwise you could leak it).

> transfer ownership of the exception to the runtime, and then 
> use a `scope` catch block, such that the runtime can be sure it 
> owns the only reference

Well 	
     try throw new Exception("foo");
     catch(scope Exception ex) {}

fails with

Error: basic type expected, not scope
Error: found scope when expecting )
Error: semicolon expected, not )
Error: found ) instead of statement
Error: unrecognized declaration

but I think that was done for compatibility with other code that 
could store the exception.
But regardless

     try throw new Exception("foo");
     catch(Exception ex) {}

will allocate a reference counted exception with malloc(?) (you 
can probably edit druntime to make it use some other scheme) 
which will deallocate at the end of the scope.

> at the end of the catch block and it can free the exception 
> rather than transfer it to the GC?

> How did it fail?

It used to not actually do the thing it was supposed to do (as in 
it would still call the GC). It was fixed.