DIP 1021--Argument Ownership and Function Calls--Community Review Round 1

[Nick Treleaven]

On Thursday, 18 July 2019 at 11:16:02 UTC, ag0aep6g wrote:
> I'm not sure if there's a meaningful difference between your 
> code and mine. You're calling the destructor explicitly with 
> `destroy`, So it's practically the same as my `clear` method, 
> no?

I think the difference is that destroy takes an S by ref, so the 
compiler could raise an error about p potentially outliving s:

void maybeBad(ref S s); // might call s.destroy
scope p = s.get;
s.maybeBad;
// p may now be invalid

But I don't think this is proposed by this DIP or supported by 
DIP 1000.

> Anyway, I think we're on the same page: The goal of DIP 1021 
> seems to be to allow marking calls to `free` (and similar 
> functions) as @trusted in certain situations. The DIP should 
> say this, and give an example.

I realized my example shouldn't mark the destructor as @trusted, 
because of the DIP limitation section:
https://github.com/dlang/DIPs/blob/793f83911fdc8c88c6ef34e6a36b5e11e3e574e5/DIPs/DIP1021.md#limitations

> That's a common hack, but strictly speaking it's an invalid use 
> of @trusted. An @trusted function must be safe regardless of 
> where it's called from. I.e., it must also be safe when called 
> from within the same module.

So we need C++ private then.

> But finding a solution to that problem is probably outside of 
> the scope of the DIP being discussed.

Yes.