Discussion Thread: DIP 1040--Copying, Moving, and Forwarding--Community Review Round 1

[deadalnix]

On Tuesday, 16 March 2021 at 09:14:58 UTC, Walter Bright wrote:
> Postblit's problems arose from it not having access to both 
> objects. The opAssign does have access to both, and the 
> qualifiers can be applied to both parameters, so I don't see a 
> barrier to it working.
>

YES!

This is why it is unsuitable for copies. During a copy, there are 
2 objects.

Any solution, that provide to objects when moving, a situation 
where only one object exists, will just open a can of worm of the 
same nature as postblit for copies opened.

This is self evident. This is so obvious that I don't know how to 
unpack it any further.

Pretend you have one object when you have two => problems.
Pretend you have two objects when you have one => problems.

> An opAssign gives the implementer complete control over the 
> operation of it, including when and how destruction takes place 
> of the original destination's contents.

That break all the invariant provided by the ctor/dtor mechanism 
and because struct are composable (you can use structs as member 
of structs) then the mess is not bounded to the one struct you 
are toying with.