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

[deadalnix]

On Wednesday, 17 March 2021 at 17:14:04 UTC, Q. Schroll wrote:
> It might be a stupid question, but why have move assignment in 
> the first place? In C++, there's the copy-and-swap idiom[1]. 
> Maybe it's obvious why it does not apply in D, but if using a 
> swap function makes implementing a copy assignment and move 
> assignment trivial, why not requiring opSwap instead of 
> opAssign for an elaborate move object?
>

This isn't a stupid question, this is THE question. It is easy to 
assume things are necessary because other went there and did it, 
but I find that questioning these assumptions is how the greatest 
design ideas came up.

D's pure attribute is the perfect example of this.

> [1] 
> https://stackoverflow.com/questions/3279543/what-is-the-copy-and-swap-idiom

Doing this has major issue: it require all movable structs to 
have a null state (as in C++) or make other unsavory tradeofs 
(see 
https://forum.dlang.org/post/bkfqchwpnonngjrtybbe@forum.dlang.org 
for a more thorough explanation).

Nevertheless, if the struct naturally has a null state, this is 
indeed a very good way to do it.