Discussion: Rvalue refs and a Move construtor for D

[kinke]

On Friday, 6 September 2019 at 15:18:24 UTC, Suleyman wrote:
> On Thursday, 5 September 2019 at 22:57:27 UTC, kinke wrote:
>> [...]
>
> Seems workable. It only affects the case of moving an lvalue to 
> an value parameter. But here are my observations:
>
> 1. `move` should always do what `forward` did in your example. 
> It shouldn't touch the lvalue, for simply removing a call to 
> the move constructor, because T.init is not always considered a 
> valid state and resetting to T.init is a "move" operation but 
> an uncontrolled one which leaves the object in an invalid 
> state. Which why making __move behave just like rvalue ref is 
> more sensible like your first attempt did.

The problem here is the extended lifetime of the by-value 
parameter. Example:

T global;

void foo(T param) { /* make T own a huge buffer */ }

void caller()
{
     foo(move(global));
     // without destructing and resetting to T.init, `param` 
continues to live
     // and keeps the huge buffer alive, until `global` is 
reassigned or destructed
}

> 2. What you have proposed so far only replaces rvalue ref 
> parameters. You haven't tackled rvalue ref returns. What do you 
> propose as an alternative.

I never thought about that, I don't think I've ever returned an 
rvalue ref in C++. Give me some time to think about it.