DIP 1016--ref T accepts r-values--Formal Assessment

[Andrei Alexandrescu]

On 1/24/19 3:01 PM, kinke wrote:
> On Thursday, 24 January 2019 at 09:49:14 UTC, Manu wrote:
>> We discussed and concluded that one mechanism to mitigate this issue
>> was already readily available, and it's just that 'out' gains a much
>> greater sense of identity (which is actually a positive side-effect if
>> you ask me!).
>> You have a stronger motivation to use 'out' appropriately, because it
>> can issue compile errors if you accidentally supply an rvalue.
> 
> `out` with current semantics cannot be used as drop-in replacement for 
> shared in-/output ref params, as `out` params are default-initialized on 
> entry. Ignoring backwards compatibility for a second, I think getting 
> rid of that would actually be beneficial (most args are probably already 
> default-initialized by the callee in the line above the call...) - and 
> I'd prefer an explicitly required `out` at the call site (C# style), to 
> make the side effect clearly visible.
> 
> I'd have otherwise proposed a `@noRVal` param UDA, but redefining `out` 
> is too tempting indeed. ;)

It seems to me that a proposal adding the "@rvalue" attribute in 
function signatures to each parameter that would accept either an rvalue 
or an lvalue would be easy to argue.

- No exposing existing APIs to wrong uses
- The function's writer makes the decision ("I'm fine with this function 
taking an rvalue")
- Appears in the function's documentation
- Syntax is light and localized where it belongs
- Scales well with number of parameters
- Transparent to callers

Whether existing keyword combinations ("in", "out", "ref" etc) could be 
used is a secondary point.

The advantage is there's a simple and clear path forward for API 
definition and use.


Andrei