The liabilities of binding rvalues to ref

[Steven Schveighoffer]

On Thu, 09 May 2013 13:45:16 -0400, Peter Alexander  
<peter.alexander.au at gmail.com> wrote:

> On Thursday, 9 May 2013 at 17:27:41 UTC, Steven Schveighoffer wrote:
>> Short answer: no matter what, we have to find a way to be able to write  
>> one function that takes by ref, and accepts both rvalues and lvalues.   
>> If ref const is that way, so be it.  If auto ref is that way, so be  
>> it.  If it's plain-old ref, I'm fine with that too.
>
> Allowing plain-old ref to bind to rvalues would be a massive mistake in  
> my opinion. See case (2) in the original post.

The case for it is operator overloads.  Note that all operator overloads  
are defined WITHIN types, and all operator overloads take 'this' by  
reference.  Without the hack currently in the compiler that 'this' is  
allowed to bind by ref to rvalues, D would be near useless.

Consider that today, this doesn't work for custom structs, unless you  
allocate the result on the heap.

((a + b) + (c + d))

The original case 2 looks bad, but not very common.  With by-ref turning  
into by-value, you are in effect changing the API and functionality.  It  
would just mean that you wouldn't make that change, or change the type  
name at the same time (so it wouldn't just compile on existing code).  I  
think someone later mentioned that if 'fix' was a method, it would break  
even today (with ref-binds-to-this hack).

You should also consider that even if we break existing code by changing  
by-ref into by-value, there's no guarantee the person maintaining the code  
does the right thing.

> It seems that 'auto ref' would be suitable, provided we can find a way  
> for it to work with normal functions (in a sensible way, not like  
> templates).

I think first we have to prove that auto ref is worth the extra cognitive  
load.  Every overload style we add has a large multiplicative effect on  
boilerplate.

As I said before, I'd be ok with any solution, as long as we can bind  
rvalues to some form of ref.

-Steve