temporary objects are not allowed to be pass by ref anymore

[Georg Wrede]

Andrei Alexandrescu wrote:
> Jarrett Billingsley wrote:
>> On Sun, Apr 19, 2009 at 8:41 AM, Denis Koroskin <2korden at gmail.com> 
>> wrote:
>>> What's a rationale behind an issue described bug 2621?
>>> http://d.puremagic.com/issues/show_bug.cgi?id=2621
>>>
>>> Why isn't it allowed anymore?
>>>
>>> It broke quite a lot of my code. And while it is fixable by doing
>>>
>>> auto tmp = someFunctionThatRetunsStruct();
>>> someMethodThatAcceptsStructByReference(tmp);
>>>
>>> it looks ugly and unnecessary.
>>
>> I just thought of something.  Why the hell should we keep C++'s "const
>> ref" anyway?  When you use "const ref" it means you want it to be
>> read-only and fast to pass large structures.  But why should the onus
>> of passing value types byref be on the programmer?  Why not make it so
>> "const valuetype" will pass byval for smaller values and byref for
>> larger, completely freeing the programmer from this tedious crap?
>> It's not something that I care about, and the threshold of byval vs.
>> byref differs from platform to platform.
>>
>> Let's nip this in the bud right now.  A const value type parameter
>> should automatically decide whether to pass by reference or not.
> 
> I suggested that change. First let me clarify that the suggestion above 
> cannot work. Due to aliasing, leaving it to the compiler to choose 
> between by-value and by-reference leads to functions breaking when the 
> size of the object changes, which is unacceptable.
> 
> struct S { int x; ... }
> void foo(const S s1, ref S s2)
> {
>    s1.x = 5;
>    s2.x = 6;
> }
> 
> S s;
> foo(s, s); // messed up
> 
> Now onto why ref was disallowed to bind to an rvalue. This is because 
> some functions take things by ref intending to change them. Passing an 
> rvalue is in such cases a bug.
> 
> I agree there are functions that only want to use ref for speed 
> purposes. I suggested Walter to use ref? for those. ref? means it could 
> be an rvalue or an lvalue.

Aside from the real issue at hand, ref? nees some other notation. Using 
a question mark here makes it look appalling, and kludgy. One also feels 
it's an interrogative of some kind.