temporary objects are not allowed to be pass by ref anymore

[Yigal Chripun]

On 19/04/2009 22:52, 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.
>
>
> Andrei

that does not compile.
you tried to change a *const* struct...