temporary objects are not allowed to be pass by ref anymore

["Jérôme M. Berger"]

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Yigal Chripun wrote:
> 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...
> 
	Change it to:
int foo (const S s1, ref S s2)
{
   s2.x = 6;
   return s1.x;
}

S s;
s.x = 3;
x = foo (s, s); // messed up

		Jerome
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