Why do struct literals count as lvalues?

[kenji hara]

2011/8/19 Timon Gehr <timon.gehr at gmx.ch>:
> On 08/19/2011 02:16 PM, Steven Schveighoffer wrote:
>>
>> On Thu, 18 Aug 2011 18:56:01 -0400, bearophile
>> <bearophileHUGS at lycos.com> wrote:
>>
>>> Jonathan M Davis:
>>>
>>>> I see _zero_ reason to
>>>> treat a newly constructed object as any different from one which is
>>>> returned
>>>> from a function.
>>>
>>> I have not followed fully this thread, so I am not sure what you are
>>> talking about, so sorry if I am misunderstanding the whole topic.
>>>
>>> I assume you want to disallow code like:
>>>
>>> struct Vect {
>>> float[4] data = 0.0;
>>> // lot of operator overloading here, all arguments are by ref
>>> }
>>> void foo(ref Vect f) {}
>>> void main() {
>>> foo(Vect([1,2,3,4]));
>>> }
>>>
>>> Such code is common. In foo() and in all the Vect operators ref is
>>> required for performance. This code is quite handy. Forcing me to
>>> define and assign a Vect to a temporary variable in main is not handy.
>>>
>>> Think about writing expressions that use Vects with operator
>>> overloading, that use ref everywhere. If you can't pass and give Vects
>>> defined inside the expressions the code becomes quite more hairy.
>>
>> I believe auto ref is slated to fix this problem.
>>
>> -Steve
>
> auto ref currently treats struct literals as lvalues too. (therefore, as
> ref). And passing by value would be considerably less efficient for large
> structs.
>
> As I understand it, the only issue with allowing literals and function
> results as lvalues is that generic code cannot detect if it is working with
> a temporary or not. I don't know if this would be useful in D though.
>
> each code segment of the form:
>
> void foo(ref S);
> ...
> foo(S(...));
>
> is equivalent to one explicitly declaring the temporary:
>
> {auto __temp=S(...); foo(__temp);}
>
> The difference is that the first is more pleasant to write. If temporaries
> would become rvalues everyone would always have to write the second form
> manually. So imho it is just a syntax sugar issue.
>
> I'd actually argue that ref-passing should work for arbitrary function
> results too.

As far as I know, Andrei's original 'auto ref' means always receive
argument *by reference* either it is lvalue or rvalue, and it works
with non template function.

But, unfortunately, it doesn't exist in current D.

Kenji Hara