Copy elision by spec

[Lars T. Kyllingstad]

On Monday, 4 November 2013 at 09:42:53 UTC, Jakob Ovrum wrote:
> On Monday, 4 November 2013 at 07:02:26 UTC, Lars T. Kyllingstad 
> wrote:
>> I was quite surprised to see that the following program 
>> compiles just fine with DMD:
>>
>>    struct S
>>    {
>>        @disable this(this);
>>        int n;
>>    }
>>
>>    S createS(int i)
>>    {
>>        S s;
>>        s.n = i;
>>        return s;
>>    }
>>
>>    void main(string[] args)
>>    {
>>        auto foo = createS(1);
>>        foo = createS(2);
>>    }
>>
>> I already knew that the compiler was allowed to elide copies 
>> on return from functions, but I thought this was an 
>> optimisation, and not part of the language proper.  I would 
>> have expected the compiler to complain that createS() can't 
>> return an S since S's postblit constructor is disabled.
>>
>> My question is therefore, is this by design?  Can I rely on 
>> this to work in the future, and on all compilers?  If this is 
>> the case, it really should be added to the spec.  (Or maybe 
>> it's there already, but I couldn't find it.)
>>
>> Lars
>
> My understanding is that your example illustrates a *move*, not 
> a *copy*. AFAICT, non-copyable structs would be next to useless 
> if we couldn't move them.

I know, and I agree.  The question is whether this is a move *by 
specification*, i.e. whether the language makes a guarantee that 
return values are always moved under certain circumstances.  If 
so, this should be mentioned in the spec, along with a detailed 
description of said circumstances.

I am using this "feature" in a program I'm working on right now.  
It would be a shame if this is a mere DMD artifact, as opposed to 
a language feature, because then I can't depend on it working in 
other compilers or in future DMD versions.  I really don't know 
any other way to solve my problem either, so I'm keeping my 
fingers crossed that this can become part of the official spec.  
For anyone interested, the actual use case is a no-arguments 
constructor for a non-copyable struct, emulated with static 
opCall():

   struct Foo
   {
       // "Constructor"
       static Foo opCall()
       {
           Foo f;
           // Initialize f.
           return f;
       }

       // Foo should not be copyable.
       @disable this(this);
   }

   // Construct a new Foo
   auto foo = Foo();