auto ref

[Steven Schveighoffer]

On Wed, 16 Dec 2009 19:05:09 -0500, Michel Fortin  
<michel.fortin at michelf.com> wrote:

> On 2009-12-16 16:46:14 -0500, Jason House <jason.james.house at gmail.com>  
> said:
>
>> Walter Bright Wrote:
>>
>>> Jason House wrote:
>>>> KennyTM~ Wrote:
>>>>
>>>>> auto const?
>>>>  I was wondering the same thing.
>>>  The const transport thing is, unfortunately, a very different problem.
>>  Of course, but it may still go through bikeshed issues. This morning I  
>> read about inout, return, vconst, aconst, sameconst, autoconst, auto  
>> const, and bikeshed. At least one of those was in jest :) auto const  
>> isn't that bad, and you obviously liked auto ref...
>
> Since this is just a special kind of const, it could be called "const^"  
> (const or a derived constness):
>
> 	const?(Object) func(const?(Object) o) {
> 		return o;
> 	}
>
> The interesting thing about it, beside not taking a keyword, is that it  
> can scale in the future if we need to add many distinct constness to the  
> same function signature:
>
> 	const?(Object) func(const?(Object) o, const?2(Object) o2, out  
> const?2(Object) o3) {
> 		o3 = o2;
> 		return o;
> 	}

This can never work.  a const?(Object) is const during the function  
execution, and cannot be assigned to.

> Not that you'd need that often, but if it does becomes necessary in the  
> future we'll still have some options.
>
> Furthermore, the concept could be extended to any type. This could be  
> useful with class hierarchies:
>
> 	Object? func(Object? o) {
> 		writeln(o.toString());
> 		return o;
> 	}
>
> 	MyObject o = func(new MyObject);
>
> Here, "Object?" means Object or a derived type.

This doesn't have the same utility as vconst, since you can't apply Object  
to other types like you can constancy.

Plus you can already do this with a template and have a virtual version of  
the func:

T func(T)(T o) if(T : Object) {func_virt(o); return o; }
protected void func_virt(Object o) {writeln(o.toString());}

-Steve