Error: xxx is not an lvalue

[Steven Schveighoffer]

On Mon, 04 May 2009 14:53:41 -0400, Unknown W. Brackets  
<unknown at simplemachines.org> wrote:

> I've always though of arrays and associative arrays as structs (which  
> really is what they are.)  Thinking that way, this behavior makes exact  
> sense - down to .length being unchangeable and associative arrays being  
> unchangeable.
>
> I mean, you wouldn't want to make it work the way you suggest by making  
> arrays and associative arrays simply pointers to their current structs  
> (just like a class would be.)  You could make such a class easily  
> yourself, anyway.
>
> If you are right, I guess the right solution would be to make the ref  
> implicit when returning (or, I suppose, passing?) such arrays and  
> associative arrays.  But that seems wrong to me too, and wouldn't come  
> free (speaking of efficiency) either.
>
> -[Unknown]
>

The underlying struct of an AA is:

struct AA
{
    AA_Implementation *impl;
}

or something like that ;)  Look at the D runtime source.

But in any case, if you return an array slice, would you not want to be  
able to update an element of the slice?

I think there is a difference between a value struct (where all internals  
are non-reference types), and a reference struct (where at least one  
element is a reference type).  A reference struct should be able to pass  
those references to other functions without having to be ref'd.  It forces  
non-transparent type wrapping if that isn't the case.  I should be able to  
replace a reference or pointer type with a wrapper struct where I can  
modify the interface, and it should be transparent that I have done so  
(except for the interface changes of course).  I think this is one of the  
goals D2 is working towards.

It makes sense that a value struct or the value types inside a reference  
struct (i.e. the length of an array) should not be changeable as rvalues,  
since you aren't going to use them later.

Both arrays (as the current implementation) and AA's are reference structs.

-Steve