constness for arrays

[David Medlock]

Don Clugston wrote:
> xs0 wrote:
> 
>> As has been discussed, the lack of something like C++ const hurts most 
>> when using arrays, as you can't code around it like with classes, 
>> structs or primitives (with the latter two you can just pass by value, 
>> for classes you can make readonly versions). The fact that inbuilt 
>> strings are also arrays makes the problem occur often.
>>
>> I was wondering whether the following would resolve that issue:
>>
>> - the top bit of arrays' .length becomes an indicator of the 
>> readonlyness of the array reference
> 
> 
> This is a really interesting idea. You're essentially chasing a 
> performance benefit, rather than program correctness. Some benchmarks 
> ought to be able to tell you if the performance benefit is real:
> 
> instead of char[], use
> 
> struct CharArray {
>  char [] arr;
>  bool readOnly;
> }
> 
> for both the existing and proposed behaviour (for the existing one, 
> readonly is ignored, but include it to make the parameter passing fair).
> 
> For code that makes heavy use of COW, I suspect that the benefit could 
> be considerable. You probably don't need to eliminate many .dups to pay 
> for the slightly slower .length.
> 
> The situation where a function only occasionally returns a read-only 
> string is probably quite common:
> 
> char [] func(int n) {
>   if (n==0) return "annoying";
>   else return toString(n);
> }
> .. and you have to .dup it for the rare case where it's a literal.
> 

Agreed , Don.

Its important to note this is two issues:

1.  A readonly property of arrays.
2.  Implementation of #1.

If Walter agrees on #1, I am sure he is best person to ask for advice on 
#2 (at least in the case of DMD).

-DavidM