Const Ideas

[Christopher Wright]

Craig Black wrote:
> "Janice Caron" <caron800 at googlemail.com> wrote in message 
> news:mailman.229.1196702116.2338.digitalmars-d at puremagic.com...
>> On 12/3/07, Craig Black <cblack at ara.com> wrote:
>>> Just to expound further, the pointer and reference syntax seems 
>>> difference
>>> since the reference is implicit.  Just think of  "const X x;" as "const 
>>> X&
>>> x;" and you will see that the same principle that applies to pointers 
>>> should
>>> also apply to references.
>> Yes, I think we're on the same page here. Of course, only classes have
>> references. Structs don't, ints don't, etc., but for classes, my
>> preference would be for all four of the following to be exactly
>> equivalent:
>>
>>    const C c;
>>    const(C) c;
>>    const C& c;
>>    const(C&) c;
>>
>> (Which leaves open the possibility of "const(C)& c;" to mean mutable
>> ref to const data").
>>
>> That's not the status quo, however
> 
> That syntax would looks OK, but I think it also makes sense to make 
> "const(C) c" equivalent to "const(C)& c;".  Walter said that this is how the 
> syntax already works.  Am I misunderstanding something?

We don't like the syntax. It makes a tiny bit of sense, with that 
description, but it's unintuitive and error-prone.

> What I don't understand is why Walter and others think that "C const c" 
> violates transitivitiy.  What do they mean by transitivity?

"C const c" -> c is a pointer that cannot be reassigned, but you can 
modify its members.

This violates transitivity, the property that you cannot follow a series 
of const pointers/references and end up with something mutable. It is 
also useful, but it isn't useful as a public interface sort-of-thing; 
it's useful at a relatively small scope. Which is why Walter hasn't 
implemented it; he doesn't see enough of a benefit yet.