Head Const

[Walter Bright via Digitalmars-d]

On 2/17/2016 3:12 PM, Jonathan M Davis wrote:
> On Wednesday, 17 February 2016 at 22:44:27 UTC, Walter Bright wrote:
>
>> It would seem that implementing headconst as a type constructor would let
>> people who wanted mutable members have their way, without introducing
>> backdoors in const.
>
> I really don't see how that's the same thing at all. The main problems with
> const pop up when what folks need is something like C++'s mutable, because they
> need to be able to do something like a reference count or a mutex which is not
> really part of the logical state of the object but is still part of the object.
> Having some form of non-transitive const allows for things like a const pointer
> to non-const data, but it doesn't help at all when what you need to do is treat
> most of the object as const while treating a small portion of it as mutable.
> Sure, that's not completely transitive, because parts of the object are mutable,
> but it's specific parts of the object, not which part of a pointer declaration
> is const and which isn't. Maybe I'm missing something, but I don't see how
> adding non-transitive const to D in order to solve the C++ declaration problem
> would help at all with the complaints that folks have with D's const. The
> complaints with D's const are almost entirely about the lack of backdoors (be
> they well-defined like a mutable attribute or a free-for-all like casting away
> const and mutating).

If the headconst was one level up, the struct can have mutating members.