any news on const/invariant?

[Robert Fraser]

Walter Bright wrote:
> Derek Parnell wrote:
>> On Mon, 26 Nov 2007 14:11:25 -0800, Walter Bright wrote:
>>
>>> Jason House wrote:
>>>> Walter Bright Wrote:
>>>>
>>>>> Denton Cockburn wrote:
>>>>>> Any update on the status of the const/invariant changes?
>>>>> Yes, it's about done.
>>>> How about documentation of how const will work.  Is this essentially
>>>> what was originally posted in the "const is broken" thread?  Or are
>>>> there new surprises in store for us?
>>> I don't remember the details of that, but the nutshell version is:
>>>
>>> 1) no more final for variables
>>
>> Is the keyword in this context now a syntax error?
> 
> It's just ignored.
> 
> 
>>> 2) no more 'head const' or 'tail const', it's all just 'const'
>>
>> Does that mean that if X is const then neither the bits in X and the bits
>> referenced by X can be modified (though the X symbol)?
> 
> Const and invariant are transitive. C++ const is not transitive, leading 
> to programmers establishing a *convention* for transitive const. But it 
> being an unenforced convention means it is not reliable.
> 
>>> 3) ditto (2) for invariant
>>
>> Does that mean that if X is invariant then neither the bits in X and the
>> bits referenced by X can be modified by anything?
> 
> Right.
> 
>>> It should be working as one would intuitively expect it to.
>>
>> If the answer is 'yes' to the couple of questions above ...
>> (a) is there a form of the syntax that allows 'X' to be modified but not
>> what it references?
> 
> Yes, transitive const applies downwards, not upwards.
> 
>> (b) is there a form that prevents 'X' from being modified but allows what
>> it references to be modified?
> 
> No. There is no language typing support for 'logical const' (i.e. const 
> structures with mutable members). Many C++ people have argued 
> passionately for logical const, but I have become strongly convinced 
> that the notion is fatally flawed.

I like it! Simple, yet it does what it needs to do.