How to break const

[Timon Gehr]

On 06/18/2012 11:04 PM, deadalnix wrote:
> Le 18/06/2012 17:29, Timon Gehr a écrit :
>> On 06/18/2012 05:14 PM, Christophe Travert wrote:
>>> Matthias Walter , dans le message (digitalmars.D:170036), a écrit :
>>>> On 06/18/2012 07:36 AM, Mehrdad wrote:
>>>>> Is it just me, or did I subvert the type system here?
>>>>>
>>>>>
>>>>> import std.stdio;
>>>>>
>>>>> struct Const
>>>>> {
>>>>>     this(void delegate() increment)
>>>>>     { this.increment = increment; }
>>>>>     int a;
>>>>>     void delegate() increment;
>>>>>     void oops() const { this.increment(); }
>>>>> }
>>>>>
>>>>> void main()
>>>>> {
>>>>>     Const c;
>>>>>     c = Const({ c.a++; });
>>>>>     writeln(c.a);
>>>>>     c.oops();
>>>>>     writeln(c.a);
>>>>> }
>>>>>
>>>>
>>>> I don't think so. When calling oops you have two references to the
>>>> object c:
>>>>
>>>> - The this-pointer of the object itself which is not allowed to change
>>>> the object in the const-call.
>>>> - The reference from within main which is allowed to change it and can
>>>> be reached via the frame pointer of the delegate.
>>>>
>>>> I see this as perfectly valid code. Of course, opinions may differ
>>>> here.
>>>
>>> But here, the frame pointer of the delegate is part of the const
>>> structure. By transitivity, the frame pointer should be const, ...
>>
>> 'By transitivity' is not a sufficient reason. What you really mean is
>> 'For the guarantee that a const pure method does not change its mutable
>> parameters'.
>
> Transitivity by itself is required to solve a wide range of problem. The
> most obvious one is controlling what is shared and what isn't using the
> type system.

That is completely unrelated.
It is impossible to justify transitivity of const for delegate context
pointers using this argument. It is far too general and the
justification for the general concept comes from a specific example
that is different from the one at hand.

The question is, what the meaning of 'const' references should be:

1. data cannot be changed transitively through the reference

2. the reference can reference both 'const' and 'immutable' data and
    'immutable' data can transitively not be changed through the
    reference.


1. requires transitive const for delegate context pointers, 2. does not.