How to break const

[Christophe Travert]

Timon Gehr , dans le message (digitalmars.D:170296), a écrit :
> On 06/20/2012 01:36 PM, Christophe Travert wrote:
>> Timon Gehr , dans le message (digitalmars.D:170288), a écrit :
>>> On 06/20/2012 09:16 AM, deadalnix wrote:
>>>> Le 19/06/2012 17:49, Timon Gehr a écrit :
>>>>>
>>>>> 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.
>>>>>
>>>>
>>>> No, 2. require 1., even if the initialization is broken.
>>>>
>>>> class Foo {
>>>>       void delegate() dg;
>>>>
>>>>       this(immutable void delegate() dg) immutable {
>>>>           thid.dg = dg;
>>>>       }
>>>> }
>>>>
>>>> Now, as delegate doesn't carry the constness of its context, an
>>>> immutable instance of Foo can refers to something that isn't immutable.
>>>
>>> Clarification: 'const' means 'const'. No other qualifiers.
>>>
>>> There is no 'const' in that example code. 'immutable' obviously needs to
>>> be transitive regardless of the particular interpretation of 'const'.
>>>
>> const means: maybe immutable.
> 
> Or maybe mutable. Therefore, interpretation '2.'
> 
>> Thus const needs to be transitive too.
> 
> Wrong. This is the (A==>B) ==> (B==>A) fallacy, where
> 
> A: 'const' is transitive
> B: 'const' references cannot modify 'immutable' data
> 
> The conclusion regarding transitivity, given interpretation 2., is that
> 'const' needs to be transitive _if_ it is actually 'immutable'.

OK, I understand what you mean. I think this behavior is dangerous. You 
have to add rules to make sure B is not violated. I am not sure this is 
worth it.

>> If you apply different rules to const and to immutable, you are breaking
>> the consistency of the langage.
>>
> 
> Certainly not. This is like saying that applying different rules to
> 'const' and mutable is breaking the consistency of the language.
> mutable is not transitive.

OK, that was not the right argument. Const is transitive according to 
the spec. Making it not transitive would break the mutable < const < 
immutable design. You're trying to make an exception for data hidden 
behind a delegate, which is a dangerous thing.

-- 
Christophe