How to break const

[Timon Gehr]

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'.