logical const without casts!

[Steven Schveighoffer]

On Fri, 30 Sep 2011 13:38:31 -0400, Michel Fortin  
<michel.fortin at michelf.com> wrote:

> On 2011-09-29 14:54:24 +0000, "Steven Schveighoffer"  
> <schveiguy at yahoo.com> said:
>
>> I just thought of an interesting way to make a logical const object   
>> without casts.  It requires a little extra storage, but works without   
>> changes to the current compiler (and requires no casts).
>>  Here's the code, then I'll talk about the implications:
>>  import std.stdio;
>>  class D
>> {
>>      D getme() { return this;}
>>      void foo() {writeln("mutable!");}
>> }
>>  class C
>> {
>>      D delegate() d;
>>      this()
>>      {
>>        auto dinst = new D;
>>        this.d = &dinst.getme;
>>      }
>>      void bar() const { d().foo();}
>> }
>>  void main()
>> {
>>      auto c = new C;
>>      c.bar();
>> }
>>  outputs:
>>  mutable!
>>  So how does it work?  It works because delegates and especially the   
>> delegate data is *not* affected by const.  So even when C is  
>> temporarily  cast to const, the delegate is not affected (i.e. it's  
>> context pointer is  not temporarily cast to const).
>>  Doesn't this poke holes in const?  Of course it does, but no more  
>> holes  than are present via another logical const scheme (i.e. using a  
>> globally  stored AA to retrieve the data).
>
> This is a hole in the transitive const, because the delegate contains a  
> pointer to mutable data. It also is a potential source of of low level  
> races since returning that type from a pure function could make it  
> immutable, which can then make this mutable data accessible to multiple  
> threads with no synchronization or atomics to protect the data's  
> integrity.

Simen Kjaeraas and Christophe brought up the same points.  I filed a bug  
on it:

http://d.puremagic.com/issues/show_bug.cgi?id=6741

-Steve