How to break const

[deadalnix]

Le 18/06/2012 16:28, Artur Skawina a écrit :
> It's fine, if you view a delegate as opaque.
>

No it isn't. You cannot ensure transitivity anywhere. This have obvious, 
and severe drawback for concurrent programing (implicit sharing is back) 
and GC performances (GC can eb crazy fast when it come to transitive 
immutable data, see OCaml's GC performances for instance).

> 'this' being const does not preclude accessing the object that 'this'
> points to via another, mutable, reference.
>
> Consider the alternative - you'd have to forbid storing any delegate
> with a non-const non-value argument inside any object.
>
> And "breaking" const would then _still_ be possible and trivial.
>

No, and your example don't demonstrate that in any way. Transitivity is 
maintained in the example below, because g isn't a member of s, and if 
it were, then the example would break at compile time.

>     import std.stdio;
>
>     S*[const(S)*] g;
>
>     struct S {
>        int i;
>        this(int i) { this.i = i; g[&this] =&this; }
>        void f() const { g[&this].i=666; }
>     }
>
>     void main() {
>        const s = S(42);
>        writeln(s);
>        s.f();
>        writeln(s);
>     }
>
> Yes, D's "pure" could help here, only it's misnamed (even if in
> this particular case that term would be fitting).
>

purity is another beast altogether.