How to break const

[Christophe Travert]

Artur Skawina , dans le message (digitalmars.D:170305), a écrit :
> On 06/20/12 09:31, Christophe Travert wrote:
>> Artur Skawina , dans le message (digitalmars.D:170191), a écrit :
>>>> The proper way to do this is not cast, it is to give the proper 
>>>> constness for all types and methods :
>>>>
>>>>    struct S {
>>>>       int i; this(int i) { this.i = i; }
>>>>       T* p; 
>>>>       void f(int i) const { this.i = i; /*p.i++;*/ }
>>>> // the commented part is illegal because S.f is const
>> 
>> I missed that. Then I disagree with your use of delegates. The solution 
>> here is to make the free f function to take a non const T*, since the 
>> function tries to modify variables from that T instance (via a 
>> delegate).
> 
> The issue is the delegate definition - what exactly is a delegate?
> My first message in this thread started with this sentence:
> "It's fine, if you view a delegate as opaque". Because if you do, then
> accesses via a delegates context pointer are no different from using
> an external reference. That message also contained a program to show
> how trivial bypassing const is, at least for impure functions. So
> banning "unsafe" delegates wouldn't really change the situation.

Having opaque delegate and no holes in the langage is a possibility, but 
not without changing the langage spec.

With opaque delegates, you can't have a pure delegate, only pure 
function pointers, because pure means you do not access external data, 
and accessing data pointed by a the opaque frame pointer of the delegate 
would be accessing external data.

You may then redefine pure for delegate as delegates having only 
immutable external data (const is not enough).

I'm definitely not in favor of that solution.