the const correctness of the this pointer

[Denis Koroskin]

On Mon, 28 Dec 2009 03:02:23 +0300, TM <ithink at therefore.iam> wrote:

> Hello,
> I was dealing with delegates when I found that the const correctness of  
> the this pointer was 'lost' when accessing a member function of a class  
> through a temporary delegate.
>
> Here is an example:
>
> class A{
>     void f () {}
> }
>
> const A a = new A;
> a.f() //error, this is normal.
> auto g = &a.f;
> g(); //no error. Is this normal ?
>
> I may be wrong, but it seems to me that as the const property of the  
> this pointer is linked to the instance of the class (here 'a'), the  
> delegate 'g' should contain a const pointer to A (because 'a' is const).
>
> More generally, I seems that there is no management of the const  
> correctness of the frame pointer of a delegate in D2
>
> What about adding the const keyword for delegates, as we do for methods  
> ? The const property of the delegate's frame pointer would be part of  
> its type.
>
> Example:
>
> void f()
> {
>     int x;
>     void g() const {
>         x ++; //would produce an error since the type of g is [void  
> delegate() const]
>    }
> }
>
> Any thoughts about this ?

It shouldn't compile: it makes no sense to can't non-const method through  
const reference, and the same applies to shared. OTOH, the following  
should just work:

class A {
     void constMethod() const {}
     void sharedMethod() shared {}
}

const A a1 = new A();
void delegate() const dg1 = &a1.constMethod;

shared A a2 = new shared A();
void delegate() shared dg2 = &a2.sharedMethod();

(in examples above, const and shared attributes are applied to both  
function pointer and frame pointer)

I already wrote about a necessity of introduction of "shared delegates" in  
a separate thread. Until then, Thread creation is broken in D.