the const correctness of the this pointer

[Steven Schveighoffer]

On Sun, 27 Dec 2009 19:02:23 -0500, 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 ?

This might be a difficult thing to fix, but it definitely *definitely*  
needs to be fixed.  The problem is that a delegate stores a function  
pointer and a context pointer.  However, it does not type the context  
pointer.  For example, if you do this:

import std.stdio;

class A
{
     void f() const {}
}

void main()
{
     const A a = new A;
     a.f();
     auto g = &a.f;
     writefln("%s", typeof(g).stringof);
}

You will get this:

void delegate()

The this pointer is hidden, and so is it's const decoration.  I would  
expect to see:

void delegate() const

I'll file a bugzilla request for this.

-Steve