On Mon, 08 Feb 2010 16:09:19 -0500, Andrei Alexandrescu
<[email protected]> wrote:
Steven Schveighoffer wrote:
On Mon, 08 Feb 2010 15:13:33 -0500, Andrei Alexandrescu
<[email protected]> wrote:
Steven Schveighoffer wrote:
On Mon, 08 Feb 2010 14:36:37 -0500, Andrei Alexandrescu
<[email protected]> wrote:
Don wrote:
I don't understand this. How does belowTop() know how to call
top()?
It calls top() through the normal interface mechanism. Inside
belowTop(), this has Stack!T type.
Actually, I think Don has a point here. A virtual function (even on
an interface) is always called with the 'this' pointer, not the
interface pointer.
That is done via an adjustment of the reference. In the case of an
interface method, no adjustment is necessary. Inside the method,
"this" has the static type of the interface and the dynamic type
whichever class implements the interface.
void foo(Stack!T st)
{
auto x = st.belowTop();
}
OK, so if st's virtual function for belowTop points to the default
implementation, there is no adjustment. But what if the actual object
*did* override the default implementation? Does it also receive the
interface pointer as 'this'? Where does the adjustment happen? What
happens if you have a reference to the actual concrete object type? Do
you have to thunk to the correct interface to pass in the expected
interface pointer? It can't be both ways.
If an object overrode the default implementation, the pointer to method
belowTop will point to code that does do the adjustment.
If I understand this correctly, calling such a "default implementation"
function is different than calling a standard interface function. And
each entry in such an interface for a overridden method will point to a
"pre function" that adjusts the 'this' reference before jumping to the
real implementation.
The vtable entries of the object itself would point to a function that
does not do the adjustment, correct?
I think all the information is available to make this work, the only issue
I see is that a function with a default implementation artificially
changes the ABI for that function. Adding a default implementation
therefore will make compiled objects incompatible, even with the same
vtable layout.
As of today, I don't see this being a problem, since you generally only
build static D programs. But I can't see a huge flaw in the idea.
-Steve
