Steven Schveighoffer wrote:
On Mon, 08 Feb 2010 14:36:37 -0500, Andrei Alexandrescu
<[email protected]> wrote:
Don wrote:
Andrei Alexandrescu wrote:
Walter has now implemented final methods in interfaces and also
contracts in interfaces, both of which I think are just awesome.
We figured that essentially he artificially disallows interfaces
from providing bodies for methods. I think that's a gratuitous
limitation; the only distinguishing quality of an interface is that
it has no state. Other than that, interfaces can always offer
overridable functions that by default offer functionality in terms
of existing interface functions. For example:
interface Stack(T)
{
void push(T);
void pop();
@property ref T top();
@property bool empty();
T belowTop()
{
auto t = top;
pop();
auto result = top;
push(t);
}
}
The default implementation of belowTop does a fair amount of work. A
particular implementation might just use that or override it with a
more efficient implementation.
Many more examples can be imagined, but I'm looking for a killer
one, or perhaps a killer counterexample (e.g. when would an
interface-defined method be really bad?)
Your thoughts welcome.
Andrei
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.
Andrei
