On Mon, 08 Feb 2010 00:37:53 -0500, Andrei Alexandrescu
<[email protected]> 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.
You forgot to return result :)
Also, it should be implemented like this to prevent a problem where a
stack with a single element is cleared before an exception is thrown.
T belowTop()
{
auto t = top;
pop();
scope(exit) push(t);
return top;
}
As another example (not sure if it's "killer"), here is a more useful
default method:
T popTop()
{
scope(success) pop();
return top;
}
I can see real use for "give me the top, and pop it off at the same
time." Now, if we make popTop final, it's 2 virtual function calls
(popTop can be inlined), but if we make it overridable, the default
implementation has 3 virtual calls, but can be optimized into 1 virtual
call if so desired. I guess it's a tradeoff between whether you think
most implementors will prefer to use some default implementation or most
will want to optimize. But in the case you expect most to optimize,
wouldn't you just provide no implementation? In the case of popTop, it's
attractive to say "your stack class only needs to define these few
primitives," but it's generally trivial to provide a popTop implementation
that is more optimized.
I've seen places where interface documentation says "implement this
function like this: ..." which is quite annoying. final functions solve
most of these, I'm not sure if we need another way to do it, but I can't
see any reason why it should be disallowed. The judgement call of whether
to provide an overridable implementation and no implementation would be
difficult. I don't see a default-but-overridable implementation being
used often.
-Steve
