Interfaces allow member definitions?

[Frustrated]

On Thursday, 30 January 2014 at 11:29:55 UTC, TheFlyingFiddle
wrote:
> On Thursday, 30 January 2014 at 11:19:58 UTC, Frustrated wrote:
>> I was, I think, able to call an interface's method. I had the
>> code like the following
>>
>>
>> interface A
>> {
>>     void foo();
>> }
>>
>> class B : A { void foo() { writeln("Hey"); } }
>> class C : A { void foo() { writeln("You"); } }
>>
>> yet, when I called a.foo(); I did not get any output. (A being 
>> of
>> type A)
>>
>>
>> Now, I was doing some weird stuff but either in the vtable for 
>> A,
>> there are empty functions that do nothing or I just happen to
>> call bogus memory that did not throw an exception.
>>
>> The real question is, do interface methods actually support
>> function definitions?
>>
>> Is there anything that stops us from actually doing
>>
>> interface A
>> {
>>     void foo() { writeln("What up!!"); }
>> }
>>
>> internally? I know member functions require a this but in this
>> case foo does not require this so it any this would work.
>>
>> Basically, does the vtable contain storage for the interface's
>> members but blocks us from using them due to the issue with 
>> this?
>>
>> If so, then shouldn't we be able to create functions in an
>> interface as long as they do not reference this? (basically
>> static functions that can be overriden as dynamic functions in
>> the class)
>>
>> e.g.,
>>
>> interface A
>> {
>>         // default behavior for foo and bar
>>         void foo() { writeln("asdasdfasdfasdf"); }
>> 	void bar() { writeln("1234"); }
>>
>> }
>>
>> class B : A
>> {
>> 	void foo() { writeln("help"); }
>> }
>>
>> void main()
>> {
>> 	A a = new B;
>>         a.foo(); // prints help
>>         a.bar(); // prints 1234
>>         B b = new B;
>>         b.foo(); // prints help
>>         b.bar(); // prints 1234
>> }
>>
>>
>> This would allow one to sort of add default behavior to an
>> interface(limited since no fields could be used but properties
>> help with it).
>>
>> basically the vtable just needs an extra spot for the interface
>> methods and calls them with null or the object it contains for
>> this... which doesn't matter since this is never used in the 
>> body
>> of the function.
>
> You can already do this using the Non-virtual interface ideom
>
>
> interface Foo
> {
>    final void bar() { writeln("Something"); }
>    void baz() { writeln("Something Else"); }
> }
>
> Note the final keyword it is imortant here. This will make bar 
> simply be a non-virutal method in Foo.
>
> If you want to provide some basic implementation but still 
> forward to a base class you can do something like this.
>
> interface Foo2
> {
>    final void bar(uint i)
>    {
>        // Does some basic stuff here.
>        bar_impl(i);
>    }
>
>    protected void bar_impl(uint i);
> }
>
>
> class A : Foo2
> {
>    protected void bar_impl(uint i)
>    {
>       //Do some specific stuff here.
>    }
> }
>
> This pattern allows you to do some basic stuff in bar and more 
> specialized stuff in bar_impl. It does require that you 
> overload bar_impl though which may not be what you want.



But this is not what I am talking about. Obviously one could use
a multitude of ways to accomplish the same task but nothing is
simple, direct, and elegant.

Do you realize you have to define two functions in the interface?

I'm not asking about a work around but if what I am talking about
can actually be done(does the vtable support this or can made to
support it?)