interface and opEquals

[Oliver]

Steven Schveighoffer Wrote:


Steve,

thank you for looking into this.

> You should use Object as the argument.  The reason is simple.  If you do not 
> use Object, then the base opEquals function is not overridden.  When you use 
> Vi as your argument, you start a new method that is overridden only by 
> derivatives of Vi. 

That puts it very clearly.

> Your example isn't a good way to demonstrate why this is 
> bad, but imagine if Vi had a base interface...

Unfortunately, I do not understand this. Could you explain a little more, or tell me were i could read up on this?

> Having said that, you do not need opEquals to be defined as a method in your 
> interface, because Object already defines it.  All classes derived from 
> Object will have a valid opEquals.  Perhaps this is why it was not working 
> for you.


> I think this code would work:
> 

The second assert fails. And that is the one i would like to work. I have the impression that it does not work because opEquals does not know what to do with Vi objects.

Then i tried to have opEquals( Object ) in the interface and wanted opEquals ( C1 test ) in the C1 class. But that did not work either. Which kind of makes sense. 

hm, i do not really understand this. Any ideas? 
Thanks,

Oliver

> import std.stdio;
> 
> interface Vi {
>     void f();
>     //int opEquals( Vi );  // ** commented out
>     //int opEquals( Object );
>     //int opEquals();
> }
> 
> class C1 : Vi {
>     void f() { writefln( "C1: ", itsInt ); }
>     this( int i ) { this.itsInt = i; }
>     int opEquals( Object o ) { // ** changed to Object
>         C1 test = cast(C1)o;
>         return test && this.itsInt == test.itsInt;
>     }
>     //int opEquals( C1 test ) { return this.itsInt == test.itsInt; }
> private:
>     int itsInt;
> }
> 
> int main() {
>     assert( 1 == 1.0 );
>     Vi myInt1 = new C1(1);
>     Vi myInt2 = new C1(1);
>     myInt1.f();
>     assert( cast(C1)myInt1 == cast(C1)myInt2 );
>     assert( myInt1 == myInt2 );
>     return 0;
> }
> 
> -Steve 
> 
>