Casting from interface to real class?

[Oliver]

Bill Baxter Wrote:

> Oliver wrote:
> > Hi Bill,
> > 
> > Bill Baxter Wrote:
> > 
> >> Bill Baxter wrote:
> >>> Is it possible to do a dynamic cast from an interface to a real class?
> >>>
> >>> SomeClass k = cast(SomeClass)anInterface;
> >>>
> >>> will that succeed if anInterface started life as a SomeClass?
> > 
> > 
> > I hope I understood your question correctly, here is an example.
> > 
> > --------------------
> > import std.stdio;
> > 
> > interface Expr {
> >     Expr type();
> >     void print();
> > }
> > 
> > Expr steSymbol;
> > 
> > class Symbol : Expr { 
> > public:
> >     this(char [] name) { this.itsName = name; }
> >     char [] getName() { return this.itsName; }
> >     Expr type() {return steSymbol;}
> >     void print() {writefln(this.itsName);}
> > private:
> >     char [] itsName;    
> >     int itsInt;
> > }
> > 
> > int main ( char [][] arg ) { 
> >     Expr e;
> >     steSymbol = new Symbol("Symbol");
> >     e = new Symbol("x");
> >     steSymbol.print();
> >     e.print();
> >     e.type().print();
> >     Symbol s = cast(Symbol)e;
> >     s.print();
> >     writefln((cast(Symbol)e).getName());
> >     return 0;
> > }
> > 
> > 
> 
> Yep, that's exactly what I was hoping I could do but thinking I 
> couldn't.  But it does work.  Happy day.
> 
> --bb


there is one, caveat though. If you overload opEquals you need to cast to object to make that work, which is very ugly in the code.

    int opEquals( Object o ) { 
        Symbol s = cast(Symbol)o;
        return s && this.itsName == s.itsName;
    }   

and calling looks ugly, something like:
assert( cast(Object)e1 == cast(Object)e2 );

I think the problem comes from the fact that interface can also be a COM interface. 

http://www.digitalmars.com/d/archives/digitalmars/D/Object_and_interface_compatibility_48235.html

should you find a way to avoid the cast, i'd be very interested to see that,


otherwise happy coding ;-)