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?

--bb

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 think I found the answer: Yes.

http://spottedtiger.tripod.com/D_Language/D_FAQ_XP.html

"""
Casting from an interface back to the class means subtracting that offset. The subtract is a little tricky since the pointer doesn't know where it came from, so the first entry in the interface's vtbl[] is an instance of the class Interface with the necessary offset. The cast is handled by the library function _d_interface_cast().
"""

--bb

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;
}

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

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 ;-)