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digitalmars.D.learn - Casting from interface to real class?

reply Bill Baxter <dnewsgroup billbaxter.com> writes:
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
Jan 20 2008
parent reply Bill Baxter <dnewsgroup billbaxter.com> writes:
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
Jan 20 2008
parent reply Oliver <oliver.ruebenkoenigREM web.de> writes:
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;
}
Jan 21 2008
parent reply Bill Baxter <dnewsgroup billbaxter.com> writes:
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
Jan 21 2008
parent Oliver <oliver.ruebenkoenigREM web.de> writes:
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 ;-)
Jan 21 2008