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D - D and covariant assignment

reply Friedrich Dominicus <frido q-software-solutions.com> writes:
As I understand this will work
class A
{
	public int foo () {return 0;}
}

class B: A
	public int foo(){ return 1;}
}


A a = new B();
a.foo -> yields 1

But that won't work
class A {
     public int foo () { return 0; }
}

class B: A{
     public int foo () { return 1; }
     public int bar() { return 0;}
}

int main (){
     A a = new B();
     printf("a.foo() = %d\n", a.foo());
     printf("a.bar() = %d\n", a.bar());
     return 0;
}

So D allows sort of covariant assignment, but for accessing a functoin 
defined in a descendant the function to be accessed must be part
of the ancestor too. In the second example bar must be defined in A too.

Is that correct?

Regards
Friedrich
Aug 12 2003
next sibling parent "Mike Wynn" <mike.wynn l8night.co.uk> writes:
"Friedrich Dominicus" <frido q-software-solutions.com> wrote in message
news:bhakdt$o1s$1 digitaldaemon.com...
 As I understand this will work
 class A
 {
 public int foo () {return 0;}
 }

 class B: A
 public int foo(){ return 1;}
 }


 A a = new B();
 a.foo -> yields 1

 But that won't work
 class A {
      public int foo () { return 0; }
 }

 class B: A{
      public int foo () { return 1; }
      public int bar() { return 0;}
 }

 int main (){
      A a = new B();
      printf("a.foo() = %d\n", a.foo());
      printf("a.bar() = %d\n", a.bar());
      return 0;
 }

 So D allows sort of covariant assignment, but for accessing a functoin
 defined in a descendant the function to be accessed must be part
 of the ancestor too. In the second example bar must be defined in A too.

..... so A a; although a can hold a reference to B or any other sub class of A you have to cast a to the subclass before using those "messages" or "methods" i.e.
      printf("a.bar() = %d\n", (cast(B)a).bar());

class C { .. } you could not do cast(C)a as there is not way that `a` can be a subclass of C (single inheritance only) the types and inheritance tree for types are all determinded at compile time late binding is performed via a virtual function table (vtbl) so a.foo() will be converted to evoke a->vtbl[index of method foo]( a, params to foo ); the compiler has to be able to determine the vtbl index at compile time (not runtime) you may have class D : A { void ping() { } int bar() { return 44; } } a could be a B or D; the vtable entry for bar might be 2 (B) 3 (D) etc ...
Aug 12 2003
prev sibling parent reply Ilya Minkov <midiclub 8ung.at> writes:
Friedrich Dominicus wrote:

 So D allows sort of covariant assignment, but for accessing a functoin 
 defined in a descendant the function to be accessed must be part
 of the ancestor too. In the second example bar must be defined in A too.
 
 Is that correct?

These things must be obvious to you! BTW, how's your own "Q" language doing? In your example "a" is of type "A", which doesn't have a "bar()" defined. If you think you know that a is actually of type B, you can go ahead and do so: int main (){ A a = new B(); printf("a.foo() = %d\n", a.foo()); printf("a.bar() = %d\n", cast(B) (a.bar()) ); return 0; } cast will reward you with an exception, if you were wrong. Can you do such acrobacy in Eiffel or Q as you have tried to suggest with your example? -i.
Aug 12 2003
next sibling parent Mark Evans <Mark_member pathlink.com> writes:
Ilya Minkov says...
BTW, how's your own "Q" language doing?

That's one more for the obscure languages list. The only Q that I had known about lived here. http://www.musikwissenschaft.uni-mainz.de/~ag/q/q.php Friedrich Dominicus' Q language lives here http://www.q-software-solutions.com/about.shtml (Almost all single-letter language names are taken, for those contemplating a new one.) Mark
Aug 12 2003
prev sibling parent reply Friedrich Dominicus <frido q-software-solutions.com> writes:
Ilya Minkov wrote:
 Friedrich Dominicus wrote:
 
 So D allows sort of covariant assignment, but for accessing a functoin 
 defined in a descendant the function to be accessed must be part
 of the ancestor too. In the second example bar must be defined in A too.

 Is that correct?

These things must be obvious to you! BTW, how's your own "Q" language doing?

Well I had a wrong example on my mind. I rewrote the stuff and found the that D is correct and I was wrong.
 
 Can you do such acrobacy in Eiffel or Q as you have tried to suggest 
 with your example?

remember me talking about Q years ago. Q kinda works, not all too much I admit.... Regards Friedrich
Aug 14 2003
parent "Daniel Yokomiso" <daniel_yokomiso yahoo.com.br> writes:
"Friedrich Dominicus" <frido q-software-solutions.com> escreveu na mensagem
news:bhfhdv$2ikm$1 digitaldaemon.com...
 Ilya Minkov wrote:
 BTW, how's your own "Q" language doing?

Well I had a wrong example on my mind. I rewrote the stuff and found the that D is correct and I was wrong.
 Can you do such acrobacy in Eiffel or Q as you have tried to suggest
 with your example?

remember me talking about Q years ago. Q kinda works, not all too much I admit.... Regards Friedrich

Hi, I'm also interested in Q, it had some nice ideas in it. Perhaps you could start a open-source project on Q, compiling it to D ;). Best regards, Daniel Yokomiso. "Sincerity is the key. If you can fake that, you've got it made." - George Burns --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.510 / Virus Database: 307 - Release Date: 14/8/2003
Aug 22 2003