• "Carlos Santander B." <carlos8294 msn.com> Sep 13 2003
• Mike Wynn <mike l8night.co.uk> Sep 13 2003

"Carlos Santander B." <carlos8294 msn.com> writes:

Is there a chance this could ever work?

class A {}
class B:A {}

class C {
    A component;
    A getComponent() { return component; }
}

class D:C {
    B component;
}

void main() {
    B b=new B;
    D d=new D;
    d.component=b; // 1
    printf("%.*s\n",d.getComponent().classinfo.name); // 2
}

Put like this, I get an access violation in 2. If I change 1 to
d.C.component=b; then it works fine. But I don't want nor that neither to
override getComponent for each derivation of C.

-------------------------
Carlos Santander


---

Checked by AVG anti-virus system (http://www.grisoft.com).
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Mike Wynn <mike l8night.co.uk> writes:

Carlos Santander B. wrote:
 Is there a chance this could ever work?
 
 class A {}
 class B:A {}
 
 class C {
     A component;
     A getComponent() { return component; }
 }
 
 class D:C {
     B component;
 }
 
 void main() {
     B b=new B;
     D d=new D;
     d.component=b; // 1
     printf("%.*s\n",d.getComponent().classinfo.name); // 2
 }
 
 Put like this, I get an access violation in 2. If I change 1 to
 d.C.component=b; then it works fine. But I don't want nor that neither to
 override getComponent for each derivation of C.


you will do the layout of D is
D{
	C{
		B{
			A{
			// a has no members
			}
		
		// B has no members
		}
	A component; // visible to C.getComponent
	}
B companent; // new member (not change if the type of C.component;
it is this item that d.component refers to
}

allowing D to redefine the class of `component` would require a `write 
barrier` in C to stop the following occuring

void func( C c, A a ) { c.componant = a; }

D d; func( d, new A() ); // would attempt to write an A to a B!


you can do ....

class C {
	private A comp;
	public void setComp( A c ) { comp = c; }
	public A getComp() { return comp; }
}

class D:C {
	public void setComp( A c )
	in {
		if ( c !== null ) {
			B b = cast(B)c
			assert( b );
		}
	}
	body {
		super.setComp( c );
	}
	public B getComp() { return cast(B)super.getComp(); }
}