TITLE: "Avoiding" downcasts in container classes using unions

PROBLEM: lowry@watson.ibm.com (Andy Lowry)

[...] 

The idea is that the repository can hold items of various classes,
each in the guise of an Item.  Any code that gets items out of the
repository knows (some how, not important) what class it really
belongs to, and wants to cast it to that class to get at its full
functionality.

Is there any good way to do that, when some of the items in the
repository are of classes that are virtually derived from Item?


RESPONSE: maxtal@extro.ucc.su.OZ.AU (John MAX Skaller), 30 Mar 93


[ ... ] you are missing that you need to redesign so you
        dont need to downcast. A union is better.

[ ... ]

	class AnItem {
		enum {t1, t2, t3} type;
		union {
			Item *i;
			Derived1 *d1;
			Derived2 *d2;
			Derived3 *d3;
		};
	} it;

	switch(it.type)
	{
		case t1: it.d1->....
		case t2: it.d2->....
		case t3: it.d3-> ....
	}

[ An alternative to this is to put virtual methods in Item, as shown below

		virtual Derived1* d1() CONST { return NULL}
		virtual Derived2* d2() CONST { return NULL}
		virtual Derived3* d3() CONST { return NULL}

  and have the derived classes override their particular method to return
  "this". The drawback of this is that the base class is polluted with
  knowledge of derived and must be modified everytime a new derived class
  is added. -adc ]