But wait, there's more. Here's the real problem. I also want to create a Copy() method that will copy this expression and its subexpressions. Something like:

	Expression *
Expression::Copy (
	Expression		*parent)
	Expression		*copy;
	int			 i, n;

	copy = ???? (parent);

	n = OpTerm ();
	for (i = 0; i < n; i++)
		copy->sub_exp[i] = sub_exp[i]->Copy (copy);

	return copy;
The problem is what to put in the "???" part. How do I call the constructor for actual class implementing the abstract class. I conclude that I can't do it directly, but I'm hoping there's some technique for getting at it.

My first attempt was to declare an Alloc(parent) method on the Expression class. Then in the subclasses I would do:

class ExpAdd : public Expression
			 ExpAdd (Expression *parent) : Expression (parent) {}
	Expression *	 Alloc (Expression *parent)
		return ExpAdd (parent);
The problem is I then have to do this same thing in every single subclass. There are dozens of operator types -- that's a lot of duplicated code. Is there a way to use a template to get rid of the noxious bit of boilerplate?