Apples, oranges, and pointers to the most derived c++ class
- by Matthew Lowe
Suppose I have a bunch of fruit:
class Fruit { ... };
class Apple : public Fruit { ... };
class Orange: public Fruit { ... };
And some polymorphic functions that operate on said fruit:
void Eat(Fruit* f, Pesticide* p) { }
void Eat(Apple* f, Pesticide* p) { ingest(f,p); }
void Eat(Orange* f, Pesticide* p) { peel(f,p); ingest(f,p); }
OK, wait. Stop right there. Note at this point that any sane person would make Eat() a virtual member function of the Fruit classes. But that's not an option, because I am not a sane person. Also, I don't want that Pesticide* in the header file for my fruit class.
Sadly, what I want to be able to do next is exactly what member functions and dynamic binding allow:
typedef list<Fruit*> Fruits;
Fruits fs;
...
for(Fruits::iterator i=fs.begin(), e=fs.end(); i!=e; ++i)
Eat(*i);
And obviously, the problem here is that the pointer we pass to Eat() will be a Fruit*, not an Apple* or an Orange*, therefore nothing will get eaten and we will all be very hungry.
So what I really want to be able to do instead of this:
Eat(*i);
is this:
Eat(MAGIC_CAST_TO_MOST_DERIVED_CLASS(*i));
But to my limited knowledge, such magic does not exist, except possibly in the form of a big nasty if-statement full of calls to dynamic_cast.
So is there some run-time magic of which I am not aware? Or should I implement and maintain a big nasty if-statement full of dynamic_casts? Or should I suck it up, quit thinking about how I would implement this in Ruby, and allow a little Pesticide to make its way into my fruit header?
