Policy-based template design: How to access certain policies of the class?
- by dehmann
I have a class that uses several policies that are templated. It is called Dish in the following example. I store many of these Dishes in a vector (using a pointer to simple base class), but then I'd like to extract and use them. But I don't know their exact types.
Here is the code; it's a bit long, but really simple:
#include <iostream>
#include <vector>
struct DishBase {
int id;
DishBase(int i) : id(i) {}
};
std::ostream& operator<<(std::ostream& out, const DishBase& d) {
out << d.id;
return out;
}
// Policy-based class:
template<class Appetizer, class Main, class Dessert>
class Dish : public DishBase {
Appetizer appetizer_;
Main main_;
Dessert dessert_;
public:
Dish(int id) : DishBase(id) {}
const Appetizer& get_appetizer() { return appetizer_; }
const Main& get_main() { return main_; }
const Dessert& get_dessert() { return dessert_; }
};
struct Storage {
typedef DishBase* value_type;
typedef std::vector<value_type> Container;
typedef Container::const_iterator const_iterator;
Container container;
Storage() {
container.push_back(new Dish<int,double,float>(0));
container.push_back(new Dish<double,int,double>(1));
container.push_back(new Dish<int,int,int>(2));
}
~Storage() {
// delete objects
}
const_iterator begin() { return container.begin(); }
const_iterator end() { return container.end(); }
};
int main() {
Storage s;
for(Storage::const_iterator it = s.begin(); it != s.end(); ++it){
std::cout << **it << std::endl;
std::cout << "Dessert: " << *it->get_dessert() << std::endl; // ??
}
return 0;
}
The tricky part is here, in the main() function:
std::cout << "Dessert: " << *it->get_dessert() << std::endl; // ??
How can I access the dessert? I don't even know the Dessert type (it is templated), let alone the complete type of the object that I'm getting from the storage.
This is just a toy example, but I think my code reduces to this. I'd just like to pass those Dish classes around, and different parts of the code will access different parts of it (in the example: its appetizer, main dish, or dessert).
