Re: '*' cannot appear in a constant-expression problem

On 2008-10-24, Stefano Sabatini <stefano.sabatini@caos.org> wrote:


[...]


Sorry it had tons of erros, check below the new version.


Well I found a solution, even if I'm not sure I really understood it.

If I define the type like this:
typedef Pizza *(* pizza_creator_fn_ptr)();

and then use the map like this:
static map<std::string, pizza_creator_fn_ptr> creators;

then it seems to work fine.

A short explanation would be nice.

New version here:
-----------------------------------8<-------------------------------------
#include <string>
#include <iostream>
#include <map>

class Pizza {
public:
    virtual void get_price() = 0;
};
 
class HamAndMushroomPizza: public Pizza {
public:
    virtual void get_price() {
        std::cout << "Ham and Mushroom: $8.5" << std::endl;
    }

    static Pizza* create_pizza() {
        return new HamAndMushroomPizza;
    }
};
 
class DeluxePizza : public Pizza {
public:
    virtual void get_price() {
        std::cout << "Deluxe: $10.5" << std::endl;
    }

    static Pizza* create_pizza() {
        return new DeluxePizza;
    }
};
 
class SeafoodPizza : public Pizza {
public:
    virtual void get_price() {
        std::cout << "Seafood: $11.5" << std::endl;
    }

    static Pizza* create_pizza() {
        return new SeafoodPizza;
    }
};

typedef Pizza* (*pizza_creator_fn_ptr)(void);

class PizzaFactory {
private:
    static std::map<std::string, pizza_creator_fn_ptr> creators;

    void init() {
        creators["Deluxe"] = &DeluxePizza::create_pizza;
        creators["Ham and Mushroom"] = &HamAndMushroomPizza::create_pizza;
        creators["Seafood"] = &SeafoodPizza::create_pizza;
    }

public:
    static PizzaFactory* get_instance()
    {
        static PizzaFactory * instance = 0;
        if (!instance) {
            instance = new PizzaFactory;
            instance->init();
        }
        return instance;
    }

    static Pizza* create_pizza(const std::string& type) {
        std::map<std::string, pizza_creator_fn_ptr>::iterator it;
        if ((it = creators.find(type)) != creators.end())
            return (*(it->second))();
        else
            return 0;
    }
};

//usage
int main() {
    PizzaFactory* factory = PizzaFactory::get_instance();
    Pizza *pizza = 0;

    pizza = factory->create_pizza("Default");
    pizza->get_price();
    delete pizza;

    pizza = factory->create_pizza("Ham and Mushroom");
    pizza->get_price();
    delete pizza;

    pizza = factory->create_pizza("Seafood Pizza");
    pizza->get_price();
    delete pizza;
}
-----------------------------------8<-------------------------------------

The code has still a problem related to the use of the static map
which isn't found by the compiler, but this is another problem.

Thanks for your attention.