'*' cannot appear in a constant-expression problem

Hi all, I'm encountering this while trying to implement a factory
singleton method to generate objects.

The singleton has a static map which binds a static creation function
defined in each class to the type of the object to be created.

Here it is the code, which is a modification of the wikipedia C++
factory example code:

----------------------------------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;
    }
};
 
class PizzaFactory {
private:

    static std::map<std::string, (Pizza *)(*)()> creators;

    init() {
        map["Deluxe"] = DeluxPizza::create_pizza;
        map["Ham and Mushroom"] = HamAndMushroom::create_pizza;
        map["Seafood"] = SeafoodPizza::create_pizza;
    }

public:
    PizzaFactory* get_instance()
    {
        static PizzaFactory instance = 0;
        if (!instance) {
            instance = new PizzaFactory;
            instance.Init();
        }
        return instance;
    }

    static Pizza* create_pizza(const std::string type) {
        PString type = config.GetAttribute("type");
        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 static map declaration syntax is somehow wrong, and after hitting
my head sometime I still can't get out of it.

I'm using g++ 4.3.1, and the syntax error I get is this:

make PizzaFactory2; and PizzaFactory2
g++ -I/home/stefano/opt/reilabs/include -I/home/stefano/include -O0 -g -ggdb PizzaFactory2.cxx -c -o PizzaFactory2.o
PizzaFactory2.cxx:50: error: `*' cannot appear in a constant-expression
PizzaFactory2.cxx:50: error: a function call cannot appear in a constant-expression
PizzaFactory2.cxx:50: error: `*' cannot appear in a constant-expression
PizzaFactory2.cxx:50: error: a function call cannot appear in a constant-expression
PizzaFactory2.cxx:50: error: a function call cannot appear in a constant-expression
PizzaFactory2.cxx:50: error: template argument 2 is invalid

The exact line of the error is:
    static std::map<std::string, (Pizza *)(*)()> creators;

which I interpret as:
      a static map from string to a static method pointer which takes no
      parameters and returns a pointer to a Pizza object.

What am I missing or what I'm doing wrongly?

Regards and many help in advance.