工厂类的典型C++实现是否有缺陷?

我需要在C++中实现工厂类,但是当我思考这个问题的时候,我发现了一个我无法解决的大问题,我发现,所有工厂实施的例子都有同样的缺陷。我可能是错的,但请告诉我原因。

所以这里是简单的“典型”工厂实现,它允许我注册新对象而不更改工厂类。

//fruit.h
class Fruit
{
protected :
  int count;
public :
  Fruit(int count) : count(count) {}
  virtual void show() = 0;
};

// factory.h
/** singleton factory */
class Factory
{
  typedef Fruit* (*FruitCreateFunction)(int);
  static Factory* factory;
  std::map<std::string, FruitCreateFunction> registeredFruits;
public :
  static Factory& instance()
  {
    if (factory == NULL)
      factory = new Factory();
    return *factory;
  }
  bool registerFruit(const std::string& name, Fruit* (createFunction)(int))
  {
    registeredFruits.insert(std::make_pair(name, createFunction));
    return true;
  }
  Fruit* createFruit(const std::string& name, int count)
  {
    return registeredFruits[name](count);
  }
};

//factory.cpp
Factory* Factory::factory = NULL;

//apple.h
class Apple : public Fruit
{
  static Fruit* create(int count) { return new Apple(count); }
  Apple(int count) : Fruit(count) {}
  virtual void show() { printf("%d nice apples\n", count); };  
  static bool registered;
};

// apple.cpp
bool Apple::registered = Factory::instance().registerFruit("apple", Apple::create);

//banana.h
class Banana : public Fruit
{
  static Fruit* create(int count) { return new Banana(count); }
  Banana(int count) : Fruit(count) {}
  virtual void show() { printf("%d nice bananas\n", count); };  
  static bool registered;
};

// banana.cpp
bool Banana::registered = Factory::instance().registerFruit("banana", Banana::create);

// main.cpp
int main(void)
{
  std::vector<Fruit*> fruits;
  fruits.push_back(Factory::instance().createFruit("apple", 10));
  fruits.push_back(Factory::instance().createFruit("banana", 7));
  fruits.push_back(Factory::instance().createFruit("apple", 6));
  for (size_t i = 0; i < fruits.size(); i++)
    {
      fruits[i]->show();
      delete fruits[i];
    }
  return 0;
}

好吧,这段代码看起来很花哨,也很有用,但下面是:

C++标准不允许我定义全局(静态)变量将被定义的顺序。

这里有3个静态变量

Apple::registered;
Banana::registered;
Factory::factory;

这个 Factory::factory 指针需要定义为空 之前 苹果(或香蕉)::注册变量,或 Factory::instance 方法将使用未初始化的值,并且行为不可预测。

那么,我为什么不来这里?代码真的只有在意外情况下才起作用吗?如果是,我应该如何解决这个问题?