//友元没有继承性，没有传递性,所以在animal中定义友元类是无效的class animal{public:animal(){};virtual ~animal(){};};class Cat:public animal{friend class animalFactory;private:Cat(){}private:string m_name;string m_color;public:void about(){cout<<"my name is "<<m_name<<",my color is "<<m_color<<endl;}~Cat() override{}};class Dog:public animal{friend  class animalFactory;private:Dog(){}private:string m_name;string m_color;int m_age;public:~Dog() override{}void about() const{cout<<"i am a dog,my name is "<<m_name<<",my color is"<<m_color<<",i am "<<m_age<<"years old"<<endl;}};class animalFactory{public:static animal* createCat(string name,string color){Cat* p_cat=new Cat;p_cat->m_color=color;p_cat->m_name=name;return p_cat;}public:static animal* createdog(string name,string color,int age){Dog* p_dog=new Dog;p_dog->m_name=name;p_dog->m_color=color;p_dog->m_age=age;return p_dog;}};void friend_test(){animal* cat=animalFactory::createCat("cat","black");Cat* real_cat=dynamic_cast<Cat*>(cat);real_cat->about();animal* dog=animalFactory::createdog("dog","white",1);Dog* real_dog=dynamic_cast<Dog*>(dog);real_dog->about();}//运行结果如下：//my name is cat,my color is black
//i am a dog,my name is dog,my color iswhite,i am 1years old