Inheritance and friend, static member relationship

5.继承与友元

友元关系不能继承,也就是说基类友元可以访问基类的私有和保护成员,But cannot access subclass private and protected members

例如:You don't necessarily know your dad's friends

class Student;//声明class Person{public:    //声明Display函数是Person类的友元    friend void Display(const Person& p, const Student& s);private:    string _name;};​class Student: public Person{private:    int _stuid;};void Display(const Person& p, const Student& s){    cout << p._name << endl; //可以访问    cout << s._stuid << endl;//不可以访问}

若想让Display函数也能够访问派生类Student的私有和保护成员,只能在派生类Student当中进行友元声明

class Student : public Person{public:    //声明Display函数是Student类的友元    friend void Display(const Person& p, const Student& s);protected:    int _stuid; };

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6.继承与静态成员

基类定义了static静态成员,则There is only one such static member in the entire inheritance hierarchy.无论派生出多少个子类,都只有一个static成员实例 .

例如:当基类PersonA static member variable is defined in count,派生类Student和Teacher都继承了基类Person的成员,But in the whole inheritance system,There is only one static member variablecount

We can pass this static member variablecount的数值,Get the number of objects created

As through the derived class/The base class creates an object,Both need to call the constructor of the base class/The copy constructor initializes some members of the base class,So we can in the base class constructor/Accumulate in the copy constructor_count的值,You can see how many objects were created

//基类class Person{public:    //基类的构造函数    Person()    {        ++_count;    }    //基类的拷贝构造函数    Person(const Person& p)    {        ++_count;    }public:    static int _count;//静态成员变量protected:    string _name;};int Person::_count = 0;//静态成员变量在类外初始化​//派生类class Student :public Person{private:    int _stuid;};​//派生类class Teacher :public Person{private:    int _teaid;};void func(Student s){}//可以通过_countThe size knows the number of objects createdint main(){    Student s1;    Student s2 = s1;    Teacher t1;    Person p;    func(s1);//Passing by value is also a copy construct    //_countmembers are the same There is only one such static member in the entire inheritance hierarchy    cout << Person::_count << endl;//5    cout << Student::_count << endl;//5    return 0;}

验证:There is only one such static member in the entire inheritance hierarchy

//All three have the same addresscout << &Person::_count << endl;cout << &Student::_count << endl;cout << &Teacher::_count << endl;

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