c++ - 友元: many classes from same parent

Question

如何在所有派生类中没有 friend 的情况下访问 privateMember？

class parent{...};  //a virtual class

class A: public parent{...};
class B: public parent{...};
class C: public parent{...};
class D: public parent{...};
class E: public parent{...};
...
//each has a function, that want access to privateMember

class MyClass{
    int privateMember;

    friend parent;
    //I know it doesnt't work, but this shows the best what I want   
}

Answer 1

保持原样(使用friend class parent)并向parent添加一个访问器函数，A，B，...将使用。它会受到保护，因此层次结构外部的函数无法使用它。

class parent {
protected:
  static int& getPrivate( MyClass & c ) { return c.privateMember; }
...
};

你必须这样做，因为友元不会扩展到派生类。