友元类继承 [英] friend class with inheritance

问题描述

如果我有两个继承类如下：

  class A 
 {
 .. 。
} 
 
 class B：public A 
 {
 ... 
} 
  

第三个类定义为A类朋友：

  class C 
 {
 friend class A; 
} 
  

我是否可以从 / code>（其也是 A 类型的对象）类C 的所有成员，已经定义 class B 首先是朋友Class？

解决方案

friend ship既不是继承也不是可传递的。它是两个类之间严格一对一的关系。

  A类{
朋友class B; 
 int Aries; 
}; 
 
 class B {
 friend class C; 
 int Taurus; 
}; 
 
 class C {
 int Leo; 
 void Capricorn（）{
 A a; 
 a.Aries = 0; // this wont work，C is not a friend of A. 
 //友谊不是传递的
} 
}; 
 
 class D：public C {
 void Gemini（）{
 B b; 
 b.Taurus = 0; // this wont work，D不是B的朋友。
 //友谊不是继承的
} 
}; 
 
 class E：public B {
 void Scorpio（）{
 C c; 
 c.Leo = 0; // this wont work either，friendship is not inherited 
} 
}; 
  

参考：C ++编程语言Bjarne Stroustrup

更多解释（我的）：如果 friend 船不是一对一，它将是封装的结束。请注意， B 类只能访问 A 的成员 private 如果 A 的类声明将 B 声明为 friend 。 B 不能强制朋友在 A >



现在，如果友谊可以继承，那么有人只需要继承 B 即可访问 A ，没有 A 在阻止它。此外，允许 friend 是可传递的会导致其他问题，因为现在 B 可能有 friend C ，他们可以有朋友 D ，一直到 Z 。所有 B ， C ， D ，... ， Z 现在可以访问 A 的私人这将是一场灾难。

If I have two Classes as follows with inheritance:

class A
{
    ...
}

class B : public A
{
    ...
}

And a third class with defined as a friend class A:

class C
{
    friend class A;
}

Will I be able to access from class B (which is also an object of type A) all members of class C as if I had defined class B the friend Class in the first place?

解决方案

friendship is neither inherited nor transitive. It is strictly one-one relationship between two classes.

class A {
  friend class B;  
  int Aries;
};

class B {
  friend class C;  
  int Taurus;
};

class C {
  int Leo;
  void Capricorn() {
    A a;
    a.Aries = 0;  // this wont work, C is not a friend of A.
                // friendship is not transitive
  }
};

class D : public C {
  void Gemini() {
    B b;
    b.Taurus = 0;  // this wont work, D is not a friend of B.
                   // friendship is not inherited
  }
};    

class E : public B {
  void Scorpio() {
    C c;
    c.Leo = 0; // this wont work either, friendship is not inherited
  }
};

Reference: "The C++ Programming Language" Bjarne Stroustrup

More explanation (mine): If friendship were not one-one, it would be the end of encapsulation. Note that B class can access private members of A only if the class declaration of A declares B as friend. B cannot enforce friendship on A.

Now, if friendship could be inherited, then someone just needs to inherit B to access private members of A, without A having any say in preventing it. Also, allowing friendship to be transitive would lead to other problems, since now B could have a friend C, who in turn could have a friend D, all the way to Z. All of B, C, D, ..., Z can now access A's private members, which would be a disaster.

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