朋友类：继承类不是朋友吗？ [英] friend class : inherited classes are not friend as well?

问题描述

在C ++中，我有一个类A，它是类B的朋友。

我看起来继承类B不是类A的朋友。 p>

这是C ++的限制还是我的错误？

这里是一个例子。编译时，我在return new Memento行上遇到错误：

Memento :: Memento：无法访问在Memento中声明的私有成员。

  class Originator; 
 
 class Memento 
 {
 friend class Originator; 
 
 Memento（）{}; 
 
 int m_Data; 
 
 public：
〜Memento（）{}; 
} 
 
 class Originator 
 {
 public：
 virtual Memento * createMemento（）= 0; 
} 
 
 class FooOriginator：public Originator 
 {
 public：
 Memento * createMemento（）
 {
 return new Memento; //不可能访问Memento的私人成员
} 
}; 
 
 void main（）
 {
 FooOriginator MyOriginator; 
 MyOriginator.createMemento（）; 
 
} 
  

我当然可以添加FooOriginator作为Memento的朋友，但那么，这意味着我必须添加所有Originator继承的类作为Memento的朋友，这是我想避免的。

任何想法？ >

解决方案

查看： http://stackoverflow.com/questions/437250/friend-scope-in​​-c/437507

投票的完全相同。

 我看起来继承的B类不是类A的朋友。
  

b $ b

正确

 我这是C ++的限制还是我的错误？ 
  

这是C ++的工作原理。我不认为这是一个限制。

In C++, I have a class A which is friend with a class B.

I looks like inherited classes of B are not friend of class A.

I this a limitation of C++ or my mistake ?

Here is an example. When compiling, I get an error on line "return new Memento":

Memento::Memento : impossible to access private member declared in Memento.

class Originator;

class Memento
{
  friend class Originator;

  Memento() {};

  int m_Data;

public:
  ~Memento() {};
};

class Originator
{
public:
  virtual Memento* createMemento() = 0;
};

class FooOriginator : public Originator
{
public:
  Memento* createMemento()
  {
    return new Memento; // Impossible to access private member of Memento
  }
};

void main()
{
  FooOriginator MyOriginator;
  MyOriginator.createMemento();

}

I could of course add FooOriginator as friend of Memento, but then, this means I would have to add all Originator-inherited classes as friend of Memento, which is something I'd like to avoid.

Any idea ?

解决方案

See: http://stackoverflow.com/questions/437250/friend-scope-in-c/437507
Voted exact duplicate.

I looks like inherited classes of B are not friend of class A.

Correct

I this a limitation of C++ or my mistake ?

It is the way C++ works. I don't see it as a limitation.

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