关于 c++ Friendship 和从 VC12 和 VC14 继承的不同行为

Question

class Base 
{
protected:
    void func1();
};

class Derived : public Base
{
friend class Third;
};

class Third
{
     void foo() 
     {
        Derive d;
        d.func1();
     }
};

我可以在 VC14 (Visual Studio 2015) 中编译代码而不会出现错误 但从 VC12 (Visual Studio 2013) 得到错误

cannot access protected member declared in class 'Base'

谁是对的？ 这种与继承的友谊的正确性是什么？

来自MSDN https://msdn.microsoft.com/en-us/library/465sdshe.aspx 或http://en.cppreference.com/w/cpp/language/friend 看来友情是不可传递的，不能被继承。但是我认为这个代码示例并非如此。

但是为什么 VC14 不会给我一个错误呢？

如果 VC14 是正确的，我该如何“修改”代码以使 VC12 也可以这样做？ 在 Derived 类中再次定义 protected func1()？

Answer 1

修正错别字后，cmets inline：

class Base 
{
protected:
    void func1();   // protected access
};

class Derived : public Base
{
  // implicit protected func1, derived from Base

  // this means 'make all my protected and private names available to Third'
  friend class Third;
};

class Third
{
     void foo() 
     {
        Derived d;
        // func1 is a protected name of Derived, but we are Derived's friend
        // we can therefore access every member of Derived
        d.func1();
     }
};

VC14 是正确的。

VC12 的可能解决方法：

class Base 
{
protected:
    void func1();
};

class Derived : public Base
{
  protected:
    using Base::func1;

  private:
    friend class Third;
};


class Third
{
     void foo() 
     {
        Derived d;
        d.func1();
     }
};

另一种可能的解决方法（使用基于密钥的访问）

class Third;
class Func1Key
{
  private:
    Func1Key() = default;
    friend Third;
};

class Base 
{
protected:
    void func1();
};

class Derived : public Base
{
public:  
  void func1(Func1Key) 
  {
    Base::func1();
  }
};


class Third
{
     void foo() 
     {
        Derived d;
        d.func1(Func1Key());
     }
};