友谊与传承

Question

I'm working on a small project and i'm sort of stuck because I don't really understand how friendship and inheritance interact with each other. I'll show you some sample code.

namespace a
{
    class Foo
    {
    public:
        Foo(int x) : m_x(x) {}
    protected:
        friend class b::Derived;
        friend class a::Base;
        int m_x;
    };

    class Base
    {
    public:
        Base(Foo foo) : m_foo(foo) {}
    protected:
        Foo m_foo;
    };
}
namespace b
{
    class Derived : public a::Base
    {
    public:
        Derived(a::Foo foo)
            : Base(foo)
        {
            m_foo.m_x;
        }
    };
}

e0265: at line 29: member a::Foo::m_x (declared at line 10) is inaccessible

Apparently Derived can't access protected members of Foo, seemingly because Derived::m_foo is a derived member, so constructing Derived will fail. Can anyone explain this in detail to me?

Answer 1

Apparently Derived can't access private members of Foo, seemingly because Derived::m_foo is a derived member, so constructing Derived will fail.

Sorry, this is not an apparent misunderstanding of friend.

A friend class can access any attribute.

You have an unrelated coding error ... the comments reveal that you are missing an initialization (of Base::m_foo) in Base. Fix that, add a few data items to Foo, then run your demo:

#include <iostream>
using std::cout, std::endl; 

class Foo
{
public:
   Foo(int x): m_x(x){}
   ~Foo(){}

   int m_z;   // add public

protected:
   int m_y;   // add protected

private:      // change to private
   friend class Derived;
   int m_x;
};

class Base
{
public:
   Base() : m_foo(0) // add m_foo Initialization (with 0)
      {}
   virtual ~Base(){}
protected:
   Foo m_foo;
};

class Derived : public Base
{
public:
   Derived(Foo foo)
      {
         foo.m_y   = 11;
         foo.m_z   = 22;
         std::cout << foo.m_x << "   "
                   << foo.m_y << "   "
                   << foo.m_z << std::endl;  //friend class can access
      }
};

class T914_t // ctor and dtor compiler provided defaults
{
public:
   int operator()(int argc, char* argv[]) { return exec(argc, argv);  }

private: // methods

   int exec(int , char** )
      {
         Foo     f(99);
         Derived d(f);

         return 0;
      }

}; // class T914_t

int main(int argc, char* argv[]) { return T914_t()(argc, argv); } // call functor

---

Typical output from class Derived ctor accessing private, protected, and public data attributes:

99 11 22

Answer 2

I found the problem. The namespace b and therefore the class derived were not visible to the friend declaration in Foo. When I Forward declared b and derived everything worked as intended and derived could access private/protected members.