友谊与传承

Question

我正在做一个小项目，但有点卡住了，因为我不太明白友谊和继承是如何相互作用的。我将向您展示一些示例代码。

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

显然Derived不能访问Foo的protected成员，貌似因为Derived::m_foo是派生成员，所以构造Derived会失败。谁能给我详细解释一下？

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.

对不起，这不是朋友明显的误会。

朋友 class 可以访问任何属性。

您有一个不相关的编码错误...评论显示您在 Base 中缺少初始化（Base::m_foo）。解决这个问题，向 Foo 添加一些数据项，然后运行你的演示：

#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

来自 class 的派生 ctor 访问私有、受保护和 public 数据属性的典型输出：

99 11 22

Answer 2

我发现了问题。命名空间 b 和因此派生的 class 对于 Foo 中的友元声明是不可见的。当我转发声明 b 并派生时，一切都按预期工作并且派生可以访问 private/protected 成员。

友谊与传承

Friendship and inheritance

c++

inheritance

friend