如何从“D类”对象访问“A类”的变量"a“

Question

我有一个B类，它继承A级，我有一个C类，它继承B类和A类；

在A类中有一个名为"a“的变量，我必须在类D中访问它，但不能使用B类的分辨率运算符，例如d.B::a；

#include<bits/stdc++.h>
using namespace std;

class A{
    public:
        int a;
};

class B : public A{
    public:
        int b;
};

class D : public B, public A{
    public:
        int d;
};

int main() {
    D d;
    d.B::a = 1; // This is working correctly on this path class D -\> class B -\> class A
    d.a = 2;
/*
But This line gives the following error
Non-static member 'a' found in multiple base-class subobjects of type 'A':
class D -> class B -> class A
class D -> class A
*/
    return 0;

}

int main() {
D d;
d.D::a = 2; //This is not working correctly on this path class D -\> class A
d.A::a = 2; //This is not working correctly on this path class D -\> class A
//How do I use Scope Resolution to access variable "a" from second path ?!
return 0;
}

Answer 1

B已经从A继承，所以D只应该从B继承：

  class A{
       public:
           int a;
  };

  class B : public A{
        public:
            int b;
  };

  class D : public B{
        public:
            int d;
  };

如果您有另一个继承自A的类C，并且希望D从B和C继承，那么您将拥有金刚石问题。然后，为了避免模棱两可，你应该写：

  class A{
       public:
           int a;
  };

  class B : virtual public A{
        public:
            int b;
  };

  class C : virtual public A{
        public:
            int b;
  };

  class D : public B, public C{
        public:
            int d;
  };

Answer 2

虚拟继承是解决问题的关键词。

试试这个：

#include<bits/stdc++.h>
using namespace std;

class A{
    public:
        int a;
};

class B : virtual public A{
    public:
        int b;
};

class D : public B, virtual public A{
    public:
        int d;
};

int main() {
    D d;
    d.B::a = 1; // This is working correctly on this path class D -\> class B -\> class A
    d.a = 2;

    std::cout << d.a;
    
/*
But This line gives the following error
Non-static member 'a' found in multiple base-class subobjects of type 'A':
class D -> class B -> class A
class D -> class A
*/
    return 0;

}