C++ 中的嵌套类、继承和共享指针

Question

The program below fails, obviously, in the return expression:

#include <memory>

class Base {
    public:

        class Nested {
            public:
                int c;
        };
};

class A : public Base {
    public:

        class Nested : public Base::Nested {
            public:
                int c = 1;
        };
};

class B : public Base {
    public:

        class Nested : public Base::Nested {
            public:
                int c = 2;
        };
};

int main() {
    std::shared_ptr<Base> X = std::make_shared<A>();

    return X::Nested.c;
};

How can I get Nested.c value of X?

In other words, I have one base class ( Base ) and two derived classes ( A and B ). Each derived class has a nested class ( Nested ). I want to called Nested.c from an instance X , which is dynamically selected as one of the derived classes.

Answer 1

Probably just a misconception about nested classes. A Nested class does not magically add members to it's parent class. You need to do that manually, for example:

class Base {
  public:

    class Nested {
        public:
            int c;
    };

    Nested nested; // member of nested class Base::Nested.
};

Keep in mind that Base::Nested , A::Nested and B::Nested are all different classes. Allthough they look similiar they are not related at all.

---

Maybe the following is what you want:

 #include <memory> class Base { private: class Nested { public: int c; }; Nested nested; // member of nested class Base::Nested. public: virtual int getC() const { return this->nested.c; } }; class A : public Base { private: class Nested { public: int c = 1; }; Nested nested; // member of nested class A::Nested. public: int getC() const override { return this->nested.c; } }; class B : public Base { private: class Nested { public: int c = 2; }; Nested nested; // member of nested class B::Nested. public: int getC() const override { return this->nested.c; } }; int main() { std::shared_ptr<Base> X = std::make_shared<B>(); return (*X).getC(); };

Each class has it's own member of it's own nested class and returns c with a virtual getter.