覆盖嵌套 class 的虚拟 function

Question

Let's say I have the following class:

class Base {
public:
    class Nested {
        virtual void display() {
            std::cout << "Not overridden" << std::endl;
        }
    };
    Nested N;
};

And I have another class named Derived that inherits from the class Base .

Is it possible to override the display() method declared inside the Nested so that in the Derived class it does this instead:

void display() {
    std::cout << "Overridden" << std::endl;
}

If so, how?

If not, what other options do I have?

Answer 1

Nested class (from cppreference ):

The name of the nested class exists in the scope of the enclosing class, and name lookup from a member function of a nested class visits the scope of the enclosing class after examining the scope of the nested class. Like any member of its enclosing class, the nested class has access to all names (private, protected, etc) to which the enclosing class has access, but it is otherwise independent and has no special access to the this pointer of the enclosing class.

and since c++11

Declarations in a nested class can use any members of the enclosing class, following the usual usage rules for the non-static members.

So sloppy speaking, nesting classes is about names and accessing members. Thats it.

A class deriving from Base does not inherit a method display , because Base has no method display . Concerning inheriting from Base there is little difference to:

class Nested {
    virtual void display() {
        std::cout << "Not overridden" << std::endl;
    }
};

class Base {
public:
   Nested N;
};

class Derived : public Base {};

Derived inherits the member N , but no methods, because Base has no methods (apart from the special compiler generated ones).