C ++-在父类中定义子类

Question

While working on a personal project, I have found myself in a situation where I want to create reusable chunks of code for child classes of my abstract parent class that can implement some of it's pure virtual methods. For that purpose I have created a an intermediate parent class inheriting after the primary child class. However, due to the nature of the final child classes within the project it doesn't make sense for the intermediate parent class to be accessible to any object other than the child classes. As a result I have created this type of class structure:

class Foo
{
protected:
    class Bar : protected Foo
    {
    public:
        void a()
        {
            //implementation of a()
        }
    };

public:
    virtual void a() = 0;
    virtual void b() = 0;
};

class Beep : public Foo, private Foo::Bar
{
    void b()
    {
        //implementation of b()
    }
};

(I am aware of the diamond inheritance problem and other issues caused by inheriting from an ancestor class via multiple sources. For the purpose of this question, please assume it to be a non-issue as it is not the focus.)

The above code results in the 'incomplete type not allowed' error upon attempting to define Foo as the parent of Bar . Is there a way to restructure the code to remove the error while maintaining the desired class/accessibility structure?

While I am interested in how one would achieve what I described (by this point mostly due to curiosity), I will accept alternative solutions as long as they result in a matching behavior, that is: code implementing parts of an abstract base class only visible/accessible to objects inheriting after said base class.

Answer 1

You can forward declare Bar and define it after Foo .

class Foo
{
protected:
    class Bar;

public:
    virtual void a() = 0;
    virtual void b() = 0;
};

class Foo::Bar : protected Foo
{
public:
    void a()
    {
        //implementation of a()
    }
};