如何让类相互继承？

Question

I have two classes ClassOne , ClassTwo each one in a separated file .h and .cpp as follow:

// ClassOne.h file
#include <ClassTwo.h>
class ClassOne : public ClassTwo {
protected:
    type m_string; // Required for "ClassTwo"
}

// ClassTwo.h file
#include <ClassOne.h>
class ClassTwo : public ClassOne {
public:
    method1(); // <---|
    method2(); // <- Required to be available in class ClassOne.
    method3(); // <---|
}

As you saw in the previous code the two classes inherit each other, but that code occurs errors error C2504: 'ClassOne': base class undefined and error C2504: 'ClassTwo': base class undefined .

The purpose of that is I want the member's functions of "ClassTow" to be available in "ClassOne" but also "ClassTwo" needs a member variable in "ClassOne".

How to make the two classes inherit from each other?

Answer 1

Maybe the CRTP would solve your problem. First, create the class ClassOne as a template that uses a type ClassTwo as a parameter. This type shall be the class that inherits from ClassOne :

template<typename ClassTwo>
class ClassOne
{
public:
    void methodOne() {
        if (you need to use methods of ClassTwo) 
             static_cast<ClassTwo*>(this)->methodTwo();
    }
};

Then define ClassTwo as the one that inherits ClassOne specifying the template parameter with itself:

class ClassTwo : public ClassOne<ClassTwo>
{
public:
    void methodTwo() {
        if (you need to use methods of ClassOne) 
             methodOne();
    }
};

Update: that is a hard way to solve the problem. There are other even simpler ones, for example to use virtual methods:

class ClassOne
{
public:
    void methodOne() {
        if (you need to use methods of ClassTwo) 
             methodTwo();
    }
    virtual void methodTwo() = 0;
};

class ClassTwo : public ClassOne
{
public:
    void methodTwo() override {
        if (you need to use methods of ClassOne) 
             methodOne();
    }
};

Both approaches have their own pros and cons. We need to know your task in details to advise which one is better.

Answer 2

How to make classes inherit each other?

A class cannot possibly inherit its child. A base can only be specified in the class definition, and a class cannot be used as a base unless it has been specified. There is no way to order the two definitions in such way that both are before each other. Thus, this is not possible.

Such inheritance would also be paradoxical because a class object contains the base as a sub object within itself. As such, you would end in situation where there is One inside Two inside One inside Two inside One inside Two inside One inside Two inside One inside Two inside One inside Two inside... can you see where this is going? It is going nowhere. The object is infinitely large because it contains infinitely deep inheritance hierarchy.

To express this in another way: Inheritance hierarchy can be seen as a directed graph. An inheritance hierarchy graph cannot contain a cycle.

---

The purpose of that is I want the member's functions of "ClassTow" to be available in "ClassOne" but also "ClassTwo" needs a member variable in "ClassOne".

You can have two classes, both of which having access to all of the data members and the functions like this:

struct ClassOne {
    // all the member functions
    // all the data members
};

struct ClassTwo : ClassOne {
    // nothing here
};

Now you have two classes, both of which have access to the same data members and member functions.