C++ Using through an interface a class that inherits from several others

Question

I am learning C++ and I am confused about the multiple inheritance and the interface classes.

I want to have a class that inherits from several others. Also, I want to use that derived class through an interface. So I suppose, the derived class should extend the base classes, and the derived interface should extend the base interfaces. I would have done that in other languages, but I think C++ does not work like that.

This is the code I thought should work:

#include <iostream>

using std::cout;
using std::endl;

class
    Base1Itf
{
    public:
        virtual void blue() = 0;
};

class
    Base1Abs
:
    public Base1Itf
{
    public:
        void blue()
        {
            cout << "blue" << endl;
        }
};

class
    DerivedItf
:
    public Base1Itf
{
    public:
        virtual void red() = 0;
};

class
    Derived
:
    public Base1Abs,
    public DerivedItf
{
    public:
        void red()
        {
            cout << "red" << endl;
        }
};

int main()
{
    DerivedItf* d = new Derived();
    d->red();
    d->blue();
    delete d;
    return 0;
}

This is the compiler error I get:

src/test.cpp: In function ‘int main()’:
src/test.cpp:49:30: error: invalid new-expression of abstract class type ‘Derived’
  DerivedItf* d = new Derived();
                              ^
src/test.cpp:35:2: note:   because the following virtual functions are pure within ‘Derived’:
  Derived
  ^~~~~~~
src/test.cpp:10:16: note:       virtual void Base1Itf::blue()
   virtual void blue() = 0;
                ^~~~

In the example only a base class is implemented, but there will be many more.

What I am doing wrong? Thank you.

EDIT

If I remove the Base1Abs inheritance of Base1Itf for avoiding the Diamond Problem, the compiler shows the same error.

You cannot create an object of a class with a [pure virtual function](https://www.geeksforgeeks.org/pure-virtual-functions-and-abstract-classes/) — kesarling He-Him, May 21 '20 at 08:22
@d4rk4ng31 Yes, I know that by reading the compiler error. But I don't know how to do what I am trying, if it is possible at all. — Joan Botella, May 21 '20 at 08:23
@d4rk4ng31 Yes, I know, but as the title says, I want to use the derived class through an interface. — Joan Botella, May 21 '20 at 08:26
You have to implement `blue()` in `Derived`. [LIVE](https://wandbox.org/permlink/RTNzOugUcVGG4E2v) — songyuanyao, May 21 '20 at 08:27
I don't want the rest of my application to know which exact concrete class are they using. — Joan Botella, May 21 '20 at 08:28
@JoanBotella, [If you are interested](https://chat.stackoverflow.com/rooms/214316/chat-room-for-https-stackoverflow-com-questions-61930209-c-using-through-an-int) — kesarling He-Him, May 21 '20 at 08:28
@songyuanyao I don't want to implement the base code into the derived class. — Joan Botella, May 21 '20 at 08:29
@JoanBotella You have to implement pure virtual method in derived class; otherwise it'll be abstract class and can't be constructed. `Derived` inherits from `DerivedItf` which inherits from `Base1Itf`; and pure virtual `Base1Itf::blue` doesn't get implemented in this inherit branch. — songyuanyao, May 21 '20 at 08:31
As I said before, the Derived class should know the least possible about the base classes implementation. Reimplementing their methods it is not ok — Joan Botella, May 21 '20 at 08:34
In PHP I have done something like this by creating several traits and interfaces for them. The base class uses all the traits and implements an interface that extends all the base interfaces. — Joan Botella, May 21 '20 at 08:37
Take a look at [diamond problem](https://www.freecodecamp.org/news/multiple-inheritance-in-c-and-the-diamond-problem-7c12a9ddbbec/) in C++ — kesarling He-Him, May 21 '20 at 08:44
Related: [When to use virtual destructors?](https://stackoverflow.com/questions/461203/when-to-use-virtual-destructors) — Phil Brubaker, May 21 '20 at 09:09

kesarling He-Him · Accepted Answer · 2020-05-21T09:32:55.217

This is the well known diamond problem in C++. This is how you solve it:

#include <iostream>

using std::cout;
using std::endl;

class Base1Itf {
public:
    virtual void blue() = 0;
    virtual ~Base1Itf() { }
};

class Base1Abs : virtual public Base1Itf {
public:
    void blue() override {
        cout << "blue" << endl;
    }
    virtual ~Base1Abs() { }
};

class DerivedItf : virtual public Base1Itf {
public:
    virtual void red() = 0;
    virtual ~DerivedItf() { }
};

class Derived : public Base1Abs, public DerivedItf {
public:
    void red() override {
        cout << "red" << endl;
    }
    virtual ~Derived() { }
};

int main() {
    DerivedItf* d = new Derived();
    d->red();
    d->blue();
    delete d;
    return 0;
}

It is also recommended to use virtual destructors in inheritance.

You see, what happens here is that classes Base1Abs and DerivedItf both inherit a copy of blue. Now, when you inherit another class Derived from these 2, that class inherits 2 copies of blue and then the compiler starts wondering which copy is to be called. So, you inherit the 2 classes virtually resulting in only one copy of blue getting inherited

Some interesting reading about the virtual inheritance: https://en.wikipedia.org/wiki/Virtual_inheritance — Joan Botella, May 22 '20 at 08:39
@JoanBotella, Learn from literally anywhere but wikipedia... trust me, I speak from experience... — kesarling He-Him, May 22 '20 at 09:27
I would suggest something like [this](https://en.cppreference.com/w/cpp/language/derived_class) for starters — kesarling He-Him, May 22 '20 at 09:28

C++ Using through an interface a class that inherits from several others

1 Answers1