Multiple inheritence using parent class method override

Question

Let's say I've got the following class hierarchy, wherein I have an interface and then further specializations of that interface that guarantee more functionality:

class parent_interface {
 public:
  virtual void foo() = 0;
};

class child1_interface : public parent_interface {
 public:
  virtual void bar() = 0;
};

class child2_interface : public parent_interface {
 public:
  virtual void baz() = 0;
}

And then I have concrete classes that provide that functionality:

class parent : public parent_interface {
 public:
  void foo() override;
};

class child1 : public parent, public child1_interface {
 public:
  //using parent::foo; ??
  void bar() override;
};

The problem is if I try to instantiate a child1 object, I get a compiler error that

cannot declare field instantiation_name to be of abstract type because the following virtual functions are pure within child1: void foo()

I can fix this by creating a method within child1 called foo, that just calls parent::foo, but I'm wondering if there is a cleaner solution to this problem.

Answer 1

You need to specify virtual inheritance, so that the compiler knows to combine the base classes back into one. This should compile for you (see it work):

class parent_interface {
 public:
  virtual void foo() = 0;
};

class child1_interface : public virtual parent_interface {
 public:
  virtual void bar() = 0;
};

class parent : public virtual parent_interface {
 public:
  void foo() override
  {
  }
};

class child1 : public parent, public child1_interface {
 public:
  //using parent::foo; ??
  void bar() override
  {
  }
};

For more information, you can read about Virtual Base Classes here.