Derived class that inherits virtual class c++

Question

class vehicle{
...
};
class motorVehicle: virtual public vehicle{
...
};
class twoWheels: virtual public vehicle{
...
};
class motorcycle: public motorVehicle, public twoWheels, virtual vehicle{//(1)
....
};

(1) Why does class motorcycle has to inherit class vehicle, when it is already contained in classes motorVehicle and twoWheels? In the explanation from the book it is written that class motorcycle has to inherit class vehicle in order to make sure that the constructor of the base class (vehicle) will be called. If this explanation is correct, I don't understand why it says "virtual vehicle"? What type of inheritance is this?

Answer 1

First of all, the solution to the problem given, surely is a solution but in my humble opinion not the best.

This is to solve the so called diamond inheritance problem. For this it's important to understand virtual inheritance and the difference between public and private inheritance.

Public inheritance means the derived class can also be considered as and casted to the inherited class. In your particular case a motor vehicle can be casted to a vehicle. Had the inheritance been private. This would not have been possible.

So the same applies again to motorcycle. It can be considered twoWheels and to motorVehicle. But since both of those can be considered a vehicle a problem arises. What if you want to cast the motorCycle to a vehicle. Should it first cast to twoWheels, and then to vehicle. Or it first to motorVehicle and then to vehicle.

And that's not all, any variable or function declared in vehicle, can also be called in motorcycle. But should it call those of twoWheels and then to motorVehicle.

So in essence there are two vehicles being constructed and this creates an ambiguity. To solve this virtual inheritance is introduced. This allows you to initialize any class in the inheritance chain yourself. For this it's not necessary to inherit from the initialized class yourself.

And that's why i don't agree with the solution presented. It would be sufficient to write the motorcycle constructor like this:

motorcycle::motorcycle():
    vehicle(),//n.b. this order is Not relevant, vehicle will be constructed first. No matter the order in this list.
    twoWheels(),
    motorVehicle()
{



}