When is virtual inheritance a good idea?

Question

I'm making a game GUI API where each widget inherits from the Widget class. I was thinking, when others make there own widgets, they might not be fully satisfied with the base class. They might want to add getTheme() for example. Would it be a good idea to make all my widgets virtually inherit from Widget then so that this is possible?

Thanks

Answer 1

Just because the user would add their own methods to a child class doesn't mean you need to use virtual inheritance. You would use it if, in your library, you have a single base class with multiple children, and people could inherit from multiple child classes at once (for example mixin rather than substitution).

Answer 2

To resolve a diamond-shaped inheritance problem. (B and C both inherit from A. What happens to A's attributes in D that itself inherits from B and C?)

A client of your library could see a RedWidget and a FlyingWidget, and might want to combine them into a RedFlyingWidget.

User would have to specify one of the base classes to be virtual when inheriting. But that is not responsibility of a library maker.

OOP flows better with single-implementation inheritance, so that's what I'd use throughout a library.

There are also "upside-down inheritance" trees, as described by Alexandrescu's excellent "Modern C++ Design." They allow clients to pull in more functionality in a form of mix-ins that are called policies.

Programming with policies allows for greater ability to combine functionality, at the expense of syntactical cleanliness. Think STL implementation, for example.

Answer 3

When is virtual inheritance a good idea?

That's a design question.

For your Widgets, I would say Yes, multi-derived classes should have the option to be just 1 Widget.