A use for multiple inheritance?

Most uses of full scale Multiple inheritance are for mixins. As an example:

class DraggableWindow : Window, Draggable { }
class SkinnableWindow : Window, Skinnable { }
class DraggableSkinnableWindow : Window, Draggable, Skinnable { }

etc...

In most cases, it's best to use multiple inheritance to do strictly interface inheritance.

class DraggableWindow : Window, IDraggable { }

Then you implement the IDraggable interface in your DraggableWindow class. It's WAY too hard to write good mixin classes.

The benefit of the MI approach (even if you are only using Interface MI) is that you can then treat all kinds of different Windows as Window objects, but have the flexibility to create things that would not be possible (or more difficult) with single inheritance.

For example, in many class frameworks you see something like this:

class Control { }
class Window : Control { }
class Textbox : Control { }

Now, suppose you wanted a Textbox with Window characteristics? Like being dragable, having a titlebar, etc... You could do something like this:

class WindowedTextbox : Control, IWindow, ITexbox { }

In the single inheritance model, you can't easily inherit from both Window and Textbox without having some problems with duplicate Control objects and other kinds of problems. You can also treat a WindowedTextbox as a Window, a Textbox, or a Control.

Also, to address your .anotherClass() idiom, .anotherClass() returns a different object, while multiple inheritance allows the same object to be used for different purposes.

I find multiple inheritance particularly useful when using mixin classes.

As stated in Wikipedia:

In object-oriented programming languages, a mixin is a class that provides a certain functionality to be inherited by a subclass, but is not meant to stand alone.

An example of how our product uses mixin classes is for configuration save and restore purposes. There is an abstract mixin class which defines a set of pure virtual methods. Any class which is saveable inherits from the save/restore mixin class which automatically gives them the appropriate save/restore functionality.

But they may also inherit from other classes as part of their normal class structure, so it is quite common for these classes to use multiple inheritance in this respect.

An example of multiple inheritance:

class Animal
{
   virtual void KeepCool() const = 0;
}

class Vertebrate
{
   virtual void BendSpine() { };
}


class Dog : public Animal, public Vertebrate
{
   void KeepCool() { Pant(); }
}

What is most important when doing any form of public inheritance (single or multiple) is to respect the is a relationship. A class should only inherit from one or more classes if it "is" one of those objects. If it simply "contains" one of those objects, aggregation or composition should be used instead.

The example above is well structured because a dog is an animal, and also a vertebrate.

Most people use multiple-inheritance in the context of applying multiple interfaces to a class. This is the approach Java and C#, among others, enforce.

C++ allows you to apply multiple base classes fairly freely, in an is-a relationship between types. So, you can treat a derived object like any of its base classes.

Another use, as LeopardSkinPillBoxHat points out, is in mix-ins. An excellent example of this is the Loki library, from Andrei Alexandrescu's book Modern C++ Design. He uses what he terms policy classes that specify the behavior or the requirements of a given class through inheritance.

Yet another use is one that simplifies a modular approach that allows API-independence through the use of sister-class delegation in the oft-dreaded diamond hierarchy.

The uses for MI are many. The potential for abuse is even greater.

Java has interfaces. C++ has not.

Therefore, multiple inheritance can be used to emulate the interface feature. If you're a C# and Java programmer, every time you use a class that extends a base class but also implements a few interfaces, you are sort of admitting multiple inheritance can be useful in some situations.