C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 25:
25.1 How is this section organized?
25.2 I've been told that I should never use multiple inheritance. Is that right?
25.3 So there are times when multiple inheritance isn't bad?!??
25.4 What are some disciplines for using multiple inheritance?
25.5 Can you provide an example that demonstrates the above guidelines?
25.6 Is there a simple way to visualize all these tradeoffs?
25.7 Can you give another example to illustrate the above disciplines?
25.8 What is the "dreaded diamond"?
25.9 Where in a hierarchy should I use virtual inheritance?
25.10 What does it mean to "delegate to a sister class" via virtual inheritance?
25.11 What special considerations do I need to know about when I use virtual inheritance?
25.12 What special considerations do I need to know about when I inherit from a class that uses virtual inheritance?
25.13 What special considerations do I need to know about when I use a class that uses virtual inheritance?
25.14 One more time: what is the exact order of constructors in a multiple and/or virtual inheritance situation?
25.15 What is the exact order of destructors in a multiple and/or virtual inheritance situation?
[25.7] Can you give another example to illustrate the above disciplines?

This second example is only slightly different from the previous since it is more obviously symmetric. This symmetry tilts the scales slightly toward the multiple inheritance solution, but one of the others still might be best in some situations.

In this example, we have only two categories of vehicles: land vehicles and water vehicles. Then somebody points out that we need amphibious vehicles. Now we get to the good part: the questions.

  1. Do we even need a distinct AmphibiousVehicle class? Is it also viable to use one of the other classes with a "bit" indicating the vehicle can be both in water and on land? Just because "the real world" has amphibious vehicles doesn't mean we need to mimic that in software.
  2. Will the users of LandVehicle need to use a LandVehicle& that refers to an AmphibiousVehicle object? Will they need to call methods on the LandVehicle& and expect the actual implementation of those methods to be specific to ("overridden in") AmphibiousVehicle?
  3. Ditto for water vehicles: will the users want a WaterVehicle& that might refer to an AmphibiousVehicle object, and in particular to call methods on that reference and expect the implementation will get overridden by AmphibiousVehicle?

If we get three "yes" answers, multiple inheritance is probably the right choice. To be sure, you should ask the other questions as well, e.g., the grow-gracefully issue, the granularity of control issues, etc.