Inheritance -- multiple and virtual inheritance

Part of C++ FQA Lite. To see the original answers, follow the FAQ links.

This section is about multiple inheritance. While inheritance and virtual functions are among the most useful (that is, the least useless) C++ features, C++ multiple inheritance is at the other end of the spectrum.

[25.1] How is this section organized?

FAQ: The FAQ section contains both "high-level" issues (the meaning & purpose of the language constructs) and "low-level" issues (the detailed semantics & implementation of the language constructs), in that order. Be sure to understand the high-level stuff before delving into the low-level stuff.

FQA: This FQA section is organized exactly like the other FQA sections - by copying the structure of the FAQ sections.

The FAQ's note about the two levels of discussion is equally applicable to all C++ features/programming languages/software systems/formal definitions in the world. But it's located here and not elsewhere for a reason. The reason is that C++ multiple inheritance makes very little sense even by C++ standards. In other words, the cases where you can express a useful high-level idea using the actual features used to implement multiple inheritance in C++ are rare. So people end up not using it, or abusing it, but fail to use it "right", because it's unclear what "right" means in this context.

The FAQ apparently believes that this situation can be fixed by explaining what "right" means. Let's sit back and watch.

[25.2] I've been told that I should never use multiple inheritance. Is that right?

FAQ: THESE PEOPLE REALLY BOTHER ME!! How can they know what you should do without knowing what you want done?!?!

FQA: This rage may provoke some sympathy. We've all met people who don't seem to be doing anything useful themselves and compensate for it by getting in others' way and telling them how to do their job, their ultimate goal apparently being to stop any useful work around them. In particular, language features which should really never be used are rare. However, language features which should be used really rarely and with careful consideration are more common, and C++ multiple inheritance is one of them.

There are two kinds of problems with multiple inheritance - "static" and "dynamic".

The "static" problems have to do with compile-time name lookup. Suppose you derive a class C from classes A and B, and both have a method called name. Which one will get called when someone calls name using a pointer to a C object? If you override name in the class C, did you override A::name, B::name or both? What happens if the two name functions accept arguments of the same type? What happens if they don't?

The "dynamic" problems have to do with the way objects of the class C are actually built, and the run time effects related to it. Basically, a C object will contain an A sub-object, and a B sub-object, and those two will be completely unrelated. So if you have a pointer to a C object, and you (silently) upcast it to A*, and write code assuming that you get an object which is derived from both A and B, and cast the A* to B*, the program will crash or worse. What you should have done is first cast the object to C* and then to B*, since without knowing the definition of C, there's no way to figure out the location of the B sub-object given the location of the A sub-object. This is one trap even a pretty experienced C++ programmer can fall into.

You can memorize the sharp edges, or you can stay away from the whole thing. Pick your poison.

[25.3] So there are times when multiple inheritance isn't bad?!??

FAQ: Sure! Sometimes (but not always) using multiple inheritance will lower all kinds of costs. If this is so in your case, use it! If it isn't, don't blame multiple inheritance - "good workmen never blame their tools".

FQA: No, there are no such times, it's a poorly designed language feature. But it could be that sometimes the other options available in C++ are even worse.

While we're at it, let's clarify the whole blame issue. If you choose to use a tool not suitable for your job, you shouldn't blame the tool. Well, actually, you should blame the vendor of the tool if it was advertised as something it wasn't. Was C++ ever advertised that way, for example, what about support for object-oriented programming?

Well, we don't even need to discuss that, because a programming language is not exactly a tool. It is more accurately described, well, as a language. The key difference between tools and languages in the context of "blame" is choice. You probably don't choose to speak English - you do so in order to communicate with all the other people speaking English. When a bunch of people do something because other people do it, too, it's called "network effects". For example, if you want to work on a project for reasons having nothing to do with computer linguistics, and the project uses C++, you'll have to use C++, too. No choice.

So that's the difference between a language and a tool. Still, you wouldn't blame English because it's so hard to learn or inconsistent or whatever, would you? Well, the difference between a programming language and a natural language is that the latter is, um, natural, so there's nobody to blame, while the former was actually designed by someone (well, usually). The other difference is the cost of an error. People usually recover from bad English, computers tend to be less tolerant.

And this is why you seem to have more ground to "blame the language" than to "blame the tools" in the general case. Of course you may not like the whole attitude of "blaming" things, etc.; everybody is free to feel any way they feel like or something. But that has nothing to do with being a "good workman" (which itself has an irksome sound to it, mind you).

[25.4] What are some disciplines for using multiple inheritance?

FAQ: There's a long answer saying 3 things. First, you should normally do it to achieve polymorphism, not base class code reuse. Second, the classes you multiply inherit from should normally be pure abstract. Third, you should consider using the "bridge pattern" or "nested generalization" - alternatives to MI described below.

FQA: The guidelines are pretty good, except for maybe "nested generalization", which is really a way to work around the deficiencies of the C++ object system rather than a reasonable way to model anything.

If you like "disciplines" without any reasoning having to do with the actual problem at hand, here's some more for you. The FAQ's guidelines are a special case of "don't use designs which would only work in one programming language" (footnote: especially if that language is C++). Specifically, the FAQ's guidelines pretty much summarize the rules for using multiple inheritance in Java, so your design would be implementable in at least two languages, which is a good sign. The reasoning behind the avoid-designs-tied-to-one-language rule is that if something is really good, many languages would have it, and if your design depends on something only available in one language, it's probably bad because it probably depends on a bad thing. This is the point where people loving the unique feature of language X scream that this reasoning is completely moronic, but we already knew that, because we promised our reasoning wouldn't refer to the specific problem at hand, which isn't very bright by itself.

If you're into real reasoning and not just "disciplines", one nice thing about having the base classes pure is that this way, you don't have to think about a whole class of questions related to reimplementation of methods. For example, if you inherit a RectangularWindow from a Rectangle and a Window, and Rectangle isn't pure and it has a perfectly good resize method, is this method still good for RectangularWindow or do you want it to resize the window, which the implementation in the base class obviously won't do? And what if you can't really override the Rectangle::resize method because it isn't virtual? The problem with reusing code from the base class is that multiple inheritance frequently breaks that code.

However, following these guidelines won't necessarily eliminate the problems with multiple inheritance mentioned above.

[25.5] Can you provide an example that demonstrates the above guidelines?

FAQ: There's a very long discussion of an example with different kinds of vehicles having different kinds of engines. The FAQ proposes to use MI, or the "bridge pattern" or "nested generalization". "Bridge pattern" means that vehicle objects keep pointers to engine objects, and users can pass many different kinds of engine to an object of the same vehicle class. "Nested generalization" means that you have many classes derived from Vehicle (like Plane), and then for each such derived class there's a bunch of classes derived from it to represent the different kinds of engine (like OilPoweredPlane). Trade-offs are discussed in great detail.

FQA: The "bridge pattern" (a fancy name for the special case of aggregation when your member object has virtual functions) looks good here, since, um, a vehicle has an engine and stuff. And hence multiple inheritance looks wrong, since an OilPoweredPlane isn't a kind of an OilPoweredEngine.

I don't feel like arguing with the FAQ's lengthy statements, since the issue isn't worth it. The cases when you deal with the definition of non-trivial object models are relatively rare. And when you do it, you have enough time to consider what stuff you really want the model to support, and then think about the different possibilities to define the model and check if each possibility really supports that stuff. I think that trying to memorize special cases (call them "patterns" or whatever) of object models is basically like trying to formalize common sense, which doesn't really work.

Are you still with me after this blasphemy? Then let's look at one non-problem mentioned by the FAQ - the fact that with aggregation ("bridge pattern"), you can't specialize algorithms such that a specific combination of vehicle and engine exhibits a special behavior. In languages which support multimethods, doing that is trivial (multimethods are like virtual functions, but they are dispatched at run time based on the types of all arguments, not just the first argument). And in C++, you can emulate multimethods using double dispatching (ugly, especially when it becomes triple, quadruple and other such kinds of dispatching, but still possible).

[25.6] Is there a simple way to visualize all these tradeoffs?

FAQ: Here's a matrix with cute smilies for ya. Just don't apply it naively.

Cute matrix omitted to avoid copyright problems, as well as cuteness problems

FQA: WARNING: there's no known way to represent common sense in a tabular form at the time of writing. Therefore, if you choose to store the cute matrix anywhere in your brain, you do it at your own risk.

[25.7] Can you give another example to illustrate the above disciplines?

FAQ: Yes - consider the case with land & water vehicles, when you also need to support amphibious vehicles. This case is more "symmetric" than the previous example, so multiple inheritance becomes more preferable. Still, you have to make sure you really want it by asking various questions (for the list of questions, follow the link to the FAQ).

FQA: Um, "symmetry" is an interesting aspect of this to focus on, but anyway, an amphibious vehicle is both a land vehicle and a water vehicle, while an oiled powered plane is a plane, but is not an oil powered engine. So yes, multiple inheritance seems more appropriate, and yes, it's wise to think about the things you ultimately want your object model to support before defining it.

We'll use the opportunity to show how to model this problem effectively using multiple inheritance (implementation of multiple interfaces by the same class) without really using C++ multiple inheritance (and thus avoiding some of its problems). I'm not saying that this always better than real C++ multiple inheritance, just that it sometimes can be.

class AmphibiousVehicle {
  class WaterVehicleImpl : public WaterVehicle {
    WaterVehicleImpl(AmphibiousVehicle* p) { /* save p */ }
  // similarly, there's a LandVehicleImpl class derived from WaterVehicle
  WaterVehicleImpl _water;
  LandVehicleImpl _land;
  AmphibiousVehicle() : _water(this), _land(this) {}
  WaterVehicle& getWaterVehicleIF() { return _water; }
  LandVehicle& getLandVehicleIF() { return _land; }

This way, you write more code than with multiple inheritance, which is bad. It gets even uglier if you want to simulate virtual inheritance, which is bad if WaterVehicle and LandVehicle inherit from a non-abstract base class Vehicle (not necessarily recommended by itself). And you have to call get functions instead of implicit upcasts, which may be considered good or bad. And there are no problems such as collisions between names of the members of the base classes (which is good).

[25.8] What is the "dreaded diamond"?

FAQ: It's when there are circles in the inheritance graph. Here's the simplest case: Derived1 and Derived2 are inherited from Base, and Join is inherited from both Derived1 and Derived2. The circle in the graph may look like a diamond if your imagination works that way.

The problem is that Join objects have two Base sub-objects, so each data member is kept twice. Which is why the diamond is called "dreaded".

The resulting ambiguities can be resolved. For example, when you have a Join object and refer to its _x variable inherited from Base, you can tell the compiler which one you mean using Derived1::_x or Derived2_::x. When you upcast from Join* to Base*, you can pick one of the two Base sub-objects by first casting the pointer to Derived1* or Derived2*. But this is almost always not the right thing to do. The right thing to do is usually to tell the compiler to keep a single sub-object.

FQA: Most C++ programmers out there don't understand why would anyone say (Derived1*)pJoin in a context where a Base* is expected. This by itself is a good reason to avoid having two sub-objects of Base in Join.

If you feel that things are getting pointlessly complicated at this point, it may be an indication of good taste.

[25.9] Where in a hierarchy should I use virtual inheritance?

FAQ: At the top of the dreaded diamond - when you derive from Base, you should say:

class Derived1 : public virtual Base { ... };
class Derived2 : public virtual Base { ... };

Note: when you define Join, you can't convince the compiler to keep a single Base sub-object - it will do whatever the definitions of Derived1 and Derived2 tell it to do. That is, when you define the classes derived from Base, you must plan ahead to support circles in the inheritance graph.

FQA: Let's put aside the question whether the support for both options - one and two Base sub-objects - is a good thing, and concentrate on the way C++ gives you to choose between these options. Doing it "at the top of the diamond" is annoying, because you have to think about the entire hierarchy when you define the classes close to its top. That is, either all derived classes will have several Base sub-objects or all of them will have one (forcing the users and the implementers of derived classes to deal with the problems of virtual inheritance).

In general, it is a special case of the generic C++ principles of specifying everything in terms of types and their attributes, as well as having the user deal with the low-level details related to underlying language feature implementation.

[25.10] What does it mean to "delegate to a sister class" via virtual inheritance?

FAQ: If you have a diamond-like hierarchy with virtual inheritance, and Base has two virtual functions f and g, then Derived1 can implement f, Derived2 can implement g and Derived1::f can call Derived2::g by simply saying g(); or (more verbosely and equivalently) Base::g(); - that is, without knowing anything about the existence of Derived2.

This is a "powerful technique".

FQA: "Powerful". What exactly can you do this way that can't be done equally well or better in ways more clear to the average developer?

What's that? You say that you only care about the enlightened wizards (variant: the set of wizards consists of a single person - yourself), not the mediocre droids from the rank-and-file? Well, I'll leave the interesting discussion of your personality aside. I'll leave it aside in order to point out that some of the people whose programming abilities I admire can't be bothered to learn the quirks of C++ anywhere near my level. My level, in turn, isn't itself anywhere near "complete" knowledge of this wonderful language.

[25.11] What special considerations do I need to know about when I use virtual inheritance?

FAQ: Usually virtual inheritance is a good idea only if the virtual base class and classes derived from it have little or no data.

BTW, even if they have no data at all, using virtual inheritance can still be better than non-virtual inheritance. For example, if you have two Base sub-objects (with no members), you can end up with two pointers to the different sub-objects, and comparing them would tell you that these are two different objects, which they aren't, at some level. Quote: "Just be careful - very careful".

FQA: Yeah. Be vewy, vewy caweful...

The FAQ's advice is a special case of its other advice about inheritance - data in base classes interacts badly with MI. And as the FAQ correctly points out, not having data in base classes doesn't solve all of the problems.

[25.12] What special considerations do I need to know about when I inherit from a class that uses virtual inheritance?

FAQ: Derived classes call the constructors of their virtual base classes directly. In particular, when a virtual base class has no default constructor, you have to call its constructor explicitly in the initialization lists of the constructors of the derived class.

If the base class follows the FAQ's advice about not having data in virtual base classes, then the base class probably has a trivial default constructor and you don't have to care about the issue when you define derived classes.

FQA: If the base class follows the FQA's advice to avoid non-trivial constructors and use initialization functions when needed, you don't have to worry about initialization of derived classes with virtual base classes, either.

[25.13] What special considerations do I need to know about when I use a class that uses virtual inheritance?

FAQ: Don't use downcasts using the C-like syntax (Derived*)pBase. Use dynamic_cast<Derived*>(pBase).

The answer is unfinished according to a "TODO" remark in it.

FQA: The problem seems to be that with virtual inheritance, the offset that must be added to pBase to make it pDerived depends on the classes derived from Derived (like the Join class from the "dreaded diamond" example). So the compiler can't generate code adding a constant offset, which is what C-style casts do when it comes to class hierarchies (upcasting and downcasting).

Why does the code silently compile to a wrong program, despite the fact that C++ already forced us to inform the compiler that we have virtual inheritance at the definitions of the classes involved in the cast operation (not the definition of the Join class which isn't necessarily visible at the context where the cast operation is compiled)? Why doesn't the compiler produce an error message or generates correct code as if we used dynamic_cast?

The answer is simple: in C++, the compiler compiles random meaningless things because it can't be bothered not to.

[25.14] One more time: what is the exact order of constructors in a multiple and/or virtual inheritance situation?

FAQ: So and so.

FQA: I don't want to summarize it, because why would anyone want to know that? Well, except maybe to suppress stupid compiler warnings about the orders of things in initialization lists not matching the actual order of construction.

Well, why would you use initialization lists?

[25.15] What is the exact order of destructors in a multiple and/or virtual inheritance situation?

FAQ: The reverse order of construction.

FQA: Right. But you probably shouldn't write code that depends on these things. Your colleagues may get annoyed.

Copyright © 2007-2009 Yossi Kreinin
revised 17 October 2009