Inheritance -- what your mother never told you

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

Note: section names are copied verbatim from the FAQ. The document is not based on the assumption that it was your mother who told you the other things about inheritance.

[23.1] Is it okay for a non-virtual function of the base class to call a virtual function?

FAQ: Sometimes it is. Suppose you have a class Animal with a non-virtual getAwfullyExcited() method. In your system, all animals do it similarly:

void Animal::getAwfullyExcited()
  makeExcitedNoises(); // all animals make different noises
  cout << "And here comes the dance!" << endl; // all animals always warn people before they dance
  danceExcitedly(); // all animals dance differently

So there you have it - an orderly hierarchy of animals, each getting excited according to the standard procedure in its own unique way.

FQA: The trouble with this whole thing in C++ is that you can't easily tell which functions can be overridden and which can't, and what "override" means.

The default is "can't" - you have to explicitly say virtual to enable overriding. Unless you derive your class from a base class having virtual functions - when you override those functions, the virtual keyword becomes optional. But even if the function in the base class is not virtual, you still can override it - except that now the binding is static: if you call a function through a pointer statically typed as Base*, the base class implementation is called, but if you call it through a Derived*, the derived class implementation is called. So it's a different kind of "overriding". And naturally you can have more than one link in the chain of derived classes.

Back to our question, it is generally OK to have a common base class implementation call functions defined in the derived classes - in many cases that does exactly what you want. However, in the specific case of C++ virtual functions it may become hard to figure out which functions are virtual and which are not.

So it's sometimes better to separate Animal (having only pure virtual functions) and EmotionalBehavior (having methods taking an animal and calling its functions to implement various common rituals). No, it's probably not a good idea to add this rule to your local Coding Conventions Document - if the class is relatively small and simple, there's no point in creating more code by splitting it to several classes. The thing is that C++ classes with many methods, some virtual and some not, especially when they are part of complex hierarchies with lots of overriding of both kinds, end up making people wonder why this piece of code was called. Why, why, why?..

[23.2] That last FAQ confuses me. Is it a different strategy from the other ways to use virtual functions? What's going on?

FAQ: Yes, there are two different strategies related to the use of virtual functions. In the first case, you have a common non-virtual method in the base class, but use virtual methods to implement the parts which do differ. In the second case, you have virtual methods implemented differently in derived classes, but they call common non-virtual methods in the base class. These common bits can also be implemented somewhere else - not necessarily in the base class.

Sometimes you use both strategies in the same class - one for some of the methods, the other for other methods. That's OK, too.

FQA: Well, sort of, yeah. Is it just me or did we really learn nothing at all? This lengthy discussion follows quite directly from first principles. virtual functions allow different derived classes to implement a method differently. All functions, including non-virtual, can be called once or more from various places. Does something totally obvious from these rules deserve the pompous name "strategy"?

No, it's not just the FAQ. This is very common in the "software engineering" community, especially in the C++ subculture. Let's look at an example. You've probably seen methods that create new objects of classes derived from a common base. Such methods are useful because their caller doesn't have to be aware of the different derived classes. For example, you can have a createMovieReader(filename) method that checks the file type and creates an MPEGReader or an AVIReader or whatever, and whoever calls createMovieReader doesn't have to care about the many different kinds of readers. Is this worth a special term accompanied by a whole discussion? Well, it has a term - it's the Factory Method Design Pattern. Sometimes you have an abstract base class with nothing but Factory Methods. This has a name of its own, too - it's the Abstract Factory Design Pattern.

The names used by people reveal a lot about them. For example, the people living close to North Pole have a two-digit number of names for "snow". Clearly, the reason is that their visual diet is composed primarily of snow, so they know a lot about the different kinds of snow and never confuse them. But trees - those they come across once in a lifetime. No point in having many names for trees. A tree, you know, that dark, high thing with messy stuff sticking out.

This tells us something about the intellectual diet of people calling trivial combinations of basic language constructs "strategies" and "patterns". Of course these people love C++ - look at all those different string classes, and all those ways to implement still new ones! So much snow to play with.

Disclaimer: my knowledge of anthropology is approximately zero. Therefore, it's better to consider the North Pole example above to be hypothetical than to make critical decisions assuming it's literally true.

[23.3] Should I use protected virtuals instead of public virtuals?

FAQ: Well, first of all avoid strict rules saying "never do this, always do that". As a rule of thumb, experience tells that most of the time virtual functions are best made public, with two notable exceptions. First, there are functions which are supposed to be called only from the base class, the way described in the previous two FAQs. And second, there's the Public Overloaded Non-Virtuals Call Protected Non-Overloaded Virtuals Idiom:

class Base
  void overloadingTotallyRules(int x)   { butWeDontWantToOverloadVirtualFunctions_int(x); }
  void overloadingTotallyRules(short x) { butWeDontWantToOverloadVirtualFunctions_short(x); }
  virtual void butWeDontWantToOverloadVirtualFunctions_int(int);
  virtual void butWeDontWantToOverloadVirtualFunctions_short(short);

This solves an important problem: overloading totally rules, but we don't want to overload virtual functions. The Public Overloaded Non-Virtuals Call Protected Non-Overloaded Virtuals Idiom makes life easy for class users and the implementors of derived classes! Clearly you, the author of the base class, absolutely must clobber your code with this nonsense to make everyone's life better! Think about the reduced maintenance costs and the benefit of the many, you selfish code grinder.

FQA: Here's a simple answer to your question, trivially following from the definitions and without the need to create a taxonomy of rules and exceptions. C++ access control is for clarity: you want to make it clear which parts of the class are supposed to be used and which are just implementation details. This can make it easier to understand your code and prevents people from using the bits that you might want to change later.

Therefore, if you don't think that anyone outside of the class hierarchy is going to use a virtual method, make it protected. Otherwise, make it public. In particular, you can start with protected and change it to public if it turns out you were wrong. That's all. Why is any special case of this particularly interesting?

As to the One Kind Of Function Calls Another Kind Of Function Idiom - remember the snow example? Well, now we are deep inside a pile of snow, coining names for each individual snowflake. I wonder if there's a Function Is Passed A Parameter To Configure Its Operation Idiom. No, there probably isn't - after all, it's not Object-Oriented.

More importantly, the "idiom" is a non-solution for the problems with overloading. All we got is extra layers of code piling up. Ultimately, the people will have to read this code to figure out what the program does, and the layers of indirection adding no functionality at all will confuse them and occupy their short-term memory instead of other, actually useful details. Why not simply avoid overloading in this case?

Talking about "reducing maintenance costs" doesn't by itself actually reduce any maintenance costs, you know.

[23.4] When should someone use private virtuals?

FAQ: Probably never. It confuses people, because they don't think private virtuals can't be overridden, but they can.

FQA: The FAQ is right - the behavior of private virtual is ridiculous; it compiles, but for the wrong reasons. Note that there are many, many more things in C++ that primarily confuse people. One excellent reason to avoid C++ altogether. I'm not joking. Nothing funny about it. Well, maybe it is funny that C++ developers are confused for a living, but that's a cruel kind of humor.

[23.5] When my base class's constructor calls a virtual function on its this object, why doesn't my derived class's override of that virtual function get invoked?

FAQ: Suppose you have a base class called Base, calling a virtual function f in its constructor. Then, when objects of a derived class called Derived are created, Base::virt is called from Base::Base, not Derived::f.

The reason is that when Base::Base executes, the object is still of type Base. It only becomes an object of type Derived when the code in Derived::Derived is entered. If you wonder why C++ works this way, consider the fact that Derived::f could access uninitialized members of the class Derived if it could be called from Base::Base, which runs before Derived::Derived initializes the members of Derived.

Luckily, C++ doesn't let this happen, preventing subtle errors!

FQA: Here's what actually happens. Derived::Derived calls Base::Base, and then it sets the vptr to point to the Derived vtable. Setting the vptr before calling Base::Base wouldn't work, because Base::Base sets the vptr to point to the Base vtable. Base::Base doesn't know that it's called in order to ultimately initialize a Derived object; the code of Base::Base and Derived::Derived is compiled separately (correction). That's what "the object is still of type Base" really means.

Now, there's a valid argument against describing the mechanisms of the implementation in order to explain the behavior of a system. The system is built by people for other people. Its behavior is supposed to make sense. If it doesn't, and can only be explained by looking into the implementation instead of the needs of the user, it's a problem in the system. Sometimes such problems seem to be inevitable. But whenever possible, it's best to discuss why a certain behavior is reasonable in the situation as perceived by the user, and only talk about the implementation when absolutely necessary. However, this argument is rarely applicable to C++.

First, C++ makes very little sense from the user's perspective. It only makes sense from the perspective of the language designer, provided that several axioms of questionable value are added to it (such as "a language must look compatible with C, although it doesn't have to really be compatible", "all built-in types should be those found in C, unless they are a new kind of pointer", etc.). For example, it is reasonable that Base::Base can't call Derived::f (well, sort of - it depends on what you think about the way C++ handles object construction in general). But is it reasonable that Base::Base can call a virtual method Base::f by simply saying f()? How often is that what you want? This question is irrelevant, because in C++, if something becomes technically possible as a side-effect of some language mechanism, it tends to be legal. For example, why does private virtual from the previous FAQ compile? The answer is simple: why not?

The second reason to describe behavior in terms of implementation is that run time errors cause C++ programs to crash in ways undefined at the semantical specification level. It is theoretically possible to read the entire code of the program and find the semantically illegal code. In practice, you'll have to find the error by looking at the execution of the program (when you are lucky) or at a snapshot of its final state before the death (when you're not), and trying to understand how things can behave this way. And you can't do that without understanding the implementation.

[23.6] Okay, but is there a way to simulate that behavior as if dynamic binding worked on the this object within my base class's constructor?

FAQ: Yes. It's an Idiom, of course. Namely, the Dynamic Binding During Initialization Idiom.

One option is to have a virtual init function to be called after construction. Another option is to have a second hierarchy of classes, which doesn't always work, but... (sorry, I couldn't make it through all the code listings. If you like lots of hierarchies of classes and find this solution interesting, please follow the link to the FAQ's answer).

The first approach has the problem of requiring an extra function call upon initialization. We can rely on the self-discipline of the programmers (the self-discipline is especially important when exceptions can be thrown by init - make sure you release the allocated object properly), or we can wrap the construction and the init call in a single create function returning a pointer to the object. The latter rules out allocation on the stack.

FQA: We seem to be making progress. Let's see: we got rid of the allocation on the stack, so we no longer need to know the private members of classes. And if we had garbage collection, exception safety would no longer be a problem, either. Too bad C++ classes and memory management can't be changed. Or can they?

How about trying another programming language?

[23.7] I'm getting the same mess with destructors: calling a virtual on my this object from my base class's destructor ends up ignoring the override in the derived class; what's going on?

FAQ: Again, you're being protected by the compiler. Protected from yourself! You could access members that were already destroyed if the compiler let you do what you want.

When Base::~Base is called, the type of the object is changed from Derived to Base.

FQA: Thanks for the protection. At least the behavior is symmetrical.

Protect me from access to destroyed objects through dangling references in the general case next time, will you?

[23.8] Should a derived class redefine ("override") a member function that is non-virtual in a base class?

FAQ: You can do that, but you shouldn't.

Experienced programmers sometimes do that for various reasons. Remember that the user-visible effects of both versions of the functions must be identical.

FQA: Here's a trade secret: not so experienced programmers do that, too. For the umpteen time: why does this compile? This particular case doesn't seem to be an unintended side effect; it's a feature with elaborate design (there's all this nonsense with using names from the base class shadowed by functions with the same name, etc.).

The redefinition-looking-like-an-override is overloading on steroids: it's even less useful and has even higher obfuscation potential. And it's only one of the zillions of various name binding rules - the bits of C++ making it impossible to decipher what code is actually called by f(x).

[23.9] What's the meaning of, Warning: Derived::f(char) hides Base::f(double)?

FAQ: What do you think it means? You're going to die.

It's like this: Derived::f(char) doesn't override Base::f(double) - it hides it. In other words, it makes it impossible to call Base::f(double) via a Derived*. At the same time, Derived::f(char) can not be called via Base*.

FQA: "You're going to die", warns the FAQ. Well, according to a popular theory, all of us are. However, this interesting fact doesn't seem to belong here. C++ surely is depressing, but I'm not aware of any data showing a causal relationship between using C++ and suicide or lethal brain damage.

No, really, why is hiding a name a lethal bug? Sure, it makes the program more obscure, but, duh, this is C++. The worst thing that can happen is that someone will call Base::f with a char and Base::f(double), not Derived::f(char) will get called. So what? Similar things happen with overloading without any inheritance involved. Do you really know the implicit conversion rules between different types, and can predict what happens when you pass an int to an overloaded f function which only has a char and a double version? I bet you can't do it when things get a little bit more complicated. No normal person can.

C++ overloading is a nightmare. Using it is naive or stupid, depending on one's experience. Many languages, both statically and dynamically typed, don't have compile time overloading based on argument types. Have you ever heard a programmer using such a language complain about it, or become very enthusiastic about C++ overloading? What problem does overloading solve? OK, suppose it improves clarity (a very questionable claim). Do you really think that overloading should come together with a ton of implicit conversion rules, and a type system with a ton of special cases (cv-qualifiers, pointers/arrays/references, single & multiple inheritance, templates...)? When overloading does come with the extra ton of rules, does it still make the program more clear? What's that? "Compile time polymorphism", you say? You mean C++ templates? I see what you're up to. Happy debugging, pal.

[23.10] What does it mean that the "virtual table" is an unresolved external?

FAQ: Well, as you know, "unresolved external" means that there's a function or a global variable that your code declares and uses, but never defines. A virtual table is a global variable declared implicitly by a typical C++ implementation for each class with at least one virtual function.

Now, normally when you forget to define a virtual function, you'll get an "unresolved external" error saying that this function is missing. But in many implementations, if you forget the first virtual function, the whole virtual table will become "unresolved". That's because these implementations define the virtual table at the translation unit where the first virtual function is implemented.

FQA: GAAA!! So THAT'S why you have to define static class variables explicitly in your .cpp file, but don't have to define vtables explicitly! This is sooo STUPID! I'm shocked. Wait a second. Let me recover.

OK, here's how it works. In C++, there's really no such thing as "a header file defining the class" and "a .cpp file implementing its functions". It's just a convention. According to the rules, the definition of the class members can be spread across several "translation units", and each "translation unit" can in turn be spread across several files #including each other. This is inherited from C, and interacts badly with the new features in C++.

For example, consider those virtual functions. You probably need a table of those, which is similar to a global C variable. Of course you don't want to have the C++ programmer implement the table manually, the way they'd do it in C - or else what's the point of the virtual keyword? But where should the vtable be implemented? In the .cpp file of a class? Where's that?

With vtables, there's a "solution". After all, we don't need a vtable unless we have some virtual functions, do we? Well, then, one of these functions has to be the first one. Why not place the vtable near the definition of that function? This is a good place, because it won't get #included twice (or the user will get a "multiple definition" error anyway).

And then there are the static class variables. Where do we stuff those? Well, um, there seems to be no good place at all; a class could consist of a single static member. No anchor in the sea of source code floating in the file system. Oh, well, we'll have the user choose a place for a duplicate Type Class::member_name; variable definition. A little bit more typing is not that bad.

I wonder what they do with virtual functions implemented in the body of a class though. I always wondered about those. Of course they can generate many vtables and throw away all copies but one the way they do with templates. But that won't work with a linker only supporting C. Maybe they didn't allow to implement non-inline functions in the body of a class before the brave decision to add features to the linker. Anyone knowledgeable about the history of the subject is welcome to enlighten me. However, the knowledge of the history of the subject can't possibly make this whole business seem less stupid to a language user.

[23.11] How can I set up my class so it won't be inherited from?

FAQ: You can have the constructors private. The objects will have to be created by public functions delegating to the constructor.

Alternatively, you can comment the fact that you don't want the class to be inherited from: // if you inherit from this class, I'll hunt you down and kill you.

There's a third solution (WARNING - this one can rot your brain): you can inherit your class from a private virtual base class, which has a private constructor and declares your class a friend. This way, if someone tries to inherit from you, they'll need to directly call the base class of the constructor, which won't compile, since the constructor is private. This can add an extra word of memory to the size of your objects though.

FQA: C++ doesn't have final. It probably isn't a big deal, especially considering the fact that if it did have final, it would be little more than a comment, just like private is little more than a comment. So the comment-instead-of-a-keyword is probably the best approach.

The other approaches yield cryptic compile time error messages. Many people in the C++ community believe that a cryptic compile time error message is a good thing. This is probably reasonable if you only care about theory ("early fault detection is good"), and ignore practice (easy fault detection is good, and running a program should be easier than deciphering strongly encrypted C++ compiler error messages, which tend to come in large cascades).

Having no real experience with other languages helps one to stick to this bizarre point of view. C++ compiles very slowly, so people who never worked with anything else don't think that running a program is easy. C++ code is also hard to debug, so people develop a fear of run time errors.

[23.12] How can I set up my member function so it won't be overridden in a derived class?

FAQ: Use a /* final */ comment instead of a final keyword. It's not a technical solution - so what? The important thing is that it works.

FQA: So why does C++ have private? It's nothing but a comment recognized by the compiler.

The real answer is as follows. If a function is not virtual, it shouldn't be overridden, as the FAQ itself has already explained. So the lack of a virtual keyword is effectively equivalent to a final keyword. Now if C++ didn't allow to override non-virtual methods, and if it would consistently require to use the virtual keyword in all virtual function declarations, it would be pretty clear which functions shouldn't be overridden in most cases.

If you look for non-technical solutions to overriding problems, consider banning non-virtual overrides in your organization. I didn't give this one a deep thought; quite frequently when you try to "fix" C++ by banning some of its features, but not others, your rules backfire. And I can't know whether coding conventions work in your organization at all (believe me, there are huge and very disciplined organizations where coding conventions are not really followed, and their primary effect is programmers feeling anger or guilt; another nice option is people mindlessly following "best practices", wreaking havoc in an orderly way). All I'm saying is that non-virtual override seems both useless and largely independent of the rest of the language at first glance, so not using it looks like a good thing.

Copyright © 2007-2009 Yossi Kreinin
revised 17 October 2009