Inheritance -- abstract base classes

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

C++ abstract base classes are not bad at all by C++ standards. However, they are quite poor by OO standards.

[22.1] What's the big deal of separating interface from implementation?

FAQ: Interfaces are the most important thing possessed by a company. Defining interfaces is hard, throwing together an implementation is easy. Designing interfaces takes lots of time and is done by people who are paid lots of money.

You wouldn't want these valuable artifacts clobbered by implementation details, would you?

FQA: If interface is decoupled from implementation, you can have more than one implementation of the same interface. This way, code using the interface can work with all those implementations. For example, the same code may be used to render a document to a screen, a printer and a file. Which is excellent, and is a very widely acknowledged and basic fact.

It's amazing that the FAQ's answer fails to mention this. The FAQ's claim about the relative importance of interfaces and implementations is unbelievable, too. Here's an interface for you:

class SearchEngine
  // search the documents pointed by URLs in the inputURLs stream, search the query string
  // in them and emit the URLs sorted by relevance to the outputURLs stream (at most maxResults URLs).
  virtual void find(const string& query, istream& inputURLs, ostream& outputURLs, int maxResults) = 0;

Now, that took about 1 minute. It's your turn to "throw together an implementation", which shouldn't take more than 30 seconds according the FAQ's argument, should it? Go.

If you don't like this example ("there are more complicated interfaces in a real search engine"), think about matchScore = faceRecognition(suspectImage, testImage), fft(outputArray, inputArray, size), text = ocr(imageWithHandWriting), render(screen,htmlPage), write(file,buffer,size)... The idea that "implementations are trivial" can only come from working on extremely stupid applications or example programs for text books.

Of course many interfaces should be decoupled from implementations - but that's because they hide a much more complicated implementation, and/or because they have more than one implementation, and/or because you want to change the implementation without affecting the client code. Surprisingly enough, C++ abstract classes actually allow you to achieve all these things.

Of course they don't fix many hard problems with C++ - there's no real run time type information, for example. And adding a function to an abstract class changes the binary interface, which is frequently unacceptable. But abstract classes are better than nothing, as opposed to the many C++ features which are in fact worse than nothing.

[22.2] How do I separate interface from implementation in C++ (like Modula-2)?

FAQ: Use an abstract base class.

FQA: What do you mean "like Modula-2"?

If you want to be able to work with many different implementations selected at run time, abstract base class is the way to go. Alternatively, you can roll your own "abstract base classes" by using structures containing function pointers, emulating "vtables". This results in extra code, but it's more portable (C binary calling conventions on a given processor architecture tend to be shared by more tools than C++ ones).

If you don't need dynamic implementation selection, you can use a header file with forward declarations of types, and have the types defined in the implementation file(s). This way you can change the implementation details without recompiling the calling code (which you can't do with C++ private members). In particular, you can build several versions of your program with different implementations of this interface without recompiling the entire program.

[22.3] What is an ABC?

FAQ: An abstract base class.

Logically, it corresponds to an abstract concept, such as Animal. Specific animals, like Rat, Skunk and Weasel, correspond to implementation classes derived from Animal.

Technically, a class with at least one pure virtual function is abstract. You can't have objects of abstract classes, you can only create objects of classes derived from them that implement all the pure virtual functions.

FQA: ABC is also an especially annoying abbreviation.

Here are a couple of things you can't do with a C++ abstract base class, and which are supported in other OO languages. You can't iterate over the methods of an abstract base class (neither at compile time nor at run time). So you can't easily utilize the decoupling between the interface and the implementation by automatically generating wrapper classes that log function calls or pack the arguments and send them to an object in another process and/or on another machine. You also can't ask a random object whether it implements the given interface (in language terms, whether its class is derived from a given abstract base class).

If the only language you use heavily is C++, it's possible that you don't realize that you might want to do these things at all, or even memorized elaborate arguments "proving" that these things should not be done. But hey - at least abstract classes don't make anything worse than it already was in C. Quite an achievement for a C++ feature.

[22.4] What is a "pure virtual" member function?

FAQ: It's a virtual function which (normally) has no implementation in the base class, and (always) must be implemented in a derived class to make it non-abstract. The syntax is: virtual result funcname(args) = 0;.

This declaration makes the class abstract - it is illegal to create objects of this class. A derived class that doesn't implement at least one pure virtual function is still abstract. Only derived classes without any unimplemented pure virtual functions can be instantiated (you can create objects of these classes).

FQA: The syntax is ridiculous. You may wonder what virtual result funcname(args) = 1999; means. Well, it means nothing; it doesn't compile.

One possible motivation for this syntax is to save a keyword (such as pure or abstract). The more new keywords there are, the "less" C++ is compatible with C, or with C++ code written before the new keyword was introduced (well, strictly speaking, either two things are compatible, or they are not, but we'll ignore that for the moment). That's because C and C++ have grammars which disallow to use keywords as identifiers. This problem is shared by the majority of programming languages.

However, C++ does have a huge amount of new keywords. This makes one wonder why the very rarely used explicit (not to mention export) is a keyword, while the many different uses of the keyword static are in fact collapsed into a single keyword instead of using new keywords for the new uses. And why not use the silly, but standard "reserved namespace" (names prefixed with two underscores or a single underscore followed by a capital letter are reserved for the compiler), the way it's (mostly) done in C99? Then we'd have __explicit built-in and explicit would be a #define you'd #include from <shiny_new_useless_cxx_features> (no trailing .h, of course). This way, you give a normally looking keyword to people who need it, and don't break the code of those who don't.

Here's a proposal for the next C++ standard: let's define two keywords, __0 and __1. With a token sequence composed of these two keywords, we can express anything (actually, one keyword is enough, but that's just too verbose). Then we won't ever need any new keywords. As the pure virtual syntax example shows, the readability loss is a small price to pay for the forward, backward and downward compatibility achieved using this approach.

[22.5] How do you define a copy constructor or assignment operator for a class that contains a pointer to a (abstract) base class?

FAQ: If you don't want to make a "deep copy" of that pointer, there's nothing special about this case.

If you do want to make a deep copy of the pointer (which you normally should do when the class "owns" it), the abstract base class should have virtual copy and assign methods you can call from your copy constructor and assignment operator.

FQA: This is a small example of how C++ forces you to write code which could be generated automatically. You can avoid the problem by avoiding the ownership approach to lifetime management (RAII), but figuring out that you need these virtual functions and implementing them is easy for people who understand C++ well.

The trouble is that most people using C++ don't understand it very well. C++ has the lethal combination of looking all nice and simple on the surface and having lots of traps installed under the surface, waiting for a victim. The only chance of the victim is to have a very good understanding of the underlying problems of C++. With that understanding, it's easy to see that, um, you have to make a deep copy since you're the "owner", and an object pointed by two owners is going to be wiped out twice by the destructor, and we don't want that, so let's copy the object - costly, but safe, and, um, we can't just copy the object, because we don't know its type, but wait, the object itself does know its type, so we need it to help us with the copying, so we need to add these virtual copy functions to this hierarchy of classes. It's a good thing we did it now before the interfaces became stable and adding functions became a royal pain, causing recompilation of calling code. Or did we?

The problem with all this reasoning is that it has little to do with whatever you're ultimately trying to achieve in your program. So many people who think more about what their program does than about the way their programming language works won't see this coming, and do something wrong. No, it doesn't mean that the people primarily focused on the programming language are more productive than others. Of course they aren't uniformly less productive, either. Some of the people with a programming language focus spend most of their time choosing between func(obj) and obj.func(), some don't; it depends on the person, and people can change, further complicating matters.

Of course this problem exists with all computer languages and, more generally, with all software systems, and, more generally, with all formal systems. Basically there's the ice - the things compatible with human common sense - and the cold water under it, into which you can fall when the rules governing the system contradict your common sense. So what's so special about C++?

Well, in C++, the very thin ice is covered with paint, and the water is deep enough to drown.

Copyright © 2007-2009 Yossi Kreinin
revised 17 October 2009