Object oriented programming is extremely popular these days. It’s so popular that some people aren’t even aware of alternatives to it. The reason its popular is clear: OOP works well a lot of the time. Traditional object oriented programming styles have some significant disadvantages in some circumstances, however. Some of the alternatives are worth looking into when those disadvantages become awkward.
As part of the static analysis project that I’m working on, I’m trying to use alternatives wherever I can. In part, this is because the advantages are substantial for the project I’m working on, but I’m also curious how far it can go and what will happen when I push it too far.
So far, I love it. Inheritance is a relationship between classes that is only a small step from C++ friendship. The derived class becomes extremely tightly coupled to the base class, and, in some cases, the base class can even become coupled to the derived class. The temptation to add functionality to the base class can be large as well.
I’ll talk about two ways of partially avoiding inheritance. Both of them rely on compile time polymorphism and cannot (or should not) be used if runtime polymorphism is required.
How often do OO programmers actually need the runtime polymorphism that they enjoy so much? Certainly it’s needed some of the time, but it’s not nearly as useful as it looks at first. For example, my static analysis project contains a different class for each of several dozen different Lua constructions. This sounds a lot like a case for inheritance, but it’s only got one virtual function in the whole project, and that’s there only to merge the C and C++ code. If the whole thing had been in C++, it wouldn’t have been needed.
Free Functions
Many programmers in the OOP world will happily add new functions to the public interface of classes whenever they can. For example, the C++ string class that PC-Doctor uses has a public member function to return a uuencoded version of the string. This is an extreme case, but it’s still worth looking at.
In this case, we’ve got some functionality that we want to give to a small number of users of a class. There aren’t many places in our code base that use this function, and essentially everything uses the string class. Furthermore, the function can easily be implemented using the existing public interface to the string. In this case, it’s pretty obvious that everyone would be happier if the function was a free function. As a free function, most users of the string won’t ever have to see it. In fact, they don’t have to include the appropriate header file or even link to the appropriate module. The interface to string becomes a bit easier to understand for the majority of string users, too.
Furthermore, if you wanted to implement uuencoding for multiple types of strings, you could still do it. For example, if you had different types for UTF-8 and UTF-16 strings, you could overload the free function for both them.
As I said, this was an extreme case. There are disadvantages to free functions, however. Let’s look at the other extreme for a bit. For this, let’s see what Herb Sutter has to say about free functions in Exceptional C++ Style. He claims that all you should construct your interface to classes by exposing the minimal set of public member functions with everything else as a free function. His argument in the book also looks at a string class, std::basic_string.
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With that in mind, I’ve made the 40 classes that form the abstract syntax tree for Lua into classes that only have constructors, destructors, and access to internal data. This is, perhaps, a bit more than even Herb Sutter would recommend. I give public access to all of the member data so that there isn’t a chance that any member functions could be needed.
I’ve written most of the code, and it works fine. However, it hasn’t survived the maintenance phase, and it’s not clear that it will.
However, I’m not going to change the way I write normal application code. At PC-Doctor, I’ll write many more classes that get used by only a few other classes. I won’t create many free functions for them. Why?
First, free functions clutter the namespace they reside in. I only want that function to be seen by the small number of programmers that might be interested in the class I just wrote. Considerably more people will see the namespace that the class is in.
Let’s look at an example. Suppose I want to write a function that appends an alert to a collection of alerts that will be shown to the user. The function might be named “append”. If this is a free function, it’ll get lost. I want users of my AlertCollection class to know that they can append alerts to it. I don’t want them to have to dig around. My users are not going to be using it for long, and they don’t want to remember the details later. They just want a complete catalog of everything they can do so they can quickly do what needs to be done.
If a programmer at PC-Doctor decided that they were fans of handling alerts and they wanted to actually remember my API, then putting some of the functions as free functions is, again, a problem. There’s one extra bit of information that the user has to remember for each function.
Finally, languages like Java and C# can’t have free functions. This is their loss, unfortunately. While I can’t give a hard and fast rule for when to use free functions, I can say that they are a valuable tool when developing some APIs.
Visitor Pattern
Well, that was more than I wanted to write in one week. (Remember, I’m a programmer!) The visitor pattern will have to wait until next week. It’s probably worth a whole post on its own anyway. It’s tricky to use effectively, but it’s occasionally extremely effective.
