Lambda the Ultimate

Because a public getter and setter for a private class variable is little better.

The primary evil of global variables is unexpected side-effects and dynamic scoping removes that evil. It effectively turns them into arguments that get implicitly passed to all functions. It also has one extra advantage.

Consider a pretty-printer implemented with the visitor patter. You need to store the indentation level in the visitor object, this basically makes it an evil global variable and requires that you write lots of error-prone code to make sure side-effects are undone - even in the presence of exceptions and functions with multiple return points. You basically end up rolling your own dynamic scope. One alternative is to make your visitor infrastructure capable of passing extra paramerts but then your visitor's all messed up and more paramters are going to result in a lot of stack twiddling, especially as they would have to be passed by value.

The same goes for anything else involving callbacks where some state has to be passed through the callback.

It's a pity that more languages don't support it.

I've read a little bit about dynamic scoping and I found the concept somewhat difficult/dangerous. Could you explain how it actually helps with safety ?

I don't speak for fergal, but in a nutshell, dynamic scoping definitely has some evilness itself, but it is certainly less evil than state (as typically used by a "typical" programmer). If dynamic scoping can replace a use of state, then that is almost certainly a win. If dynamic scoping itself can also be gotten rid of, all the better.

Eiffel has something akin to a more refined friend declaration. You can specify what classes can see what features. A friend a la C++ can see all private data.

Section 5. here is a good grounding.

Not a great fan of Eiffel but I always like this aspect of it. I think there's problems with derived classes changing visibility though.

A global variable (a mutable storage cell) holds state that may be read and written from anywhere and is used to pass state implicitly. Properly sequencing accesses to a global variable is usually very important.

Usually a class is a declaration (comparable to a constant) that may be read from anywhere, but is not (usually) written to and is not used for passing state implicitly. There is (usually) no need to sequence accesses to a class.

Your comment that classes are read-only and are thus more like constants is true only in a few OO languages. It's true in Java, for instance, but not in Ruby or Python (or Smalltalk or Self, from what I understand).

Also, it doesn't take into account stateful effects on classes. Many classes have static fields with getters and setters (or constant fields which point to objects with getters and setters of their own), in which case a class isn't just a single global variable. It's actually an entry point to a whole universe of global state.

Finally, even in Java there's a very strong need to sequence accesses to a class in a wide variety of cases. Consider for instance the subtleties involved in class initialization. Just a couple of weeks ago I spent a lot of time tracking down a race condition involving a complex web of static initialization blocks in a Java application.

Your comment that classes are read-only and are thus more like constants is true only in a few OO languages. It's true in Java, for instance, but not in Ruby or Python (or Smalltalk or Self, from what I understand).

Well, this depends on what you mean by read-only. What I mean is that a class declaration specifies the components (methods, variables, etc...) of a class and the set of components usually won't change after the declaration. This is the case in basically all statically typed OO languages (ignoring AOP extensions). As you say, changing the set of components of a class is possible in some (dynamically checked) languages, but even then it is recommended practise to make class declarations complete and avoid mutating classes (adding and removing methods, for instance) after the fact, because such mutations can be very confusing. And, this is why I said "usually" more than once.

The rest of what you say is irrelevant in this respect. Many OO languages allow you to have (global) variables at class scope and it should not be news to anyone that it can cause problems just like global variables.

While I mostly agree with your comments, I think it's important to realize that even "real world" objects in other engineering disciplines do suffer from the equivalent of "global variables" that components are not explicitly wired to, and "magical" connections to shared "information". Two examples of this kind of thing are the radiative and convective thermal environment of a system, and the electromagnetic environment of a system.

Now, in many cases, these things are not really an issue. But in some designs electromagnetic coupling between different components is a major concern, and design for electromagnetic compatibility becomes a major focus of design efforts. In other cases, thermal interference between components is an issue, and it becomes necessary to add extra thermal control elements, such as insulation or thermal straps, to maintain the correct thermal environment. In both of these example situations, it isn't feasible to just plug components together and go. In fact, my own experience is that these implicit interfaces between components and subsystems are a significant contributor to the difficulties of design and integration for "real" systems (such as spacecraft).

In other disciplines, these problems are managed through careful analysis during the design phase. In the software world we have two rational options: we can, like other disciplines, perform careful analysis (using various formalisms); or, since we are dealing with purely artificial constructs, we can eliminate the globals and make our lives much easier. Sadly, the default choice seems to be a third option: use globals, and skip the careful analysis. That, at least IMHO, is why component-based software has run into problems.

Incidentally, this is exactly the problem that Odersky and Zenger try to address. I've said it before, and I guess I'll just keep saying it: I think this paper presents a very compelling approach that works today in Scala, and they make it very clear what language features enable this pattern. I definitely urge you to take a look.

(And yeah I realize this isn't the problem the original post was trying to get at...)

Aggred, Stateful Singletons are evil. They required for some complex situations where you need to have just one (generally when you have real hardware that you need strict access control on within your program). However I think the GoF made a big mistake when they put them in their book. (Though in their defense, they seem aware of all the problems, they just didn't tell you strongly enough in the book that you shouldn't use them if there is any other choice)

There is a time and place for global variables. There is a time and a place for singletons. In the real world those times and places are rare. I've debugged many programs just by removing instance() and making the program compile.

is that writing a class which declares, a priori, "there shall be only one!", is almost always a violation of separation of concerns.

The correct (IMHO!) way to implement a singleton is to write your class (or other datatype) as normal; then use an external "SingletonHolder" or "InstanceManager" to manage the single (or n if you prefer) instances of the class.

The Singleton pattern, as is currently specified, requires one to modify the class in question if one wants to change the number of allowable instances in a program. With an external instance manager; all you need to do is change the configuration of the instance manager.

Since a class is already an object factory that knows how to instantiate objects I do not understand why do you think it needs additional management classes to manage multiplicities? The requirement of creating exactly n instances ( with n>1 ) seems there for pure theoretical reasons and I beg for non-trivial use cases. At least for me your suggestion looks like bloat and overdesign I know from Java projects where colleagues and me tried to figure out the responsibilities of all the clunky Manager-, Helper-, Holder- etc. stuff.

Instantiation strategies (singletons, object pooling, ...) also fall very neatly into the domain of metaclasses. See, for example, this paper (which, now that I remember it, definitely belongs in a top list for 2005)...

Thanks Matt, for linking this. I will take a look at the paper. As for Python metaclasses that are mentioned by segphault too, Python classes reference metaclasses that customize them explicitely i.e. they accept metaclass manipulation. This is satisfying since instantiation still remains in the responsibility of the class and interfaces need not to be changed so that the code remains DRY - something I don't see with Scott Johnstons SingletonManager instances that duplictate access points for instance access/creation. Nevertheless I'm not much worried about his decoupling solution since the feared codebloat as it is present in much Java designs is obviously not there - on the contrary. Its a fair tradeoff. I should have known LtU readers better.

Kay

I don't mean for every class Foo that you want to make a singleton; you need a FooManager class.

Instead, have a pre-existing class, SingletonManager, which is global--and which will only bind to one instance of something else. By using generics/templates; this can be done without any obnoxious casting (one reason in support of the Singleton pattern in Java is that the get_impl() method can return the correct type, rather than Object).

Often time, the real problem you want to solve is NOT preventing users from creating two instances of class Foo; it's to make sure that (within some context), there is exactly one Foo which is the "official" Foo. In a properly designed class (i.e. one that isn't oozing with static non-final/const members), having additional instances of the class generally aren't harmful--the issue is ensuring unique or synchronized access to some external resource.

When a class is used to manage/synchronize access to some external resource, in many cases the IDEAL solution is for the external resource to provide its own syncronization. That isn't always an option, of course.

More sophisticated instance managers can provide more interesting semantics. For example--in the context of logfiles; you could have a method which returns the current Logger associated with a given system logfile (say, /var/log/myApp.log), creating a new Logger if one doesn't already exist. If Logger is a Singleton; no other Logger instances (pointing to different logfiles) can exist; but if Logger is a non-Singleton class whose instances are managed by an instance manager; it becomes far more useful.

Virtually every electrical component of your car is connected to the common ground a.k.a the battery's negative terminal. On older cars, that common ground is accessible globally via the car's frame.

Or consider wireless routers. Every comptuter is my house is connected to the same global resouce, the router. There is only one true router, and all communication to the outside world goes through it.

...there can be multiple grounds in a circuit, and not all are alike. Analog ground, digital ground, safety ground, etc. Even though the impedance of a huge hunk of metal (or a circuit board layer) is low, it is not zero; detectable difference in potential may exist at different points in a circuit region called "ground".

A proper and thorough discussion of this is outside the scope of LtU (and my technical ability); but be rest assured, "ground" is frequently not a singleton.

Good luck reusing your RouterSingleton in a bridge device or a computer with multiple network cards.

I have been struggling with the same exact issues you raise. In fact I was going to comment earlier about this, but after reading comments by others, I thought I was the one who misunderstood! :)

For the past several weeks I've been thinking about how inside of a class, member variables are essentially global variables regardless of their access modifiers. I was thinking about how, when working with a class, you have to keep all of these details in your head. This isn't as big of a problem as the global variables of yore, but yet, OOP is like spaghetti code in its own way. The situation is much better now, because instead of having one large platter of spaghetti you have several smaller plates of spaghetti. Some are very small and quite manageable, others are larger and much more difficult to reason about.

I concluded that a class should only contain private member variables for what it absolutely must. Everything else should be passed from method to method, much like in procedural or functional programming. For me these thoughts were interesting thoughts, and a good idea in general, but they were just thoughts: until last week. You see, last week I had to add some functionality to a class that somebody else wrote. Of course I knew that changing the behavior of the existing methods was a bad idea, so I was careful to not change anything. So, I went ahead and wrote my code, using some of the private methods of the class in my new code. I made a major mistake. I didn't write a unit test. The next day a bug showed up: I got the sequence wrong. Unit test! Oh, I should have written a test case! Just to make sure. But it was such a small change, and it was so very simple. (I know, bad excuses! Bad Ben, bad! Doh!) The problem was the methods were mucking around with the private member variables. It got me thinking: now, unit testing should have caught the error, right? But that doesn't mean the problem just goes away. If unit testing fixes everthing, then we don't need to avoid gloabal variables or anything. "Just write a unit test!" If you have to get the sequence of method calls just right (even thought it isn't obvious that the methods need to be sequenced) isn't that telling something about the design? But there was nothing wrong with the design as far as OOP goes. Yet, it leaves me with a bad taste in my mouth. I can hear everybody saying: Unit Test! But in a way, private member variables are not very amiable to testing, are they? Passing arguments into methods instead of using private member variables just seems like a better idea to me (if you can help it) as it just seems safer and more amiable to unit testing.

But in a way, private member variables are not very amiable to testing, are they?

The problem is caused by encapsulation, which is not unique to OO. However, non-static private methods additionally require a whole object to be constructed in order to be tested. This could be ok, but sometimes, these objects establish connections to databases, etc. so the unitness of unit tests becomes questionable. Now you have to create a mock object if you like to decouple your unit tests from the database, etc. etc.

Happy New Year everybody.

It looks like to me that the problem was that the code imposed a defined order to call private methods. Yes, instance variables allow this sort of bad design, but it's bad design nontheless. In my experience, when instance variables are semantically germane to the class' responsibilities they don't pose a problem.

What's wrong with the standard-OO tools, derivation, composition, maybe MI or mixins? Isn't that exactly what they do?

... At the very least, why aren't the private methods always static so that the public methods have to pass the state explicitly?

Funny, I always thought the other way round: why make private static methods at all? I mean, if it's a static method, it doesn't depend on the internals of the class and is predestined for reuse, so why hide it? Chances are, you'll be hiding away a useful tool method...

What's wrong with the standard-OO tools, derivation, composition, maybe MI or mixins? Isn't that exactly what they do?

I mean, if it's a static method, it doesn't depend on the internals of the class and is predestined for reuse, so why hide it? Chances are, you'll be hiding away a useful tool method...

If there's a reasonable demand, surely we can promote that method to the public interface. My problem is not with what is public or private, though. It's about limiting the portion of the object's state that a method has access to. I can't do anything to limit the public methods, but I can limit the private methods.

My problem is not with what is public or private, though. It's about limiting the portion of the object's state that a method has access to. I can't do anything to limit the public methods, but I can limit the private methods.

It would seem that what you really want is decent fine grained control of what state variables can be accessed by any given method/function/procedure, and that certainly is available in the right languages. In Java, using JML annotations and tools, you can declare, at the top of each method (public or private), which instance variables are assignable, or accessible from within the method (and even which other methods are callable from within the given method), and have checks that such restrictions are followed (as unit tests, or runtime checks, or even static checks with ESC/Java2). A language like SPARKAda provides similar functionality (though not with quite the same OO mentality) via global in out and derives annotations to provide control over what state variables are accessed or changed, and what inputs/other state variables any changes are derived from. SPARKAda even goes so far as to refuse a function or procedure access to any state variables unless you specifically grant it in a global annotation.

Koray, you and Benjamin raise some interesting observations, and something that has always bothered me about mainstream OO languages. It seems to me that it's far too easy to basically get yourself in a sphagetti mess of coupling and state because it's so easy to just throw another instance variable and another random method into a class.

I understood that as his meaning as well.

It sounds like the classes have some very obvious "code smells" that can be used to direct refactoring to improve the design.

1) Groups of methods that use only a subset of the instance variables. It sounds like the methods and variables they use should be in a separate class.

2) If the use of instance variables becomes as confusing as system-wide global variables, then the class is much, much too big. It should be split into multiple, collaborating classes. Other smells, such as the one above, will indicate where to split the big class.

So, yes, instance variables are like global variables in a way. The way to avoid the problems inherent in global variables when doing OO programming is by divide and conquer. Just as a global variable is not a problem in a small program, so an instance variable is not a problem in a small class.