From STUPID to SOLID Code!

The only other case I can think of is when the object represents a true hardware resource (e.g. a flash queue controller). Although even that doesn't technically need to be a singleton. Another case where I've used one is when I'm caching channels to other servers created by a factory function.

I know what you are saying, and agree. But consider: I have state that I truly need to be global - something I read from an INI file, the command line, or internally calculated, it doesn't much matter. Is not the alternative to a singleton having that state replicated all over the place? And then you introduce model/view/ signals, have to deal perhaps with race conditions, and so on. Then, just try to grep for the existence of that state in your code.

Like anything, it's a tradeoff. Let's say I have an app with 10 OpenGL windows, and I want to simultaneously control the lighting model for all at once. A singleton gives me an easy way to do that, but then the windows have to poll in some manner (which is perhaps not an issue, since they are in a render loop anyway). But I am brittle - introduce the requirement that each window handles light differently and the singleton is toast. OTOH, introducing a signal/slot or model/view architecture may be overkill for what I am doing. This might be a small corner of the code, and I just want clarity for the few times I go in to read it. I find it extraordinarily hard to understand messaging code - where did this message come from, what was the state of that object that caused it to fire the message, and so on. I think complexity goes up at least by 1 order of magnitude when you have to maintain state and time in your mind to understand a program.

Perhaps not the best example; so consider command line arguments. It's read only, set once, but perhaps needed to be accessed by many different parts of my code. A singleton or 5 seems pretty darn clean to me. Again, not without it's tradeoffs, but the alternatives, to me, seem worse. You can pass, pass, pass, the data through methods and constructors until it gets to the code that needs the info - in which case you have infected a lot of code with rather superfluous (to them) knowledge about command line settings. That's brittle, and you end up touching a lot of code when you add something to the command line. You can do a signal/slot or model/view, which can be quite powerful, but you may be forced into a specific design by your sw stack (they have a model/view architecture which you are using for GUI or something). So your code is brittle by being coded to a specific API, and again, it rather obscures where this data comes from. Plus, this forces the command line reading code to know all about your specific API, how to format the signal, when to send it (command line reading is often done well before the whole stack is started up and going, so you can't send a signal yet). That's very brittle as well. Can you take that code, put it in a test harness, and trivially write tests for it? Probably not.

Maybe I am missing some obvious technique. But a few things in an app are in fact global. Trying to obscure that fact, to my mind, just makes the sw more brittle and a lot harder to understand and debug. Which is not in any way an argument for making non-global things global for convenience.

Do you have any advice for avoiding singletons? Consider I have a singleton that does Geographical lookups and requires a substantial amount of time to load/process data mapping. This data never changes after the initial load. Would it be better to make the singleton private and to access it via a non-singleton instance of the class (or subclass) it lives in?

Maybe because we are lazy, and we think that abbreviations save time/energy, which is obviously a wrong assumption.

Sure, but mathematical language only applies to very specific concepts. You're free to talk about the applied domain (if not pure math) using informal text.

Programs need to be interpreted by a computer, and as such need to be written in a formal language everywhere, and as such has a larger amount of concepts that need to be formally modelled. More concepts means we need more descriptive names to be able to distinguish them.

Math uses single letter/symbols usually because those letters have a very short scope, and they are always well defined before usage (or there is a pattern that any mathematician would recognize). Also, short naming is a boon in math because the mathematical objects defined are pretty constant: everyone learns the same notation and that notation becomes the basis for the next layer of abstraction.

Programming by its nature is an exercise in re-enventing the wheel (how many different semantics are there for opening a file in all the different languages in use?). Also, the sheer number of algorithms and abstractions in use in programming dwarfs that of mathematics.

I think it's because a) math started on paper, and b) the way you interact with math documents is different than the way you work with code.

Most of conventions used in math come from the time before computers; on paper, every character you don't have to write by hand is a big time-saver.

Moreover, math papers tend to be read top-down. You start at the beginning, and gradually build your understanding and "symbol-to-concept" mapping table as you go down. Whereas in code, you often just jump in the middle of a file to change something, and you don't want to read 1k lines of code in some random place to determine what f(a,b,q(z)) means. Descriptive names allow you to have all the context you need inferable locally.

Note that many good principles of programming are exercises in maintaining locality. Reducing program state, functional programming style (function output depends only on inputs, not on hidden state), writing small functions, building abstraction layers (treating one layer as a "programming language" for the layer above) - all of these allow you to understand and change the code you're working on without having to look at unrelated things.

Also margin of error: the longer the name is, the more likely you'll mistype it somewhere and screw everything up.

> Mathematicians get upset if an operation uses more then a single character or symbol to express it, and are willing to have massive amounts of overloading to achieve it.

Because context-switching and then processing concise symbols in that context is more efficient for most people's brains that processing prose.

Yes, I linked his presentation in the introduction :)

If you really want to modify global state while running a test, then you might as well use a dependency injection container and get similar behaviour, entirely generalized.

You can then have two much simpler clock classes, one for real time and one for fake time.

Personally, I prefer just passing around the clock, though - it makes dependencies between classes crystal clear.

I think that it is a matter of vocabulary, no? Reading the LSP definition in Liskov's paper is quite hard to understand, but with simpler words, it is more understandable.

> If Square and Rectangle had only getter methods (i.e., they were immutable objects), then no violation of LSP could occur.

Yeah, this is the fundamental problem; mutable objects tend to result in LSP violations when class heirarchies follow common sense is-a relationships, but mutating operations are not limited to those (if any exist) that preserve identity.

A corollary to this is that the use of default constructors very often accompanies LSP violations, as it usually implies all of the object's state is subject to mutating operations.

Care to explain your take on this point of view?

The way I see it, it couples objects to the things they should be coupled to while avoiding unnecessary coupling.

> 'Dependency Inversion' increases coupling.

This is an interesting claim. Could you explain further how this is the case (or provide references which explain it?)

Oh I am really sorry. Could you give me more details (OS, browser) so that I can fix the font?