Open/Closed Principle in Software Design

3 min read

Open/Closed principle or OCP is one of the guidelines that help software developers achieve high quality software design.

Well, it’s actually pretty hard to tell, what exactly does the term high quality software mean. Back to the OCP. Bertrand Meyer is credited as having originated the term Open/Closed Principle, which appeared in his 1988 book _ Object Oriented Software Construction_. It goes like this:

Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification.

What does it mean? When you design some piece of software, it’s vital to keep in mind possible places for future extensions. Let’s face it, the customer specifications change with a speed of a guy, who just found out he had accidentally drunk a whole bottle of extremely effective laxative. So your code is most certainly going to be a subject of change and extension. And as we know, it doesn’t end well (in either case).

What the principle implies is, that you can think a little forward (build for today, design for future, right?) and design your software so no changes are necessary when it comes to adding a new features and functionality. Let the code speak for itself:

def area(geometric_entity):
    if geometric_entity.type() == SQUARE:
        return geometric_entity.a * geometric_entity.a
    elif geometric_entity.type() == CIRCLE:
        return PI * geometric_entity.r * geometric_entity.r
        raise UnknownEntityError("I literally have no idea.")

This is a really dumb example in the first place, but it shows the key aspect of OCP. If you decide, that it’d nice to have your neat area function work with triangles as well, you need to go here and add another elif clause. Then others will come with requirements on other geometric entities and before you know it, this 6 harmless, poorly coded lines will turn into 1500-line Riemann integral solving monster (seen it happen). And if the code monster won’t eat you, your project manager definitely will…

The point is, every time you change something in your software, something else can go wrong (and according to Murphy’s laws, it usually does). There are unit tests, which are designated specifically to discover such errors (if you don’t even write unit tests, you’re either doomed, a superman or a fellow, who enjoys a quality time with a gallons of coffee and GNU debugger). But even unit tests don’t get everything and it’s generally a good idea to avoid poking the bear, if you absolutely don’t have to.

Software quality might be a little subjective and generally hard to define formally. So are, as the software quality, this guidelines (that’s right, not rules). They cannot be enforced unconditionally. But it’s definitely good to know, that they’re there. And they might help you see flaws in your design before it’s too late and save your ass from getting fired :-P. Or not. Anyway, Open-Close principle is not the only one, so stay tuned for more premium 95 octane knowledge (ok, I probably watch How I Met Your Mother waaay to much lately).

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