The Single Responsibility and Open/Closed Principle are the Same

Consider a class C and a set of potential feature changes F'.  If class C has two responsibilities, it is likely that many F in F' will only affect code for one of those responsibilities in C.

We apply an F to C but, since there are two responsibilities in C it has less closure - it was open to F and to feature changes that applied to the other responsibility.

Now consider separating responsibilities by breaking C into C1 and C2.  If F only applies to C1, C2 is closed with respect to F.

Smaller, focused abstractions lead to closure.

Comments

[Philip_schwarz]

Hey Michael,

Thanks, this clarifies the following answer you posted in [1]:

"It’s a funny thing about the design principles. I think that often if you take one, you get many of the others by implication. If you concentrate on SRP, you can end up with OCP without even thinking about it. I think the same is true of OCP.. If you really push it, you will end up with modules with single responsibilities..."

[1] http://web.archive.org/web/20091225221756/http://blog.objectmentor.com/articles/2009/02/26/10-papers-every-programmer-should-read-at-least-twice

Philip

[Mulva]

Ooer, careful, Michael, you're using that word, "Closure," that already has perfectly good credentials elsewhere. Your, "Closure," is neither the mathematical, "Closure," nor anything related to the lambda, "Closure." Just because it's, "Closed for modification," doesn't mean it's an example of a, "Closure." It's an example of, "Being closed."

[Vic Ripa]

Hi Michael,
the idea that the SOLID principles are all connected to a common notion (responding to change) has been explored here by Giorgio Sironi:

http://css.dzone.com/articles/solid-principles-are-they

That, in turn, is inspired by Carlo Pescio's work on the physics of software.

[Phillip]

I think that is an oversimplification. I'm not sure, whether I'd even agree on the statement that SRP implies OCP.

You could construct a perfectly valid component with exactly one responsibility. Let's call it a dispatcher. If that dispatcher is implemented as cascading if-then-else-statements, then you are required to change that implementation in order to introduce new features.

Via a wrapper you could argue that there is no need for a change in the dispatcher-component. But in that case you'd spread the responsibility for a dispatch on the wrapper and the dispatcher.

But I agree that small and focussed abstractions tend to improve encapsulation.

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