Encapsulation as passing on responsibility

I recently dealt with some code that I felt was not properly encapsulated, but in a sense that I’ve not seen articulated in this way before. Please enlighten me if I missed it.

Here’s a snippet:

OutputThing manipulate( InputThing input )
    ProcessingThing proc( input );
    moreProcessing( proc.intermediateResult, input );
    return OutputThing( proc );

This function takes in some input and returns some output, using a ProcessingThing to do some work.

I needed to make a change to ProcessingThing to support some functionality, and I fell foul of something I didn’t expect.

I wanted to transform my input a bit, and wrote that code inside ProcessingThing. What I didn’t expect was that the untransformed input would later be used (in the moreProcessing call). The moreProcessing code failed because the transformation I had done made the ProcessingThing inconsistent with the InputThing.

Obviously, I should have checked more carefully.

But, it struck me that there is a pattern here. We are processing something by passing it from step to step in a “production line” of code. By re-using something from an older step, we violate this metaphor of passing on responsibility, because suddenly there are two copies of our input – it wasn’t passed on at all.

This feels a bit like we’ve violated the encapsulation of input by ProcessingThing, but maybe I’m stretching the word encapsulation too far?

I feel like if ProcessingThing properly encapsulated the details of InputThing, we wouldn’t need to re-use input later.

In fact, the fix in my case was to put the moreProcessing logic into ProcessingThing, meaning there was no need to refer back to the original input. I think this supports the argument that we are talking about encapsulation.

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