Agree, there's no rule that everything has to be a class in C++.

I suppose drum.play() could default to using a stick.

I think the GP's idea is to just do play(drum, stick) as its own function and not tie it to either class or have it be a passthru into internal methods for either.

You can even make everything a class, and still provide a functional-ish interface, by using the class as a namespace with a common immutable context.

But...that was exactly the question the article had. Should drum be the enclosing class, or should stick be? What he cares about is the verb play, but he has to encapsulate it in a noun, and none of the ones actually involved dominate so clearly as to moot having to make a choice.

  musicPart.play(drum, sticks);

Basicslly you can replace the module namespace with a class namespace if desired. What module could that be?

Right. But that's just it. "musicPart" is an object. A noun. What the hell is a "Music Part"? It's a completely synthetic object you've had to create solely to namespace functions. It's like how so many large Java projects have static "Utils" classes.

What you've done is not OO; in fact, it's an anti-pattern in OO. You've created, as you correctly note, a namespace or module around static functions; there is no meaningful encapsulation of data.

My point isn't that it can't be done, it's that trying to follow best OO practices, you run into this sort of problem (which has had a lot of digital ink spilled on it). The fact there is a pragmatic solution isn't in any doubt; it's that such a solution intentionally eschews the data encapsulation that OO demands (but not imperative or functional approaches, which instead say it's the right approach to do that).

If you ever played music, you know it consists of parts. Say, a percussion part, a bass part, a rhythm guitar part, a lead guitar part, a vocal part, etc.

Why would 'play' hang off of the (presumably static) "MusicPart" then, rather than, "Player", "Musician", "Composition", etc?

Also, you realize that this whole discussion is proving the point, right?

But now the traits aren't connected with drums (or sticks) but MusicParts. Or you end up having some `drum.foo` too.

> using the class as a namespace with a common immutable context

This is why we can't have nice things.

Pray elaborate.

I guess he is talking about traits. How do you 'functionize' them?

I wasn't thinking about traits when I wrote that, but you make a great point — that's a whole another way to design things. Maybe it could have been something fancy like:

    impl<T> Sound for T where T: Percussion<Accessory=Stick>

There's also the pattern used in from+into where one implementation is wrapped to provide its counterpart on the other type. Nice to read but feels a little extra magical for someone without a lot of Rust background.

Blanket implementation is a very common idea in Rust. The standard library "owns" its own Traits and so it gets to provide blanket implementations for those, but you can do the same for your own.

Here's (really, this is literally the code) how std provides a blanket implementation of Into given From:

  impl<T, U> Into<U> for T
  where
      U: From<T>,
  {
      fn into(self) -> U {
          U::from(self)
      }
  }

There's actually a chain, From implies Into, Into implies TryFrom, then TryFrom implies TryInto.