Question

So I am just starting to make my way through The Pragmatic Bookself’s, “Programming Ruby 1.9 (3rd Edition)” and I’ve come across some code that I need a little clarification on.

If you own the book, it’s in Chapter 3’s, “Classes, Objects, and Variables,” specifically in the section about virtual attributes.

Basically, a class is defined with an initializer that sets a couple of instance variables, one of which is @price. That variable has an accessor / mutator created with attr_accessor like so:

attr_accessor :price

That class also has a virtual attribute called, price_in_cents which simply returns the value from this line:

Integer(price*100 + 0.5)

Now my question is why is price in the virtual attribute not prefixed with an @? It is clearly dealing with an instance variable. Executing the code without the @ works just the same as with; why is that?

P.S. Sorry for not just posting the code wholesale—given that this is a question about code in a book, I wasn’t sure what legal right I’d have to post.

Answer 1

That’s a receiverless message send.

In Ruby, the receiver self is implicit: you can leave it out if you want to. So, price is basically the same as self.price (ignoring access restrictions).

In other words, it’s calling the method price you defined with attr_accessor.