Instance members

Crystal Contrib
1

some thoughts

is that a little odd – that we have to prefix instance vars with all those @ signs ?

is not - use of - instance members far more common, when using classes ?

is it possible to assume inst.vars inside classes , without prefix ?

2

The @ symbol denotes that that variable is an instance variable. Just like in Ruby. However there are a few ways to access the instance variables in a class.

Take this for example:

class Foo
  property age : Int32 = 1
  @name : String = "Jim"

  def do_something
    user_age = age # Calling the getter for that ivar
    self.age = 19 # Calling the setter of that ivar

    puts "Hello #{@name}" # Accessing the ivar's value directly
    @name = "Fred" # Changing the ivar's value directly
  end
end

Not sure if that what you mean?

3

No, that always assigns to a local variable. You have to do this:

self.age = 19
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4

Oops, fixed.

5

Ah! I’ve made that mistake a few times, and wondered why it didn’t work the way I was thinking it would. Seems obvious now. In my case I often simply switch back to using the @age version.

6

I’m a HUGE fan of @ and @@ to keep track of my scope. It’s one of those things that makes Ruby/Crystal feel Stoner-friendly.

7

In most APIs you wouldn’t access ivars directly but use getters and setters instead. This allows to implement some custom logic in these methods, should that be required.
So, I think in general, accessing ivars directly should be considered a low-level interface, mostly a hack. For this purpose it’s good to have to prefix each ivar by @. It should make you wonder, whether you really should access this directly or is there some kind of accessor (or maybe there is a reason not to).

8

Just use the property macro and access the variables with a class reference. Calls the @ on the variable internally for you, that’s what I do.

However, if you are wanting to modify a property inside a class, make sure you use @

9

thanks for input.
it s more clear to me now - seems to have intent.

1 Like