Question

I’m new to programming. Right now I’m studying Ruby. To my understanding, global variables are defined in the global namespace (so outside of any classes or functions). I’m reading something and it says global variables have a $ sign before them. What does that mean? Does it mean when I define a function or class and want to reference my global variable (let’s say it is edmund = 123) I would have to reference it like this: $edmund?

so:

edmund = 123
def my_function()
  456 + $edmund
end

Also are class variables (the ones that begin with @@) like instance variables (@) where you can access them by calling them through Class.classvariable? What is their purpose?

Answer 1

Global scope is scope that covers the entire program. Global scope is enjoyed by global variables, which are recognizable by their initial dollar-sign ($) character. They’re available everywhere and creating your own global variables can be tempting, especially for beginning programmers. But they’re not always a good idea.

$gvar = "I'm a global!"
class C
    def examine_global
        puts $gvar
    end
end

c = C.new
c.examine_global # I'm a global!

---

Class variables begin with two at signs: @@var, for example. Despite their name, class variables aren’t class scoped. Rather, they’re class-hierarchy scoped. At its simplest, the idea behind a class variable is that it provides a storage mechanism that’s shared between a class and instances of that class, and that’s not visible to any other objects.

class Parent
    @@value = 100
end

class Child < Parent
    @@value = 200
end

class Parent
    puts @@value
end

What gets printed is 200. The Child class is a subclass of Parent, and that means Parent and Child share the same class variables—not different class variables with the same names, but the same actual variables. When you assign to @@value in Child, you’re setting the one and only @@value variable that’s shared throughout the hierarchy—
that is, by Parent and Child and any other descendant classes of either of them.

---

And to give credit where its due – This explanation comes from “The Well Grounded Rubyist” by David A Black, one of the best resources to learn about Ruby.