Instance @variable vs class @@variable

[Chris Fullmer]

I've got an example that helped me understand the class vs instance variable situation a little better.

If you have ever made a true "tool" in SketchUp, you would have made it its own class. So for example, my line tools plugin makes a line tools class. That class gets instantiated each time the user runs the plugin. I had used instance variables to remember the last line length entered by the user, so that the tool would rememeber the user's last input. But it only remembered their input while the tool weas active. Once they switched over to the select tool, and then back to my plugin, their input was lost because it was stored in an instance variable. So I switched them all to class variables. Then once they input something, that class variable would stay alive during their entire SU session.

Did that make sense and help at all?

Chris

[MartinRinehart]

@unknownuser said:

Classes are simple and straightforward in Ruby

That's certainly true, but please explain why:

class MyUniqueName
    # funcs and vars here
end

is better than:

module MyUniqueName
    # funcs and vars here
end

[fredo6]

Martin,

A Class allows to create instances, which are designated by variables, so very easy to manipulate.
It is also a good way to encapsulate an interface (what SU does with the Sketchup::Tool placeholder class)
Within the class statement, you can declare methods with their name, without any prefixing.

Personally, I always declare classes within modules. Classes is a way to manipulate objects, whereas module is a way to package the script and provide privacy via a namespace.

Fredo

[Jim]

For a typical plugin that uses a module strictly as a namespace, an instance variable (@anytyhing) can be thought of as "global" within the module.

module MyMod
  @var = 1
  def MyMod.one
    @var # <- this is the same @var
  end
  def self.two
    @var # <-- the same @var
  end
end

Even if the module is split across files.

# File1.rb
module MyMod
    @var ||= 1
    def MyMod.one
        @var ||= 2
        p @var
    end
end

#File2.rb
module MyMod
    @var ||= 3
    def MyMod.two
        @var ||= 4
        p @var
    end
end

Technically, instance variables are bound to self, so you need to be able to resolve what object is self to know which instance variables are in scope.

I think a class variable will have the same visibility when used the same way in a module, but for a different reason.

[MartinRinehart]

@unknownuser said:

Within the class statement, you can declare methods with their name, without any prefixing.

I find Ruby's insistence on self.xxx to define module-level functions is quite a nuisance. On the other hand, if you create a singleton instance of your class, then to manipulate anything you have to instance.xxx() whereas in the module you can call your functions without a prefix.

In practice, both my Rubies have used classes within modules but that's because I converted original module-less code to module-enclosed code, classes and all.

[MartinRinehart]

@jim said:

For a typical plugin that uses a module strictly as a namespace, an instance variable (@anytyhing) can be thought of as "global" within the module....

Even if the module is split across files.

Didn't know that you could have instance vars in modules, nor that you could split modules across files. Thanks.

[Jim]

@martinrinehart said:

Didn't know that you could have instance vars in modules, nor that you could split modules across files. Thanks.

It's one Ruby's hallmark features (for better or worser) - you can re-open a class at any time and add to it, or redefine what's there.

[NewOne]

@jim said:

For a typical plugin that uses a module strictly as a namespace, an instance variable (@anytyhing) can be thought of as "global" within the module.

I think a class variable will have the same visibility when used the same way in a module, but for a different reason.

Before starting this post (and that was the reason for the post), I had some problems with instance/class variables. I noticed that instance variables act something like "globals" inside a module, but something strange happened with class variables. I could not acces class variables from outside class. I am not sure yet what was the cause...

[NewOne]

As I leaned since now, in ruby, variables don't store data, instead they are pointing to value. But I don't know if class variables have pointers or not? Do they are static or dynamic?

[MSP_Greg]

Boy, a lot of topics in this thread. Here's an old programmer's brief thoughts...

Why use Classes - Use classes when 'things' have data/properties. I have code that exports faces from SU to a text file. One format requires all the coordinates be listed together (with an ID) in the text file. My code loops thru the faces and creates an object from each face, with layer, material, etc and coordinate info as instance variables. The loop code creates a few hashes for the coordinates; the face objects can reference that after it's created. If I didn't use classes, I'd be screwing around with crazy arrays and all sorts of stuff. Years ago, that's how one had to code, B4 OOP.

Class vs Instance - Using the above example, I use class variables for switches that affect all the objects, like what data to export. Also, the coordinate hashes are class variables.

All of my web dialogs are classes, not because I create a lot of them, but because the class holds data. Both data for the dialog, and data that exists after a modal dialog is closed, like the choices a user picked.

Keep in mind that classes are very generic and designed so that the user can't get at the internals. That's what setters/getters are for, so the designer, if needed, can validate changes made by an object's user.

HTH,

Greg

[david.]

@newone said:

@jim said:

For a typical plugin that uses a module strictly as a namespace, an instance variable (@anytyhing) can be thought of as "global" within the module.

I think a class variable will have the same visibility when used the same way in a module, but for a different reason.

Before starting this post (and that was the reason for the post), I had some problems with instance/class variables. I noticed that instance variables act something like "globals" inside a module, but something strange happened with class variables. I could not acces class variables from outside class. I am not sure yet what was the cause...

You shouldn't be able to access class variables outside a class (object instance) without using an accessor (a reader and/or writer). Same goes for instances variables. That's part of the reason for using classes. There really are no global variables except true global variables (eg, using '$' as the first character of the name). And, Module vars are not globally scoped, they are Module scope only.

Class vars are visible (or shared) to all object instances of a class. Instance vars are visible only to a specific object instance of a class. That is why I gave the simple example using the truck above. Neither is visible outside an object instance without an accessor (using either a class method or attribute accessor).

You should check out the freely available online doc... Programming Ruby for more details about classes and variables in Ruby.

[Dan Rathbun]

@martinrinehart said:

@unknownuser said:

Within the class statement, you can declare methods with their name, without any prefixing.

I find Ruby's insistence on self.xxx to define module-level functions is quite a nuisance. On the other hand, if you create a singleton instance of your class, then to manipulate anything you have to instance.xxx() whereas in the module you can call your functions without a prefix.

You don't need to prefix methodnames in modules. Use a class << self block wrapper around all the methods inside the module.

See my post: [info] Using Ruby Modules