Ruby Notes

modules and instance / class variables

If I have a module that includes two variables like this:

module Test_module

  @my_var1 = "init string1"

  @@my_var2 = "init string 2"

then I include that module in a class:

class Test_class1

  include Test_module

and create two instances of that class:

myobj1 = Test_class1.new

myobj2 = Test_class1.new

my_var1 and my_var2 will now exist as attributes of myobj1 and myobj2. However:

• my_var1 will be nil in each object, as it is an instance variable and therefore needs to be set for each instance of the Test_class object. The initialisation value in Test_module is ignored.
• my_var2 will be "init string 2" in each object (at this point) as it is a class level variable

If I run

myobj1.my_var1 = "in obj1"

myobj2.my_var1 = "in obj2"

Then each object will have a distinct vale in their my_var1 variable. However if I run:

myobj1.my_var2 = "new my_var2 value"

Then this changes the "class" (module) variable's value. As there is only ever one module, this means myobj2.my_var2 points to the same memory location and hence myobj2.my_var2 == "new my_var2 value"

What makes this a little more interesting, if I include the module in another class

class Test_class2

  include Test_module

and create an instance of that class:

myobj3 = Test_class2.new

myobj3.my_var2 ALSO points to the same memory location and hence myobj3.my_var2 == myobj2.my_var2 == myobj2.my_var1

So the @@ variables in a module are specific to the module, regardless of what classes it is included in.

Last but not least, any references to @my_var1 within the Module (other than the initialisation string!) impact the instance variable in the class that included the Module.