What is mattr_accessor in a Rails module?

Question

I couldn\'t really find this in Rails documentation but it seems like \'mattr_accessor\' is the Module corollary for \'attr_accesso

Answer 1

Here's the source for cattr_accessor

And

Here's the source for mattr_accessor

As you can see, they're pretty much identical.

As to why there are two different versions? Sometimes you want to write cattr_accessor in a module, so you can use it for configuration info like Avdi mentions.
However, cattr_accessor doesn't work in a module, so they more or less copied the code over to work for modules also.

Additionally, sometimes you might want to write a class method in a module, such that whenever any class includes the module, it gets that class method as well as all the instance methods. mattr_accessor also lets you do this.

However, in the second scenario, it's behaviour is pretty strange. Observe the following code, particularly note the @@mattr_in_module bits

module MyModule
  mattr_accessor :mattr_in_module
end

class MyClass
  include MyModule
  def self.get_mattr; @@mattr_in_module; end # directly access the class variable
end

MyModule.mattr_in_module = 'foo' # set it on the module
=> "foo"

MyClass.get_mattr # get it out of the class
=> "foo"

class SecondClass
  include MyModule
  def self.get_mattr; @@mattr_in_module; end # again directly access the class variable in a different class
end

SecondClass.get_mattr # get it out of the OTHER class
=> "foo"