How can I initialize a module's instance variables in Ruby?
RubyModuleInstance VariablesRuby Problem Overview
I have some modules where I would like to use instance variables in. I'm currently initializing them like this:
module MyModule
def self.method_a(param)
@var ||= 0
# other logic goes here
end
end
I also could call a init method to initialize them:
def init
@var = 0
end
but this would mean I have to remember to always call it.
Is there a better way of doing this?
Ruby Solutions
Solution 1 - Ruby
Initialize them in the module definition.
module MyModule
# self here is MyModule
@species = "frog"
@color = "red polka-dotted"
@log = []
def self.log(msg)
# self here is still MyModule, so the instance variables are still available
@log << msg
end
def self.show_log
puts @log.map { |m| "A #@color #@species says #{m.inspect}" }
end
end
MyModule.log "I like cheese."
MyModule.log "There's no mop!"
MyModule.show_log #=> A red polka-dotted frog says "I like cheese."
# A red polka-dotted frog says "There's no mop!"
This will set the instance variables when the module is defined. Remember, you can alwasys reopen the module later to add more instance variables and method definitions, or to redefine existing ones:
# continued from above...
module MyModule
@verb = "shouts"
def self.show_log
puts @log.map { |m| "A #@color #@species #@verb #{m.inspect}" }
end
end
MyModule.log "What's going on?"
MyModule.show_log #=> A red polka-dotted frog shouts "I like cheese."
# A red polka-dotted frog shouts "There's no mop!"
# A red polka-dotted frog shouts "What's going on?"
Solution 2 - Ruby
You can use:
def init(var=0)
@var = var
end
And it will default to 0 if you don't pass anything.
If you don't want to have to call it everytime, you could use something like this:
module AppConfiguration
mattr_accessor :google_api_key
self.google_api_key = "123456789"
...
end
Solution 3 - Ruby
i answered a similar question, you can set class instance variables doing this
module MyModule
class << self; attr_accessor :var; end
end
MyModule.var
=> nil
MyModule.var = 'this is saved at @var'
=> "this is saved at @var"
MyModule.var
=> "this is saved at @var"
Solution 4 - Ruby
Apparently it's bad form to initialise instance variables in a module in Ruby. (For reasons I don't fully understand, but pertaining to the order in which things are instantiated.)
It seems that best practice is to use accessors with lazy initialisation, like so:
module MyModule
def var
@var ||= 0
end
end
Then use var
as the getter for @var
.
Solution 5 - Ruby
for a class, I'd say the following, since the initialize is called whenever you .new a new instance of the class.
def initialize
@var = 0
end
from Practical Ruby:
> It goes on to say that a module's > initialize will be called if an > including class's initialize calls > super, but doesn't mention that this > is a consequence of how super works > everywhere, not special handling for > initialize. (Why might one assume > initialize gets special handling? > Because it gets special handling with > respect to visibility. Special cases > create confusion.)