Instance variables in Ruby

Question

Ruby 1.9

I suddenly realize I don't understand how to define and initialize an instance variable in Ruby. It must be used only within a certain class and not accessible out of a class at all, so attr_accessor or attr_reader is not what I need.

class MyClass #how do I initialize it? @my_var = 'some value' def method1 #I need to do something with @my_var puts @my_var end def method2 #I need to do something with @my_var puts @my_var end end a = MyClass.new a.method1 #empty a.method2 #empty

So I found that there is another way to do it

class MyClass #is this the only way to do it? def initialize @my_var = 555 end def method1 #I need to do something with @my_var puts @my_var end def method2 #I need to do something with @my_var puts @my_var end end a = MyClass.new a.method1 #555; it's ok a.method2 #555; it's ok

Well, is second approach the right one?

Baldrick · Accepted Answer · 2012-08-02 14:21:02Z

3

each class has an initialize() method that acts similar to a constructor in other languages, instance variables should be initialized there:

class MyClass def initialize @my_var = 'some value' end # etc... end

edited Aug 2, 2012 at 14:21

Baldrick

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answered Aug 2, 2012 at 14:18

Hunter McMillen

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4 Comments

Alexandre Over a year ago

They can be initialized only in the method (contructor) initialize()? It's my question.

tadman Over a year ago

They can be initialized in any instance method but initialize is the first method called for any given instance so it's the best place to do it.

Alexandre Over a year ago

Why can't it be initialized out of any method? class MyClass @my_var = 'some value' ;end ?

Hunter McMillen Over a year ago

Whenever you create an instance variable it gets associated with whatever the current value of self is, outside of any method if would get associated with the MyClass class object, inside of a method self refers to the current instance of MyClass. So if you initialized outside of the method it would be associated with the class itself rather than a specific instance of that class.

knut · Accepted Answer · 2012-08-02 14:31:04Z

Yes, initialize is the right way.

But you may also make:

class MyClass def method1 #I need to do something with @my_var puts @my_var ||= 555 end def method2=(x) #I need to do something with @my_var puts @my_var = x end end #Test: x = MyClass.new x.method1 #555 x.method2= 44 #44 x.method1 #44

When method1is called the first time, it initialize the variable.

Edit: It does not work as expected, when you assign nil

x = MyClass.new x.method1 #555 x.method2= nil #nil x.method1 #again 555

This version works better:

class MyClass def method1 @my_var = 555 unless defined? @my_var puts @my_var end def method2=(x) puts @my_var = x end end x = MyClass.new x.method1 #555 x.method2= nil #nil x.method1 #keeps nil

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