Access instance variable from outside the class

问题

If an instance variable belongs to a class, can I access the instance variable (e.g. @hello) directly using the class instance?

class Hello
  def method1
    @hello = "pavan"
  end
end

h = Hello.new
puts h.method1

回答1:

Yes, you can use instance_variable_get like this:

class Hello
  def method1
    @hello = "pavan"
  end
end

h = Hello.new
p h.instance_variable_get(:@hello) #nil
p h.method1                        #"pavan" - initialization of @hello
p h.instance_variable_get(:@hello) #"pavan"

If the variable is undefined (first call of instance_variable_get in my example) you get nil.

As Andrew mention in his comment:

You should not make this the default way you access instance variables as it violates encapsulation.

A better way is to define an accessor:

class Hello
  def method1
    @hello = "pavan"
  end
  attr_reader :hello  
end

h = Hello.new
p h.hello #nil
p h.method1                        #"pavan" - initialization of @hello
p h.hello #"pavan"

If you want another method name, you could alias the accessor: alias :my_hello :hello.

And if the class is not defined in your code, but in a gem: You can modify classes in your code and insert new functions to classes.

回答2:

You can also accomplish this by calling attr_reader or attr_accessor like this:

class Hello
  attr_reader :hello

  def initialize
    @hello = "pavan"
  end
end

class Hello
  attr_accessor :hello

  def initialize
    @hello = "pavan"
  end
end

Calling attr_reader will create a getter for the given variable:

h = Hello.new
p h.hello        #"pavan"

Calling attr_accessor will create a getter AND a setter for the given variable:

h = Hello.new
p h.hello        #"pavan"
h.hello = "John"
p h.hello        #"John"

As you might understand, use attr_reader and attr_accessor accordingly. Only use attr_accessor when you need a getter AND a setter and use attr_reader when you only need a getter

来源：https://stackoverflow.com/questions/12122736/access-instance-variable-from-outside-the-class

标签

ruby

instance-variables