如何从Ruby中的扩展类访问变量

Question

In most languages you can do something like the following:

class a {
    this.property = 'prop'
}

class b extends a {
    puts this.property //prints 'prop'
}

How does this work in ruby? My initial thought was something like

class A
    @property = 'prop'
end

class B < A
    puts @property
end

But this doesn't work. Ruby has the self keyword but that seems from what I can tell to be reserved for methods. Can classes inherit variables in ruby?

Answer 1

Yes they can, and here's a snippet that shows how.

class A2
  def initialize
    @foo = 42
  end
end

class B2 < A2
  def print_foo
    puts @foo
  end
end

# Prints 42
B2.new.print_foo

The above code defines a class A2 , with a constructor that defines and sets an instance variable @foo . Class B2 extends A2 , and defines a method that uses @foo .

I think the issue with your code is that @property is not assigned a value, as the assignment isn't in a method that gets called at any point.

Answer 2

Yes. You need to init instance variables in the initialize method.

See an example:

class A
  attr_accessor :property  

  def initialize
    self.property = 'prop'
  end
end

class B < A
end

puts B.new.property # prints "prop"

Answer 3

Variables starting with an @ sigil are instance variables . They belong to a particular instance (ie "object").

You have two objects in your code, one object is the class A , the second object is the class B . Each of those two objects has its own instance variables.

A has an instance variable called @property with the value 'prop' . B has no instance variables, however, uninitialized instance variables evaluate to nil instead of raising an exception, so, you don't get an error, but it evaluates to nil .

You cant' inherit variables. The only thing you can inherit in Ruby, are methods:

class A
  def self.property
    'prop'
  end
end

class B < A
  puts property
end
# prop

Ruby also has class variables that start with an @@ sigil. These are shared across the entire class hierarchy:

class A
  @@property= 'prop'
end

class B < A
  puts @@property
end
# prop

However, class variables are almost never used in Ruby, because of the wide scope they have, almost like globals.