CODEWITHSUNDEEP
ruby
kravcneger
kravcneger

Reputation: 61

Why does a class inherit from a struct

Is

class A < Struct.new(:att)
end

the same as

class A
  attr_accessor :att
end

in Ruby?

Upvotes: 4

Views: 2390

Answers (2)

7stud
7stud

Reputation: 48649

Why does a class inherit from a struct

It doesn't. Classes inherit from other classes. In ruby, to create an inheritance hierarchy you can write:

class A
  def bark
    puts "woof"
  end
end

class B < A
end

B.new.bark

--output:--
woof

But, you can also do this:

class A
  def bark
    puts "woof"
  end
end

def do_stuff
  return A
end

class B < do_stuff
end

B.new.bark

--output:--
woof

Is that a class that inherits from a method? No! Ruby lets you use an arbitrary expression for the parent class--as long as it returns a class. The code:

Struct.new(:att)

is an arbitrary expression which returns a class:

x = Struct.new(:att)
p x

--output:--
#<Class:0x007f8c3126c568>

Upvotes: 0

Amadan
Amadan

Reputation: 198446

No. It is equal to

B = Struct.new(:att)
class A < B; end

which is pretty much equivalent, but not equal to

class B
  attr_accesor :att
  def initialize(att)
    @att = att
  end
end

class A < B
end

Not equal because the value is not actually stored in @att:

B = Struct.new(:att)

class B
  def real_att
    @att
  end
end
b = B.new(32)
b.att
# => 32
b.real_att
# => nil

Personally, I don't see why one wouldn't just do A = Struct.new(:att), without the inheritance business.

EDIT: As Jordan notes in comments, there is an even nicer way to add methods to a struct: the Struct conStructor (teehee) takes a block, which executes in the context of the newly created class: whatever you define there, will be defined on the new struct.

B = Struct.new(:att) do
  def process
    #...
  end
end

Upvotes: 11

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