CODEWITHSUNDEEP
rubyconstants
dj199008
dj199008

Reputation: 1789

confusion about ruby CONSTANT

Suppose a simple ruby program about CONSTANT variable: 

OUTER_CONST = 99
class Const
  def get_const
    CONST
  end
  CONST = OUTER_CONST + 1
end

puts Const.new.get_const

I assume the result of Const.new.get_const should be nil, but the result is 100! I wonder why?

Upvotes: 1

Views: 102

Answers (2)

mdesantis
mdesantis

Reputation: 8507

get_const is a method, and you are calling it after CONST definition; so when you call it CONST is already defined.

def get_const ... end defines a method, does not execute its content; you execute its content when you call it at the Const.new.get_const line, so when CONST is already defined.

Besides: if CONST was not defined at the moment of get_const call, you would not get nil, but a NameError:

class Const
  def get_const
    CONST
  end
end

Const.new.get_const #=> NameError: uninitialized constant Const::CONST

Upvotes: 4

Agis
Agis

Reputation: 33626

This is because Ruby is dynamic and the constant lookup happens at runtime. Also keep in mind that your script is evaluated sequentially (ie. line by line).

I've added some comments for clarity:

OUTER_CONST = 99

class Const
  def get_const
    CONST
  end

  CONST = OUTER_CONST + 1
  # the CONST constant is now
  # defined and has the value 100
end

# at this point the Const class
# and CONST constant are defined and can be used

# here you are calling the `get_const` method
# which asks for the value of `CONST` so the 
# constant lookup procedure will now start and
# will correctly pick up the current value of `CONST`
Const.new.get_const # => 100

Upvotes: 2

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