How to use an overridden constant in an inheritanced class

Question

given this code:

class A
  CONST = 'A'

  def initialize
    puts CONST
  end
end

class B < A
  CONST = 'B'
end

A.new # => 'A'
B.new # => 'A'

I'd like B to use the CONST = 'B' definition, but I don't know how. Any ideas?

Greetings

Tom

Solution 1

class A
  CONST = 'A'

  def initialize
    puts self.class::CONST
  end
end

class B < A
  CONST = 'B'
end

A.new # => 'A'
B.new # => 'B'

Solution 2

I had a few issues with the solution by Konstantin Haase. When accessing the constant in an instantiated object of the inheriting class, the parent constant was used.

I had to explicitly refer to the class.

self.class::CONST

cheers

Solution 3

Sorry I couldn't get the code formatting to work in a 'comment' only in an 'answer' but this is in response to akostadinov's question to Hendrik "how is this different from his [Konstantin's] answer?"

I'd guess Hendrik was trying to access the constant from methods in his inheriting class & that depends on if it's an instance or static method. It seems to behave as you'd expect in an instance method. But maybe or maybe not how you'd expect for a static method. Even if that's not what Hendrik meant, this may be worth noting:

If you have the exact class definitions as Konstantin, but you add a method to class A like this:

def self.print_const
  puts CONST
end

Then you get A both times:

A.print_const # prints A
B.print_const # prints A

However if you define the method in A by referencing the class:

def self.print_const
  puts self::CONST
end

Then you get:

A.print_const # prints A
B.print_const # prints B

Solution 4

In case anyone finds this and is using module extension instead, just use

self::CONST

View more solutions

Comments

[nohat]

So... this means there is no way to do it without changing everywhere in class A that uses CONST?

[Konstantin Haase]

Yes. Constant lookup is usually bound at compile time.

[akostadinov]

how is this different from his answer?