In Ruby 2.4.0, the Fixnum and Bignum classes were merged into Integer, resolving a long-standing issue.

• Fixnum for small integers (range depends on implementation, usually limited to machine word size)

• Bignum for larger integers (arbitrary precision)

Let’s see this in action.

Ruby 2.3.3

$ ruby -v
ruby 2.3.3p222 (2016-11-21 revision 56859) [x86_64-darwin16]
$ irb
>> 4.class
=> Fixnum
>> (2**62).class
=> Bignum    

two integer classes are both subclasses of Integer.

>> 4.class.superclass
=> Integer
>> (2**62).class.superclass
=> Integer

So, in Ruby 2.3.3:

• Small numbers → Fixnum

• Large numbers → Bignum

• Both inherit from Integer

Ruby 2.4.0

Now, let’s see what occurs in Ruby 2.4.0:

$ ruby -v
ruby 2.4.0p0 (2016-12-24 revision 57164) [x86_64-darwin16]
$ irb
>> 4.class
=> Integer
>> (2**62).class
=> Integer

No matter how small or large the number is, Ruby now uses Integer.

This change is known as “Fixnum and Bignum Unification” (Feature #12005).

Why Did Ruby Do This?

• Simplicity – Developers no longer need to worry about whether a number is a Fixnum or a Bignum. Everything is simply an Integer.

• Consistency – Code that depended on checking Fixnum or a Bignum directly could behave inconsistently. Now, you can rely on a single class.

• Cleaner APIs – Methods like is_a?(Fixnum) or is_a?(Bignum) are no longer necessary. Instead, always check against `Integer`.

Compatibility Considerations

If your code explicitly checks for Fixnum or Bignum, you’ll need to update it. For example:

Ruby 2.3.3 code

def big_number?(n)
  n.is_a?(Bignum)
end

Equivalent to Ruby 2.4.0

def big_number?(n)
  n.is_a?(Integer) && n > (2**62)
end

Or more simply, you might just care if it’s an integer.

n.is_a?(Integer)

Other Notable Updates in Ruby 2.4.0

Beyond integers, Ruby 2.4.0 brought some other useful improvements.

1. String#match?: A faster way to test regex matches without creating a MatchData object.

"hello".match?(/h/)   # => true

1. Regexp#match?: Same optimization for regex objects.

1. Hash#transform_keys / transform_values: Easy transformations.

{ a: 1, b: 2 }.transform_keys(&:to_s)  
# => {"a"=>1, "b"=>2}

• Performance improvements:

• Faster method dispatch

• Reduced memory usage

• Better garbage collection for symbols

Final Thoughts

Removing Fixnum and Bignum might surprise developers upgrading from Ruby 2.3.x, but it’s a positive step toward making Ruby more simple and consistent.

From now on, all integers—big or small—are called Integer.

So, the next time you run:

>> 4.class
=> Integer

Just smile and remember that Ruby’s evolution made your life a little easier.

If you’re upgrading an older Ruby application, make sure to grep your codebase for Fixnum or Bignum references and replace them with Integer.

Posted on January 04, 2017 by Amit Pandya in Ruby