Objects

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Objects in Ruby

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Every class in Ruby is a subclass of Object, everything is an instance of Object.

In Ruby, everything is an Object. That means that hashes, strings, objects from user-defined classes, and even numbers and strings all have methods.

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Equality in Ruby

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Object defines a method == that compares two objects and returns true if they are equal. Two objects are equal if they have the same value; this works for all built in Ruby classes:

2 == 2 # => true
2 == 4 # => false


However, by default, an Object doesn't know how to compare itself to other objects. For instance, if you define a new class, Fraction:

class Fraction
attr_reader :numerator, :denominator
def initialize(numerator, denominator)
@numerator, @denominator = [numerator, denominator]
end
end

Fraction.new(3, 6) == Fraction.new(1, 2) # => false

YOU have to write your own == method if you want to compare your classes with '=='


class Fraction
# ...
def ==(other)
# first, check if we're comparing two fractions, #comparing two different types of objects should return false

return false unless (other.is_a?(Fraction))

(numerator / denominator) == (other.numerator / other.denominator)
end
end

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More on Equality in Ruby

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== — generic "equality"

At the Object level, == returns true only if obj and other are the same object. Typically, this method is overridden in descendant classes to provide class-specific meaning.

This is the most common comparison, and thus the most fundamental place where you (as the author of a class) get to decide if two objects are "equal" or not.

=== — case equality

For class Object, effectively the same as calling #==, but typically overridden by descendants to provide meaningful semantics in case statements.

This is incredibly useful. Examples of things which have interesting === implementations:

Range
Regex
Proc (in Ruby 1.9)
So you can do things like:

case some_object
when /a regex/
# The regex matches
when 2..4
# some_object is in the range 2..4
when lambda {|x| some_crazy_custom_predicate }
# the lambda returned true
end