Ruby comparison operators? == vs. ===

Question

What's the difference between == and ===? Which one should you use when?

Answer 1

== is used for equality in conditional statements like if, unless, etc. === is used for determining equality in case statements.

Answer 2

Both are just methods called on objects. This means that the objects decide which means what. However, there are conventions in Ruby about how these are different. Usually, == is stricter than === - a === b will almost always be true if a == b is. The best place to read about this is http://ruby-doc.org/core/classes/Object.html. Scroll down to the different sections about == and ===. Here are some of the conventions I know about:

• ==, when applied to plain Objects, will only be true if one is exactly the same as the other - if they are stored in the same memory location (this is how Ruby works internally). If the arguments are of types other than Object, though, this method will usually be overridden.
• equal? is just like == for plain Objects, but will never be overridden by subclasses.
• === is used for:
  • an is_a? alternative, backwards. String === 'str' is true.
  • matching regexes. /s[at]r*/ === 'str' is true.

You can find the specific meaning of === for various classes in the documentation for those classes, for example, the Range version is here (a synonym for include?): http://ruby-doc.org/core/classes/Range.html#M000691