Why

Today while read source code of Grape gem, I spot an interesting line:

Hash === options

This makes me wonder what is === and how it works.

There seems serval “Equality” method in Ruby. So after some Googling, I find a stack-overflow answer that states difference among them. It seems to be the Case Euqality operator, most commonly used with case...when statements.

However, another things bugs me: This operator seems to can be called on Class object as well as instance object. How it that works?

To find out, I first look up the documentation.

For Object#===, it says:

Returns true or false. (1) Like Object#==, if object is an instance of Object (and not an instance of one of its many subclasses). (2) This method is commonly overridden by those subclasses, to provide meaningful semantics in case statements. (3)

I have no problem with sentence 1, the returning value part. For sentence 2, I don’t quite get its meaning. Then for sentence 3, I guess it means Subclasses may define custom behavior for their instances. So there is no mentioning how this operator is used with Class objects.

How

With confusion bore in my mind, I read some good articles explaining Ruby inheritance hierarchy and the === operator. But none of them answer my question.

Then I read the Object#=== documentation again and finally have a clue in my mind.

Sentence 2 can be rephrased like this: If self is an instance of Object, then this operator works just like Object##, which is “pointer comparison”.

For sentence 3, since Class is a descendent of Object, it should be able to have custom behavior.

The inheritance chain of Class is: [Class, Module, Object]. And finally I found the explanation of Module#===:

Case Equality—Returns true if obj is an instance of mod or an instance of one of mod’s descendants. Of limited use for modules, but can be used in case statements to classify objects by class.

What

As a Ruby beginner who comes from C like language, I always found it’ confusing that many method can be called on instances of Classes as well as Classes themselves. You cannot just tell whether a method is class level or instance level easily.

So my conclusion is: There is no such difference regarding class or instance method. They are only instances, either born from Class or from some other classes. A method can be called on Class directly if you include it into Class. If included into class of an instance, then it’s available for instances.

Where

When

Who