How does defining [square bracket] method in Ruby work?

It's just syntactic sugar. There are certain syntax patterns that get translated into message sends. In particular

a + b

is the same as

a.+(b)

and the same applies to ==, !=, <, >, <=, >=, <=>, ===, &, |, *, /, -, %, **, >>, <<, !==, =~ and !~ as well.

Also,

!a

is the same as

a.!

and the same applies to ~.

Then,

+a

is the same as

a.+@

and the same applies to -.

Plus,

a.(b)

is the same as

a.call(b)

There is also special syntax for setters:

a.foo = b

is the same as

a.foo=(b)

And last but not least, there is special syntax for indexing:

a[b]

is the same as

a.[](b)

and

a[b] = c

is the same as

a.[]=(b, c)

Methods in ruby, unlike many languages can contain some special characters. One of which is the array lookup syntax.

If you were to implement your own hash class where when retrieving an item in your hash, you wanted to reverse it, you could do the following:

class SillyHash < Hash  def [](key)    super.reverse  endend

You can prove this by calling a hash with the following:

a = {:foo => "bar"} => {:foo=>"bar"} a.[](:foo) => "bar" a.send(:[], :foo) => "bar" 

So the def [] defined the method that is used when you do my_array["key"] Other methods that may look strange to you are:

class SillyHash < Hash  def [](key)    super.reverse  end  def []=(key, value)    #do something  end  def some_value=(value)    #do something  end  def is_valid?(value)    #some boolean expression  endend

Just to clarify, the definition of a [] method is unrelated to arrays or hashes. Take the following (contrived) example:

class B  def []    "foo"  endend B.new[] => "foo" 

the square brackets are the method name like Array#size you have Array#[] as a method and you can even use it like any other method:

array = [ 'a', 'b', 'c']array.[](0) #=> 'a'array.[] 1  #=> 'b'array[2]    #=> 'c'

the last one is something like syntactic sugar and does exactly the same as the first one. The Array#+ work similar:

array1 = [ 'a', 'b' ]array2 = [ 'c', 'd' ]array1.+(array2) #=> [ 'a', 'b', 'c', 'd' ]array1.+ array2  #=> [ 'a', 'b', 'c', 'd' ]array1 + array2  #=> [ 'a', 'b', 'c', 'd' ]

You can even add numbers like this:

1.+(1) #=> 21.+ 1  #=> 21 + 1  #=> 2

the same works with /, *, - and many more.