What does a double * (splat) operator do

Have you seen a function declared like this?

def foo a, **b ... end 

I understand that a single * is the splat operator. What does ** mean?

4 Answers

Ruby 2.0 introduced keyword arguments, and ** acts like *, but for keyword arguments. It returns a Hash with key / value pairs.

For this code:

def foo(a, *b, **c) [a, b, c] end 

Here's a demo:

> foo 10 => [10, [], {}] > foo 10, 20, 30 => [10, [20, 30], {}] > foo 10, 20, 30, d: 40, e: 50 => [10, [20, 30], {:d=>40, :e=>50}] > foo 10, d: 40, e: 50 => [10, [], {:d=>40, :e=>50}] 

That is the double splat operator which is available since Ruby 2.0.

It captures all keyword arguments (which can also be a simple hash, which was the idiomatic way to emulate keyword arguments before they became part of the Ruby language)

def my_method(**options) puts options.inspect end my_method(key: "value") 

The above code prints {key:value} to the console.

Just like the single splat operator captures all regular arguments, but instead of an array you get a hash.

Real-life example:

For example in Rails the cycle method looks like this:

def cycle(first_value, *values) options = values.extract_options! # ... end 

This method can be called like this: cycle("red", "green", "blue", name: "colors").

This is quite a common pattern: You accept a list of arguments and the last one is an options hash, which can be extract - for example - using ActiveSupport's extract_options!.

In Ruby 2.0 you can simplify these methods:

def cycle(first_value, *values, **options) # Same code as above without further changes! end 

Admittedly it's only a minor improvement if you are already using ActiveSupport but for plain Ruby the code gains quite a lot of conciseness.

In addition, you can use it in caller side like this:

def foo(opts); p opts end bar = {a:1, b:2} foo(bar, c: 3) => ArgumentError: wrong number of arguments (given 2, expected 1) foo(**bar, c: 3) => {:a=>1, :b=>2, :c=>3} 

For your case, the above answers are accurate.

However, double splat operators can also be used in Ruby language for arithmetic operations as well. For example, x^y can be written as x**y.

Refer to this stackover question for more.