Ruby 关键字参数说明

Question

Lately I saw this function signature:

   class Foo < Bar
    def initialize(arg: {})
      ...
   end

What does the keyword argument ( arg: {} ) with the curly braces means here?

Answer 1

Its just a keyword argument with an empty hash as the default value:

def initialize(arg: {})
  arg
end

irb(main):011:0> initialize().class
=> Hash

Its really strange and unidiomatic though. Before Ruby 2.0 introduced first-class support for keywords you declared a method that takes an optional options hash as:

def initialize(hash = {})

end

This argument had to be at the end of the list of. The name is not significant.

With Ruby 2.0 you can declare a method that takes any number of keywords with a double splat:

def initialize(**other_keyword_args)

end

You can combine it with positional and named keyword arguments as well:

def initialize(a, b = 2, foo:, bar: 2, **other_keyword_args)

end

Using initialize(arg: {}, ...) would make sense if there where more parameters and this one takes a hash but on its own its just strange.

Answer 2

Ruby doesn't need braces ( {} ) around a hash when passing the last argument to a method:

def foo(anything)
  anything       # => {:arg=>{}}
          .class # => Hash
end

foo(arg: {})

For more information see " Calling Methods ."