What does the `<<` (double less than) mean without an argument?

Question

I found this inside a method I want to override in the formtastic Gem. The method looks like:

def to_html
  input_wrapping do
    hidden_field_html <<
    label_with_nested_checkbox
  end
end

What does the << do on the third line? I know what it does with Arrays but here I have no idea.

Answer 1

You can read it like this:

hidden_field_html << label_with_nested_checkbox

label_with_nested_checkbox is the argument being concatenated onto the end of hidden_field_html - they've split it over two lines for 'clarity'

Answer 2

1. In class inheritance there is < used, not <<, and the former has nothing to do with method << in general.

2. Ruby has a high level of tolerance to space indenting; almost everywhere one might put any amount of spaces, including newlines, between function call and it’s argument.

E. g.:

'aaa'.
  length
#⇒ 3

and

'aaa'
  .length
#⇒ 3

are both perfectly valid.

3. << is the generic method, that might be overwritten in any class. Here it is supposedly String#<< method, that inplace appending the argument to the string receiver.

In general, one might overwrite this method in any arbitrary class:

class A
  attr_accessor :var
  def initialize
    @var = 5
  end
  def << value
    @var += value
  end
end

a = A.new
a.var
#⇒ 5
a << 37
a.var
#⇒ 42