What does << mean in Ruby?

It can have 3 distinct meanings:

'<<' as an ordinary method

In most cases '<<' is a method defined like the rest of them, in your case it means "add to the end of this array" (see also here).

That's in your particular case, but there are also a lot of other occasions where you'll encounter the "<<" method. I won't call it 'operator' since it's really a method that is defined on some object that can be overridden by you or implemented for your own objects. Other cases of '<<'

• String concatenation: "a" << "b"
• Writing output to an IO: io << "A line of text\n"
• Writing data to a message digest, HMAC or cipher: sha << "Text to be hashed"
• left-shifting of an OpenSSL::BN: bn << 2
• ...

Singleton class definition

Then there is the mysterious shift of the current scope (=change of self) within the program flow:

class A  class << self    puts self # self is the singleton class of A  endenda = A.newclass << a  puts self # now it's the singleton class of object aend

The mystery class << self made me wonder and investigate about the internals there. Whereas in all the examples I mentioned << is really a method defined in a class, i.e.

obj << stuff

is equivalent to

obj.<<(stuff)

the class << self (or any object in place of self) construct is truly different. It is really a builtin feature of the language itself, in CRuby it's defined in parse.y as

k_class tLSHFT expr

k_class is the 'class' keyword, where tLSHFT is a '<<' token and expr is an arbitrary expression. That is, you can actually write

class << <any expression>

and will get shifted into the singleton class of the result of the expression. The tLSHFT sequence will be parsed as a 'NODE_SCLASS' expression, which is called a Singleton Class definition (cf. node.c)

case NODE_SCLASS:    ANN("singleton class definition");    ANN("format: class << [nd_recv]; [nd_body]; end");    ANN("example: class << obj; ..; end");    F_NODE(nd_recv, "receiver");    LAST_NODE;    F_NODE(nd_body, "singleton class definition");    break; 

Here Documents

And, how could I forget those, Here Documents use '<<' in a way that is again totally different. You can define a string that spans over multiple lines conveniently by declaring

here_doc = <<_EOS_The quick brown fox jumps over the lazy dog...._EOS_

To distinguish the 'here doc operator' an arbitrary String delimiter has to immediately follow the '<<'. Everything inbetween that initial delimiter and the second occurrence of that same delimiter will be part of the final string. It is also possible to use '<<-', the difference is that using the latter will ignore any leading or trailing whitespace.

Mostly used in arrays to append the value to the end of the array.

a = ["orange"]a << "apple"puts a

gives this

  ["orange", "apple"] 

It's the operator which allows you to feed existing arrays, by appending new items.

In the example above you are just populating the empty array threads with 5 new threads.