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From what I know "<" is inheritance of a class from another class?

but I also see "<<" being used for the same thing from what I am seeing,.

Example: 

def catch_alarm(alarm_info, options)
  alarms = []
  alarms << AlarmMonitor.new({
        region_id: region_name,
        account_id: '10202'.
        alarm_name: alarm_info
    },
  alarms
end
Kev
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    Does this answer your question? [What is the purpose of the "class << Class" (angle brackets) syntax?](https://stackoverflow.com/questions/33453063/what-is-the-purpose-of-the-class-class-angle-brackets-syntax) – Balastrong May 27 '20 at 21:24
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    It would be tremendously helpful for your future fellow software developers if you could explain what, precisely, is unclear to you about [the documentation of `Array#<<`](https://ruby-doc.org/core/Array.html#method-i-3C-3C). Only if you explain what is wrong with the documentation will the Ruby developers be able to improve it, so that future developers do not run into the same problems you did. You will essentially be making the world a better place, and who wouldn't want that? – Jörg W Mittag May 27 '20 at 21:50

2 Answers2

3

First of all, < and << are not the same.

Ruby – like many other programming languages – uses the infix notation for binary operators, i.e.:

left-hand side operator right-hand side

For example:

1 + 2
a - b
i >= j
x << y

But unlike many other languages, operators are implemented as methods in Ruby. So it's actually:

receiver method argument

If you want to know, what alarms << AlarmMonitor.new(...) does, you have to know what kind of object alarms is. In your example, it's an array (alarms = []) so the method is Array#<<:

ary << obj → ary

Append—Pushes the given object on to the end of this array. This expression returns the array itself, so several appends may be chained together.

Note that different objects (or classes) can implement the same operator in different ways:

a = [1, 2]
a << 3       # pushes 3 onto the array, i.e. [1, 2, 3]

d = Date.new(2020, 5, 28)
d << 3       # returns a date 3 months earlier, i.e. 2020-02-28

i = 0b00001011
i << 3       # shifts the bits 3 to the left, i.e. 0b01011000

The behavior primarily depends on the receiver.

Let's briefly get back to <. In addition to methods, Ruby has keywords. Keywords sometimes work like methods, but they have their own parsing rules.

For class creation, Ruby has the class keyword:

class MyClass
  # ...
end

Which accepts an optional subclass via:

class MySubclass < MyClass
  # ...
end

Here, < is not an operator, but part of the class keyword syntax.

However, there's also a < operator for classes: Module#<. It can be used to check whether MySubclass is a subclass of MyClass: (thus mirroring the class creation syntax)

MySubclass < MyClass
#=> true
Stefan
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2

In this context << is the Array push operator, so isn’t related to the use of < for class inheritance.

simonwo
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