At Symbol

The @ symbol is used to define instance variables (@ivar) and class variables (@@cvar). We'll also see it show up in the docs to differentiate unary operators from binary operators.

Instance and Class Variables

An instance variable is prefixed with @ and is unique to the instance of the object. A class variable is prefixed with @@ and is shared across all instances of the class.

Here is an only-sorta-contrived example of using instance and class variables:

class DatabaseConnection
  @@active_connections = 0
  @@max_connections = 100

  def initialize(host, port)
    @host = host
    @port = port
    @connected = false
  end

  def connect
    if @@active_connections < @@max_connections
      @connected = true
      @@active_connections += 1
      "Connected to #{@host}:#{@port}"
    else
      "Connection limit reached"
    end
  end

  def disconnect
    if @connected
      @connected = false
      @@active_connections -= 1
      "Disconnected from #{@host}:#{@port}"
    end
  end
end

This class defines a couple class variables (@@active_connections and @@max_connections) at the top of its definition. It also defines a few instance variables (@host, @port, and @connected) when an instance is initialized.

See the Ruby docs on instance variables ↗ and class variables ↗.

Unary Operator Documentation

The Integer class, for instance, has two ways of using the - operator. The one you might be familiar with is the binary operator for subtracting another number from the subject integer (note: two operands, hence binary operator). The Ruby docs show it as #- (see Integer#- ↗). The - operator can also be used as a unary operator for negating the subject integer (4 versus -4). This is annotated in the Ruby docs as #-@ (see Integer#-@ ↗).