Default values of Ruby variables

问题

I need to know the default values of variables in ruby.

Global Variables: nil
Instance Variables: ? 
Class Variables: ?
Local Variables: ?

I looked here, but all I could find is that global variables have a nil value.

回答1:

You assign a local variable value at first assignment:

local = 1
local1 + local # NameError: undefined local variable or method `local1' for main:Object

Class variables are similar, if you use them without initialization, you get an error:

class A
  @@a
end  
# NameError: uninitialized class variable @@a in A

Class instance variables and class variables are nil by default:

class A
  def self.a
    @a
  end  
  def a
    @a
  end  
end  

> A.a
#=> nil
> A.new.a
#=> nil

回答2:

This is trivial to test yourself:

• undefined global variables evaluate to nil and generate a warning:

  defined? $foo
  #=> nil
  
  $foo
  #=> nil
  # warning: global variable `$foo' not initialized
  

• undefined local variables raise a NameError:

  defined? foo
  #=> nil
  
  foo
  # undefined local variable or method `foo' for main:Object (NameError)
  

• defined but not initialized local variables evaluate to nil:

  defined? foo
  #=> nil
  
  if false then foo = 42 end
  
  defined? foo
  #=> 'local-variable'
  
  foo
  #=> nil
  

• undefined instance variables evaluate to nil and generate a warning:

  defined? @foo
  #=> nil
  
  @foo
  #=> nil
  # warning: instance variable `@foo' not initialized
  

• undefined class variables raise a NameError:

  defined? @@foo
  #=> nil
  
  @@foo
  # uninitialized class variable @@foo (NameError)
  

• undefined constants raise a NameError:

  defined? FOO
  #=> nil
  
  FOO
  # uninitialized constant FOO (NameError)
  

来源：https://stackoverflow.com/questions/54548952/default-values-of-ruby-variables