這裏發生了什麼？ （無紅寶石）

Question

p parent.class #=> NilClass # ok. 
p !!parent # => false # as expected. 
p parent.object_id # => 17006820 # should be 4 
p parent && parent.foo # => NoMethodError foo # should be nil-guarded 

此對象從哪裏來？

Answer 1

可能是這樣的：

class BlankSlate 
    instance_methods.each do |m| 
    # Undefine all but a few methods. Various implementations leave different 
    # methods behind. 
    undef_method(m) unless m.to_s == "object_id" 
    end 
end 

class Foo < BlankSlate 
    def method_missing(*args) 
    delegate.send(*args) 
    end 

    def delegate 
    # This probably contains an error and returns nil accidentally. 
    nil 
    end 
end 

parent = Foo.new 

p parent.class 
#=> NilClass 

p !!parent 
#=> false 

p parent.object_id 
#=> 2157246780 

p parent && parent.foo 
#=> NoMethodError: undefined method `foo' for nil:NilClass 

創建BlankSlate或BasicObject是一種常見的模式（它被添加到核心紅寶石爲1.9之前的版本）。它用於創建對象，使用它們發送的任何方法做一些特殊的事情，或者將他們的行爲嚴重委託給不同的類。缺點是它可能會引入這樣的奇怪行爲。