以下内容来自 rubydoc.org
纠正一下猫叔的说法，defined? 是关键字不是一个方法，所以说“参数”不太合适。改成“语法”[Synopsis]比较好。

defined?

defined? expression tests whether or not expression refers to anything recognizable (literal object, local variable that has been initialized, method name visible from the current scope, etc.). The return value is nil if the expression cannot be resolved. Otherwise, the return value provides information about the expression.

Note that the expression is not executed.

p defined?(def x; end)   # "expression"
x                        # error: undefined method or variable

p defined?(@x=1)         # "assignment"
p @x                     # nil

Assignment to a local variable will, however, have the usually result of initializing the variable to nil by virtue of the assignment expression itself:

p defined?(x=1)          # assignment
p x                      # nil

In most cases, the argument to defined? will be a single identifier:

def x; end
p defined?(x)            # "method"

guoxiaomi 发表于 2017-6-18 06:12
会不会是，defined? 的参数是方法调用，而不是方法名（symbol对象），去掉defined?
后面的冒号看看 ...

嗯，确实是，大神啊。

会不会是，defined? 的参数是方法调用，而不是方法名（symbol对象），去掉defined?
后面的冒号看看