检查一个常数是否已经定义

This is a simple one, I hope. How do I check, in the following example, if a constant is already defined?

我希望这是一个简单的问题。在下面的例子中，如果一个常数已经被定义了，我该如何检查呢?

#this works
var = var||1
puts var
var = var||2
puts var

#this doesn't
CONST = CONST||1
puts CONST
CONST = CONST||2
puts CONST

=> 1
   1
   uninitialized constant CONST (NameError)

3 个解决方案

#1

109

CONST = 2 unless defined? CONST

See here for more about awesome defined? operator.

在这里看到更多关于可怕的定义吗?操作符。

^{P.S. And in the future I guess you'll want var ||= 1 instead of var = var||1.}

顺便说一下，我猜你以后会想要var|| = 1而不是var = var||1。

#2

const_defined? API pry> User.const_defined?("PER_PAGE") => true pry> User.const_defined?("PER_PAGE123") => false

const_defined吗?API pry> User.const_defined?("PER_PAGE") => true pry> User.const_defined?("PER_PAGE123") => false

#3

CONST ||= :default_value

the above works for me on ruby 1.9.3 but fails on 1.8... well 1.8 is ancient now.

以上在ruby 1.9.3中对我适用，但是在1.8中失败了……1。8现在已经很古老了。

#1

109

CONST = 2 unless defined? CONST