如何判断是否在模块名称空间而不是全局名称空间中定义了常量? [英] How to judge whether a Constant has been defined within a module namespace, not a global one?

问题描述

我有两个同名的const；一个是全局const，另一个是在名称空间Admin下定义的.但是我需要区分它们；全局对象已经定义，而范围对象还需要自动定义(如果尚未定义):

I have two Const with the same name; One is a global const, and the other is defined under the namespace Admin. But I need to distinguish them;The global one has already defined, and the scoped one need to auto defined if it has not been defined yet:

A = 'A Global Const'  
module Admin  
  A = 'A Const within the Admin namespace' if const_defined? 'A'  # always true and the Admin::A can never be defined!
end  
puts A  # => 'A Global Const' 
puts Admin::A  # => NameError: uninitialized constant Admin::A
# the Admin::A will never be defined.

---

但是，如果定义了全局A，则"const_defind?"部分将永远返回真相！
我什至尝试过:

---

But if the Global A is defined, the "const_defind?" part will always return ture!
I even have tried:

... if defined? A  
... if self.const_defined? 'A'  
... if Object.const_get('Admin').const_defined? 'A'  

---

总是如此！
我需要区分它们，因为我需要在A和Admin :: A两种形式中使用A；
类似于PostsController供公众使用的情况，以及Admin :: PostsController供管理员使用的情况；
帮助！

---

Always true!
I need to distinguish them because I need to use the A in A and Admin::A two forms;
Like the situation PostsController for public use, and Admin::PostsController for admin use;
Help!

推荐答案

实际上，const_defined?和const_get遍历了层次结构，对于模块而言，该层次结构包括(

Indeed, const_defined? and const_get go through the hierarchy, which for modules include (artificially) the Object class. You can use Module#constants to avoid this, though:

module Admin  
  A = 'A Const with in the Admin namespace' unless constants.include?(:A)
end

注意:在Ruby 1.8中，您要检查"A"，而不是:A

Note: In Ruby 1.8, you check against "A", not :A

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