为什么我不能访问 Ruby 方法中的局部变量? [英] Why can't I access a local variable inside a method in Ruby?

问题描述

我有一个名为 test.rb 的 Ruby 文件

I have a Ruby file named test.rb

ff="ff"
def test
  puts ff
end

我执行它，出现错误:

test.rb:3:in `test': main:Object (NameError) 的未定义局部变量或方法 `ff'

这是什么原因?有什么文档可以解释吗?

What's the reason for this? Is there any documentation to explain it?

推荐答案

在 test 方法定义中无法访问 ff 的原因仅仅是方法(使用 def 关键字)创建一个新的范围.与分别使用 class 和 module 关键字定义类和模块相同.

The reason ff is inaccessible inside the test method definition is simply that methods (created with the def keyword) create a new scope. Same with defining classes and modules using the class and module keywords respectively.

在这种情况下，main(顶级对象)的作用几乎与作用域问题完全无关.

The role of main (the top-level object) is almost completely irrelevant to the question of scope in this situation.

请注意，如果您确实希望 test 方法能够访问定义上下文中定义的局部变量，请使用 define_method(或者在您的情况下，<代码>define_singleton_method 方法)，见这里:

Note that, if you DO want your test method to have access to locals defined in the definition context, then use the define_method (or in your case, the define_singleton_method method), see here:

ff = "hi"
define_singleton_method("test") { ff }
test #=> "hi"

与 def 关键字不同，define_method 方法系列不会创建新的作用域，而是关闭当前作用域，捕获任何局部变量.

Unlike the def keyword, the define_method family of methods do not create new scopes but instead close over the current scope, capturing any local variables.

在@soup 给出的下一个示例中使用 @ff 的原因不是 main 在某种程度上是一个特殊情况"，它只是定义了一个 ivar顶级是 main 的一个 ivar，因此可以被在 main 上调用的方法访问.

The reason using @ff worked in the next example given by @soup, is not that main is somehow a "special case" it's just that an ivar defined at top-level is an ivar of main and so is accessible to a method invoked on main.

然而，test 方法与 main 的关系是什么?它不是on 的方法，只是main 本身——它实际上是一个定义在Object 类上的私有实例方法.这意味着 test 方法(作为私有方法)几乎可用于 ruby​​ 程序中的每个对象.定义在顶层 (main) 的所有方法实际上都定义为 Object 类上的私有实例方法.

What, however, is the relationship of the test method to main? It is not a method on just main itself - it is actually a private instance method defined on the Object class. This means that the test method would be available (as a private method) to nearly every object in your ruby program. All methods defined at top-level (main) are actually defined as private instance methods on the Object class.

有关 Ruby 顶级的更多信息，请参阅这篇文章:http://banisterfiend.wordpress.com/2010/11/23/what-is-the-ruby-top-level/

For more information on the Ruby top-level, see this article: http://banisterfiend.wordpress.com/2010/11/23/what-is-the-ruby-top-level/

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