如何检查是否定义了类？ [英] How do I check if a class is defined?

问题描述

如何将字符串转换为类名，但只有在该类已经存在的情况下才可以。

How do I turn a string into a class name, but only if that class already exists?

如果Amber已经 ，我可以从一个字符串到类通过：

If Amber is already a class, I can get from a string to the class via:

Object.const_get("Amber")

或（在Rails中）

"Amber".constantize

但是其中任何一个都会失败， NameError ：未初始化的常量Amber 如果Amber不是一个类。

But either of these will fail with NameError: uninitialized constant Amber if Amber is not already a class.

我的第一个想法是使用 defined？ / code>方法，但它不区分已经存在的类和不存在的类：

My first thought is to use the defined? method, but it doesn't discriminate between classes that already exist and those that don't:

>> defined?("Object".constantize)
=> "method"
>> defined?("AClassNameThatCouldNotPossiblyExist".constantize)
=> "method"

那么如何测试一个字符串是否命名一个类， （好，一个开始 / 救援块捕获NameError错误？

So how do I test if a string names a class before I try to convert it? (Okay, how about a begin/rescue block to catch NameError errors? Too ugly? I agree...)

推荐答案

const_defined？ ？

请记住，在Rails中，在开发模式下有自动加载，因此在测试时可能会很棘手：

Remember in Rails, there is auto-loading in development mode, so it can be tricky when you are testing it out:

>> Object.const_defined?('Account')
=> false
>> Account
=> Account(id: integer, username: string, google_api_key: string, created_at: datetime, updated_at: datetime, is_active: boolean, randomize_search_results: boolean, contact_url: string, hide_featured_results: boolean, paginate_search_results: boolean)
>> Object.const_defined?('Account')
=> true

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