How do I check if a class is defined?

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

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

Object.const_get("Amber")

or (in Rails)

"Amber".constantize

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

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"

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...)

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最佳答案

How about const_defined??

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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Inspired by @ctcherry's response above, here's a 'safe class method send', where class_name is a string. If class_name doesn't name a class, it returns nil.

def class_send(class_name, method, *args)
Object.const_defined?(class_name) ? Object.const_get(class_name).send(method, *args) : nil
end

An even safer version which invokes method only if class_name responds to it:

def class_send(class_name, method, *args)
return nil unless Object.const_defined?(class_name)
c = Object.const_get(class_name)
c.respond_to?(method) ? c.send(method, *args) : nil
end
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I've created a validator to test if a string is a valid class name (or comma-separated list of valid class names):

class ClassValidator < ActiveModel::EachValidator
def validate_each(record,attribute,value)
unless value.split(',').map { |s| s.strip.constantize.is_a?(Class) rescue false }.all?
record.errors.add attribute, 'must be a valid Ruby class name (comma-separated list allowed)'
end
end
end
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Another approach, in case you want to get the class too. Will return nil if the class isn't defined, so you don't have to catch an exception. 

class String
def to_class(class_name)
begin
class_name = class_name.classify (optional bonus feature if using Rails)
Object.const_get(class_name)
rescue
# swallow as we want to return nil
end
end
end


> 'Article'.to_class
class Article


> 'NoSuchThing'.to_class
nil


# use it to check if defined
> puts 'Hello yes this is class' if 'Article'.to_class
Hello yes this is class
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In rails it's really easy:

amber = "Amber".constantize rescue nil
if amber # nil result in false
# your code here
end
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It would appear that all the answers using the Object.const_defined? method are flawed. If the class in question has not been loaded yet, due to lazy loading, then the assertion will fail. The only way to achieve this definitively is like so:

  validate :adapter_exists


def adapter_exists
# cannot use const_defined because of lazy loading it seems
Object.const_get("Irs::#{adapter_name}")
rescue NameError => e
errors.add(:adapter_name, 'does not have an IrsAdapter')
end

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