CODEWITHSUNDEEP
rubyclass
Jimmy
Jimmy

Reputation:

How to check if a class already exists in Ruby

How do I check if a class already exists in Ruby?

My code is:

puts "enter the name of the Class to see if it exists"   
nameofclass=gets.chomp  
eval (" #{nameofclass}......  Not sure what to write here")

I was thinking of using:

eval "#{nameofclass}ancestors.     ....."

Upvotes: 73

Views: 51745

Answers (14)

Neeraj Kumar
Neeraj Kumar

Reputation: 7511

If it is a simple standalone class e.g.

class Foo
end

then we can use Object.const_defined?("Foo"). And, if your class is inside any module e.g.

module Bar
  class Foo
  end
end

then we can use Module.const_get("Bar::Foo").

Please note that if Module.const_get('Bar::Foo') doesn't find the class then it will raise exception. While, Object.const_defined?('Foo') will return true or false.

Upvotes: 3

Afz902k
Afz902k

Reputation: 191

Here's something I sometimes do to tackle this very issue. You can add the following methods to the String class like so:

class String
    def to_class
        my_const = Kernel.const_get(self)
        my_const.is_a?(Class) ? my_const : nil
    rescue NameError 
        nil
    end

    def is_a_defined_class?
        true if self.to_class
    rescue NameError
        false
    end
end

Then:

'String'.to_class
=> String
'unicorn'.to_class
=> nil
'puppy'.is_a_defined_class?
=> false
'Fixnum'.is_a_defined_class?
=> true

Upvotes: 6

Goulven
Goulven

Reputation: 889

None of the answers above worked for me, maybe because my code lives in the scope of a submodule.

I settled on creating a class_exists? method in my module, using code found in Fred Wilmore's reply to "How do I check if a class is defined?" and finally stopped cursing.

def class_exists?(name)
   name.constantize.is_a?(Class) rescue false # rubocop:disable Style/RescueModifier
end

Full code, for the curious:

module Some
  module Thing
    def self.build(object)
      name = "Some::Thing::#{object.class.name}"
      class_exists?(name) ? name.constantize.new(object) : Base.new(object)
    end

    def self.class_exists?(name)
      name.constantize.is_a?(Class) rescue false # rubocop:disable Style/RescueModifier
    end

    private_class_method :class_exists?
  end
end

I use it as a factory which builds objects depending on the class of the object passed as argument:

Some::Thing.build(something)
=> # A Some::Thing::Base object
Some::Thing.build(something_else)
=> # Another object, which inherits from Some::Thing::Base

Upvotes: 1

Frank Koehl
Frank Koehl

Reputation: 3176

If you want something packaged, the finishing_moves gem adds a class_exists? method.

class_exists? :Symbol
# => true
class_exists? :Rails
# => true in a Rails app
class_exists? :NonexistentClass
# => false

Upvotes: 0

Tate Johnson
Tate Johnson

Reputation: 3950

Class names are constants. You can use the defined? method to see if a constant has been defined.

defined?(String)    # => "constant"
defined?(Undefined) # => nil

You can read more about how defined? works if you're interested.

Upvotes: 11

stcorbett
stcorbett

Reputation: 613

You can avoid having to rescue the NameError from Module.const_get if you are looking the constant within a certain scope by calling Module#const_defined?("SomeClass").

A common scope to call this would be Object, eg: Object.const_defined?("User").

See: "Module".

Upvotes: 32

stackdump
stackdump

Reputation: 748

I assume you'll take some action if the class is not loaded.

If you mean to require a file, why not just check the output of require?

require 'already/loaded'  
=> false

Upvotes: 0

random-forest-cat
random-forest-cat

Reputation: 35894

Kernel.const_defined?("Fixnum") # => true

Upvotes: 8

Andrea Salicetti
Andrea Salicetti

Reputation: 2483

In just one line, I would write:

!!Module.const_get(nameofclass) rescue false

that will return trueonly if the given nameofclass belongs to a defined class.

Upvotes: 4

Mike Bethany
Mike Bethany

Reputation:

Here's a more succinct version:

def class_exists?(class_name)
  eval("defined?(#{class_name}) && #{class_name}.is_a?(Class)") == true
end

class_name = "Blorp"
class_exists?(class_name)
=> false

class_name = "String"
class_exists?(class_name)
=> true

Upvotes: 5

Hugh
Hugh

Reputation: 11

I used this to see if a class was loaded at runtime:

def class_exists?(class_name)
  ObjectSpace.each_object(Class) {|c| return true if c.to_s == class_name }
  false
end

Upvotes: 1

Rob
Rob

Reputation: 7099

defined?(DatabaseCleaner) # => nil
require 'database_cleaner'
defined?(DatabaseCleaner) # => constant

Upvotes: 10

Olly
Olly

Reputation: 3437

You can use Module.const_get to get the constant referred to by the string. It will return the constant (generally classes are referenced by constants). You can then check to see if the constant is a class.

I would do something along these lines:

def class_exists?(class_name)
  klass = Module.const_get(class_name)
  return klass.is_a?(Class)
rescue NameError
  return false
end

Also, if possible I would always avoid using eval when accepting user input; I doubt this is going to be used for any serious application, but worth being aware of the security risks.

Upvotes: 88

Sam Saffron
Sam Saffron

Reputation: 131112

perhaps you can do it with defined?

eg:

if defined?(MyClassName) == 'constant' && MyClassName.class == Class  
   puts "its a class" 
end

Note: the Class check is required, for example:

Hello = 1 
puts defined?(Hello) == 'constant' # returns true

To answer the original question: 

puts "enter the name of the Class to see if it exists"
nameofclass=gets.chomp
eval("defined?(#{nameofclass}) == 'constant' and #{nameofclass}.class == Class")

Upvotes: 55

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