Listing an Objects Methods

Table of contents:

Listing an Object s Methods

Problem

Given an unfamiliar object, you want to see what methods are available to call.

Solution

All Ruby objects implement the Object#methods method. It returns an array containing the names of the object's public instance methods:

	Object.methods
	# => ["name", "private_class_method", "object_id", "new",
	# "singleton_methods", "method_defined?", "equal?", … ]

To get a list of the singleton methods of some object (usually, but not always, a class), use Object#singleton_methods:

	Object.singleton_methods # => []
	Fixnum.singleton_methods # => ["induced_from"]

	class MyClass
	 def MyClass.my_singleton_method
	 end

	 def my_instance_method
	 end
	end
	MyClass.singleton_methods # => ["my_singleton_method"]

To list the instance methods of a class, call instance_methods on the object. This lets you list the instance methods of a class without instantiating the class:

	''.methods == String.instance_methods # => true

The output of these methods are most useful when sorted:

	Object.methods.sort
	# => ["<", "<=", "<=>", "==", "===", "=~", ">", ">=",
	# "__id__", "__send__", "allocate", "ancestors", … ]

Ruby also defines some elementary predicates along the same lines. To see whether a class defines a certain instance method, call method_defined? on the class or respond_to? on an instance of the class. To see whether a class defines a certain class method, call respond_to? on the class:

	MyClass.method_defined? :my_instance_method # => true
	MyClass.new.respond_to? :my_instance_method # => true
	MyClass.respond_to? :my_instance_method # => false

	MyClass.respond_to? :my_singleton_method # => true

 

Discussion

It often happens that while you're in an interactive Ruby session, you need to look up which methods an object supports, or what a particular method is called. Looking directly at the object is faster than looking its class up in a book. If you're using a library like Rails or Facets, or your code has been adding methods to the built-in classes, it's also more reliable.

Noninteractive code can also benefit from knowing whether a given object implements a certain method. You can use this to enforce an interface, allowing any object to be passed into a method so long as the argument implements certain methods (see Recipe 10.16).

If you find yourself using respond_to? a lot in an interactive Ruby session, you're a good customer for irb's autocomplete feature. Put the following line in your .irbrc file or equivalent:

	require 'irb/completion'
	#Depending on your system, you may also have to add the following line:
	IRB.conf[:use_readline] = true

Then you can type (for instance) "[1,2,3].", hit the Tab key, and see a list of all the methods you can call on the array [1, 2, 3].

methods, instance_methods, and singleton_methods will only return public methods, and method_defined? will only return true if you give it the name of a public method. Ruby provides analagous methods for discovering protected and private methods, though these are less useful. All the relevant methods are presented in Table 10-1.

Table 10-1. Discovering protected and private methods

Goal

Public

Protected

Private

List the methods of an object

methods or public_methods

protected_methods

private_methods

List the instance methods defined by a class

instance_methods or public_instance_methods

protected_instance_methods

private_instance_methods

List the singleton methods defined by a class

singleton_methods

N/A

N/A

Does this class define such-and-such an instance method?

method_defined? or public_method_defined?

protected_method_defined?

private_method_defined?

Will this object respond to such-and-such an instance method?

respond_to?

N/A

N/A

Just because you can see the names of protected or private methods in a list doesn't mean you can call the methods, or that respond_to? will find them:

	String.private_instance_methods.sort
	# => ["Array", "Float", "Integer", "String", "'", "abort", "at_exit",
	# "autoload","autoload?", "binding", "block_given?", "callcc", … ]
	String.new.respond_to? :autoload? # => false

	String.new.autoload?
	# NoMethodError: private method 'autoload?' called for "":String

 

See Also

  • To strip away irrelevant methods, see Recipe 10.3, " Listing Methods Unique to an Object"
  • Recipe 10.4, "Getting a Reference to a Method," shows how to assign a Method object to a variable, given its name; among other things, this lets you find out how many arguments a method takes
  • See Recipe 10.6, "Listening for Changes to a Class," to set up a hook to be called whenever a new method or singleton method is defined for a class
  • Recipe 10.16, "Enforcing Software Contracts"


Strings

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Date and Time

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Hashes

Files and Directories

Code Blocks and Iteration

Objects and Classes8

Modules and Namespaces

Reflection and Metaprogramming

XML and HTML

Graphics and Other File Formats

Databases and Persistence

Internet Services

Web Development Ruby on Rails

Web Services and Distributed Programming

Testing, Debugging, Optimizing, and Documenting

Packaging and Distributing Software

Automating Tasks with Rake

Multitasking and Multithreading

User Interface

Extending Ruby with Other Languages

System Administration



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