Object is the default root of all Ruby objects. Object inherits from BasicObject which allows creating alternate object hierarchies. Methods on Object are available to all classes unless explicitly overridden.

Object mixes in the Kernel module, making the built-in kernel functions globally accessible. Although the instance methods of Object are defined by the Kernel module, we have chosen to document them here for clarity.

When referencing constants in classes inheriting from Object you do not need to use the full namespace. For example, referencing File inside YourClass will find the top-level File class.

In the descriptions of Object’s methods, the parameter symbol refers to a symbol, which is either a quoted string or a Symbol (such as :name).


An obsolete class, use Integer


ENV is a Hash-like accessor for environment variables.

See ENV (the class) for more details.

Holds the original stdin

Holds the original stdout

Holds the original stderr

ARGF is a stream designed for use in scripts that process files given as command-line arguments or passed in via STDIN.

See ARGF (the class) for more details.

When a Hash is assigned to SCRIPT_LINES__ the contents of files loaded after the assignment will be added as an Array of lines with the file name as the key.

An alias for OptionParser.

No documentation available

A module that provides a two-phase lock with a counter.

A class that provides two-phase lock with a counter. See Sync_m for details.

This class watches for termination of multiple threads. Basic functionality (wait until specified threads have terminated) can be accessed through the class method ThreadsWait::all_waits. Finer control can be gained using instance methods.


ThreadsWait.all_waits(thr1, thr2, ...) do |t|
  STDERR.puts "Thread #{t} has terminated."

th =,...)
th.next_wait # next one to be done

Raised by Timeout.timeout when the block times out.

No documentation available

An obsolete class, use Integer


An obsolete alias of nil

An obsolete alias of true

An obsolete alias of false

DATA is a File that contains the data section of the executed file. To create a data section use __END__:

$ cat t.rb
puts DATA.gets
hello world!

$ ruby t.rb
hello world!

ARGV contains the command line arguments used to run ruby.

A library like OptionParser can be used to process command-line arguments.

The running version of ruby

The date this ruby was released

The platform for this ruby

The patchlevel for this ruby. If this is a development build of ruby the patchlevel will be -1

The SVN revision for this ruby.

The copyright string for ruby

The engine or interpreter this ruby uses.

The version of the engine or interpreter this ruby uses.

The full ruby version string, like ruby -v prints

The Binding of the top level scope

Class Methods
No documentation available
Instance Methods

Returns true if two objects do not match (using the =~ method), otherwise false.

Returns 0 if obj and other are the same object or obj == other, otherwise nil.

The <=> is used by various methods to compare objects, for example Enumerable#sort, Enumerable#max etc.

Your implementation of <=> should return one of the following values: -1, 0, 1 or nil. -1 means self is smaller than other. 0 means self is equal to other. 1 means self is bigger than other. Nil means the two values could not be compared.

When you define <=>, you can include Comparable to gain the methods <=, <, ==, >=, > and between?.

Case Equality – For class Object, effectively the same as calling #==, but typically overridden by descendants to provide meaningful semantics in case statements.

This method is deprecated.

This is not only unuseful but also troublesome because it may hide a type error.

Passes args to CSV::instance.

  #=> [["CSV", "data"]]

If a block is given, the instance is passed the block and the return value becomes the return value of the block.

CSV("CSV,data") { |c| { |a| a.include?("data") }
} #=> true

CSV("CSV,data") { |c| { |a| a.include?("zombies") }
} #=> false

The primary interface to this library. Use to setup delegation when defining your class.

class MyClass < DelegateClass(ClassToDelegateTo) # Step 1
  def initialize
    super(obj_of_ClassToDelegateTo)              # Step 2

Here’s a sample of use from Tempfile which is really a File object with a few special rules about storage location and when the File should be deleted. That makes for an almost textbook perfect example of how to use delegation.

class Tempfile < DelegateClass(File)
  # constant and class member data initialization...

  def initialize(basename, tmpdir=Dir::tmpdir)
    # build up file path/name in var tmpname...

    @tmpfile =, File::RDWR|File::CREAT|File::EXCL, 0600)

    # ...


    # below this point, all methods of File are supported...

  # ...

Returns a Digest subclass by name in a thread-safe manner even when on-demand loading is involved.

require 'digest'

# => Digest::MD5

# => Digest::SHA256

# => LoadError: library not found for class Digest::Foo -- digest/foo

Returns the class of obj. This method must always be called with an explicit receiver, as class is also a reserved word in Ruby.

1.class      #=> Integer
self.class   #=> Object

Produces a shallow copy of obj—the instance variables of obj are copied, but not the objects they reference. clone copies the frozen (unless :freeze keyword argument is given with a false value) and tainted state of obj. See also the discussion under Object#dup.

class Klass
   attr_accessor :str
s1 =      #=> #<Klass:0x401b3a38>
s1.str = "Hello"    #=> "Hello"
s2 = s1.clone       #=> #<Klass:0x401b3998 @str="Hello">
s2.str[1,4] = "i"   #=> "i"
s1.inspect          #=> "#<Klass:0x401b3a38 @str=\"Hi\">"
s2.inspect          #=> "#<Klass:0x401b3998 @str=\"Hi\">"

This method may have class-specific behavior. If so, that behavior will be documented under the #initialize_copy method of the class.

provides a unified clone operation, for REXML::XPathParser to use across multiple Object types

Defines a singleton method in the receiver. The method parameter can be a Proc, a Method or an UnboundMethod object. If a block is specified, it is used as the method body.

class A
  class << self
    def class_name
A.define_singleton_method(:who_am_i) do
  "I am: #{class_name}"
A.who_am_i   # ==> "I am: A"

guy = "Bob"
guy.define_singleton_method(:hello) { "#{self}: Hello there!" }
guy.hello    #=>  "Bob: Hello there!"

Prints obj on the given port (default $>). Equivalent to:

def display(port=$>)
  port.write self

For example:

[ 4, 5, 6 ].display


1cat[4, 5, 6]

Produces a shallow copy of obj—the instance variables of obj are copied, but not the objects they reference. dup copies the tainted state of obj.

This method may have class-specific behavior. If so, that behavior will be documented under the #initialize_copy method of the class.

on dup vs clone

In general, clone and dup may have different semantics in descendant classes. While clone is used to duplicate an object, including its internal state, dup typically uses the class of the descendant object to create the new instance.

When using dup, any modules that the object has been extended with will not be copied.

class Klass
  attr_accessor :str

module Foo
  def foo; 'foo'; end

s1 = #=> #<Klass:0x401b3a38>
s1.extend(Foo) #=> #<Klass:0x401b3a38> #=> "foo"

s2 = s1.clone #=> #<Klass:0x401b3a38> #=> "foo"

s3 = s1.dup #=> #<Klass:0x401b3a38> #=> NoMethodError: undefined method `foo' for #<Klass:0x401b3a38>

Equality — At the Object level, == returns true only if obj and other are the same object. Typically, this method is overridden in descendant classes to provide class-specific meaning.

Unlike ==, the equal? method should never be overridden by subclasses as it is used to determine object identity (that is, a.equal?(b) if and only if a is the same object as b):

obj = "a"
other = obj.dup

obj == other      #=> true
obj.equal? other  #=> false
obj.equal? obj    #=> true

The eql? method returns true if obj and other refer to the same hash key. This is used by Hash to test members for equality. For objects of class Object, eql? is synonymous with ==. Subclasses normally continue this tradition by aliasing eql? to their overridden == method, but there are exceptions. Numeric types, for example, perform type conversion across ==, but not across eql?, so:

1 == 1.0     #=> true
1.eql? 1.0   #=> false

Adds to obj the instance methods from each module given as a parameter.

module Mod
  def hello
    "Hello from Mod.\n"

class Klass
  def hello
    "Hello from Klass.\n"

k =
k.hello         #=> "Hello from Klass.\n"
k.extend(Mod)   #=> #<Klass:0x401b3bc8>
k.hello         #=> "Hello from Mod.\n"

Prevents further modifications to obj. A RuntimeError will be raised if modification is attempted. There is no way to unfreeze a frozen object. See also Object#frozen?.

This method returns self.

a = [ "a", "b", "c" ]
a << "z"


prog.rb:3:in `<<': can't modify frozen Array (FrozenError)
 from prog.rb:3

Objects of the following classes are always frozen: Integer, Float, Symbol.

Returns the freeze status of obj.

a = [ "a", "b", "c" ]
a.freeze    #=> ["a", "b", "c"]
a.frozen?   #=> true

Allows for declaring a Gemfile inline in a ruby script, optionally installing any gems that aren’t already installed on the user’s system.

@note Every gem that is specified in this ‘Gemfile’ will be ‘require`d, as if

the user had manually called `Bundler.require`. To avoid a requested gem
being automatically required, add the `:require => false` option to the
`gem` dependency declaration.

@param install [Boolean] whether gems that aren’t already installed on the

user's system should be installed.
Defaults to `false`.

@param gemfile [Proc] a block that is evaluated as a ‘Gemfile`.

@example Using an inline Gemfile

#!/usr/bin/env ruby

require 'bundler/inline'

gemfile do
  source ''
  gem 'json', require: false
  gem 'nap', require: 'rest'
  gem 'cocoapods', '~> 0.34.1'

puts Pod::VERSION # => "0.34.4"

Returns a string containing a human-readable representation of obj. The default inspect shows the object’s class name, an encoding of the object id, and a list of the instance variables and their values (by calling inspect on each of them). User defined classes should override this method to provide a better representation of obj. When overriding this method, it should return a string whose encoding is compatible with the default external encoding.

[ 1, 2, 3..4, 'five' ].inspect   #=> "[1, 2, 3..4, \"five\"]"                 #=> "2008-03-08 19:43:39 +0900"

class Foo
end                  #=> "#<Foo:0x0300c868>"

class Bar
  def initialize
    @bar = 1
end                  #=> "#<Bar:0x0300c868 @bar=1>"

Returns true if obj is an instance of the given class. See also Object#kind_of?.

class A;     end
class B < A; end
class C < B; end

b =
b.instance_of? A   #=> false
b.instance_of? B   #=> true
b.instance_of? C   #=> false

Returns true if the given instance variable is defined in obj. String arguments are converted to symbols.

class Fred
  def initialize(p1, p2)
    @a, @b = p1, p2
fred ='cat', 99)
fred.instance_variable_defined?(:@a)    #=> true
fred.instance_variable_defined?("@b")   #=> true
fred.instance_variable_defined?("@c")   #=> false

Returns the value of the given instance variable, or nil if the instance variable is not set. The @ part of the variable name should be included for regular instance variables. Throws a NameError exception if the supplied symbol is not valid as an instance variable name. String arguments are converted to symbols.

class Fred
  def initialize(p1, p2)
    @a, @b = p1, p2
fred ='cat', 99)
fred.instance_variable_get(:@a)    #=> "cat"
fred.instance_variable_get("@b")   #=> 99

Sets the instance variable named by symbol to the given object, thereby frustrating the efforts of the class’s author to attempt to provide proper encapsulation. The variable does not have to exist prior to this call. If the instance variable name is passed as a string, that string is converted to a symbol.

class Fred
  def initialize(p1, p2)
    @a, @b = p1, p2
fred ='cat', 99)
fred.instance_variable_set(:@a, 'dog')   #=> "dog"
fred.instance_variable_set(:@c, 'cat')   #=> "cat"
fred.inspect                             #=> "#<Fred:0x401b3da8 @a=\"dog\", @b=99, @c=\"cat\">"

Returns an array of instance variable names for the receiver. Note that simply defining an accessor does not create the corresponding instance variable.

class Fred
  attr_accessor :a1
  def initialize
    @iv = 3
end   #=> [:@iv]

Returns the receiver.

string = "my string"
string.itself.object_id == string.object_id   #=> true

Returns true if class is the class of obj, or if class is one of the superclasses of obj or modules included in obj.

module M;    end
class A
  include M
class B < A; end
class C < B; end

b =
b.is_a? A          #=> true
b.is_a? B          #=> true
b.is_a? C          #=> false
b.is_a? M          #=> true

b.kind_of? A       #=> true
b.kind_of? B       #=> true
b.kind_of? C       #=> false
b.kind_of? M       #=> true

Looks up the named method as a receiver in obj, returning a Method object (or raising NameError). The Method object acts as a closure in obj’s object instance, so instance variables and the value of self remain available.

class Demo
  def initialize(n)
    @iv = n
  def hello()
    "Hello, @iv = #{@iv}"

k =
m = k.method(:hello)   #=> "Hello, @iv = 99"

l ='Fred')
m = l.method("hello")   #=> "Hello, @iv = Fred"

Note that Method implements to_proc method, which means it can be used with iterators.

[ 1, 2, 3 ].each(&method(:puts)) # => prints 3 lines to stdout

out ='test.txt', 'w')
[ 1, 2, 3 ].each(&out.method(:puts)) # => prints 3 lines to file

require 'date'
%w[2017-03-01 2017-03-02].collect(&Date.method(:parse))
#=> [#<Date: 2017-03-01 ((2457814j,0s,0n),+0s,2299161j)>, #<Date: 2017-03-02 ((2457815j,0s,0n),+0s,2299161j)>]

Returns a list of the names of public and protected methods of obj. This will include all the methods accessible in obj’s ancestors. If the optional parameter is false, it returns an array of obj<i>‘s public and protected singleton methods, the array will not include methods in modules included in <i>obj.

class Klass
  def klass_method()
k =
k.methods[0..9]    #=> [:klass_method, :nil?, :===,
                   #    :==~, :!, :eql?
                   #    :hash, :<=>, :class, :singleton_class]
k.methods.length   #=> 56

k.methods(false)   #=> []
def k.singleton_method; end
k.methods(false)   #=> [:singleton_method]

module M123; def m123; end end
k.extend M123
k.methods(false)   #=> [:singleton_method]
No documentation available

Only the object nil responds true to nil?.   #=> false
nil.nil?          #=> true

Returns an integer identifier for obj.

The same number will be returned on all calls to object_id for a given object, and no two active objects will share an id.

Note: that some objects of builtin classes are reused for optimization. This is the case for immediate values and frozen string literals.

Immediate values are not passed by reference but are passed by value: nil, true, false, Fixnums, Symbols, and some Floats.  ==  # => false
(21 * 2).object_id    == (21 * 2).object_id    # => true
"hello".object_id     == "hello".object_id     # => false
"hi".freeze.object_id == "hi".freeze.object_id # => true

Returns the list of private methods accessible to obj. If the all parameter is set to false, only those methods in the receiver will be listed.

Returns the list of protected methods accessible to obj. If the all parameter is set to false, only those methods in the receiver will be listed.

Similar to method, searches public method only.

Returns the list of public methods accessible to obj. If the all parameter is set to false, only those methods in the receiver will be listed.

Invokes the method identified by symbol, passing it any arguments specified. Unlike send, public_send calls public methods only. When the method is identified by a string, the string is converted to a symbol.

1.public_send(:puts, "hello")  # causes NoMethodError
An alias for namespace

Removes the named instance variable from obj, returning that variable’s value. String arguments are converted to symbols.

class Dummy
  attr_reader :var
  def initialize
    @var = 99
  def remove
d =
d.var      #=> 99
d.remove   #=> 99
d.var      #=> nil

Returns true if obj responds to the given method. Private and protected methods are included in the search only if the optional second parameter evaluates to true.

If the method is not implemented, as Process.fork on Windows, File.lchmod on GNU/Linux, etc., false is returned.

If the method is not defined, respond_to_missing? method is called and the result is returned.

When the method name parameter is given as a string, the string is converted to a symbol.


Hook method to return whether the obj can respond to id method or not.

When the method name parameter is given as a string, the string is converted to a symbol.

See respond_to?, and the example of BasicObject.

Invokes the method identified by symbol, passing it any arguments specified. You can use __send__ if the name send clashes with an existing method in obj. When the method is identified by a string, the string is converted to a symbol.

class Klass
  def hello(*args)
    "Hello " + args.join(' ')
k =
k.send :hello, "gentle", "readers"   #=> "Hello gentle readers"

Returns the singleton class of obj. This method creates a new singleton class if obj does not have one.

If obj is nil, true, or false, it returns NilClass, TrueClass, or FalseClass, respectively. If obj is an Integer, a Float or a Symbol, it raises a TypeError.  #=> #<Class:#<Object:0xb7ce1e24>>
String.singleton_class      #=> #<Class:String>
nil.singleton_class         #=> NilClass

Similar to method, searches singleton method only.

class Demo
  def initialize(n)
    @iv = n
  def hello()
    "Hello, @iv = #{@iv}"

k =
def k.hi
  "Hi, @iv = #{@iv}"
m = k.singleton_method(:hi)   #=> "Hi, @iv = 99"
m = k.singleton_method(:hello) #=> NameError

Returns an array of the names of singleton methods for obj. If the optional all parameter is true, the list will include methods in modules included in obj. Only public and protected singleton methods are returned.

module Other
  def three() end

class Single
  def Single.four() end

a =


class << a
  include Other
  def two()

Single.singleton_methods    #=> [:four]
a.singleton_methods(false)  #=> [:two, :one]
a.singleton_methods         #=> [:two, :one, :three]

cgi_runner.rb – CGI launcher.

Author: IPR – Internet Programming with Ruby – writers Copyright © 2000 TAKAHASHI Masayoshi, GOTOU YUUZOU Copyright © 2002 Internet Programming with Ruby writers. All rights reserved.

$IPR: cgi_runner.rb,v 1.9 2002/09/25 11:33:15 gotoyuzo Exp $

Mark the object as tainted.

Objects that are marked as tainted will be restricted from various built-in methods. This is to prevent insecure data, such as command-line arguments or strings read from Kernel#gets, from inadvertently compromising the user’s system.

To check whether an object is tainted, use tainted?.

You should only untaint a tainted object if your code has inspected it and determined that it is safe. To do so use untaint.

Returns true if the object is tainted.

See taint for more information.

Yields self to the block, and then returns self. The primary purpose of this method is to “tap into” a method chain, in order to perform operations on intermediate results within the chain.

(1..10)                  .tap {|x| puts "original: #{x}" }
  .to_a                  .tap {|x| puts "array:    #{x}" }
  .select {|x| x.even? } .tap {|x| puts "evens:    #{x}" }
  .map {|x| x*x }        .tap {|x| puts "squares:  #{x}" }
No documentation available
No documentation available

Creates a new Enumerator which will enumerate by calling method on obj, passing args if any.

If a block is given, it will be used to calculate the size of the enumerator without the need to iterate it (see Enumerator#size).


str = "xyz"

enum = str.enum_for(:each_byte)
enum.each { |b| puts b }
# => 120
# => 121
# => 122

# protect an array from being modified by some_method
a = [1, 2, 3]

It is typical to call to_enum when defining methods for a generic Enumerable, in case no block is passed.

Here is such an example, with parameter passing and a sizing block:

module Enumerable
  # a generic method to repeat the values of any enumerable
  def repeat(n)
    raise ArgumentError, "#{n} is negative!" if n < 0
    unless block_given?
      return to_enum(__method__, n) do # __method__ is :repeat here
        sz = size     # Call size and multiply by n...
        sz * n if sz  # but return nil if size itself is nil
    each do |*val|
      n.times { yield *val }

%i[hello world].repeat(2) { |w| puts w }
  # => Prints 'hello', 'hello', 'world', 'world'
enum = (1..14).repeat(3)
  # => returns an Enumerator when called without a block
enum.first(4) # => [1, 1, 1, 2]
enum.size # => 42

Returns a string representing obj. The default to_s prints the object’s class and an encoding of the object id. As a special case, the top-level object that is the initial execution context of Ruby programs returns “main”.

Convert an object to YAML. See Psych.dump for more information on the available options.

Deprecated method that is equivalent to untaint.

Removes the tainted mark from the object.

See taint for more information.

Deprecated method that is equivalent to taint.

Deprecated method that is equivalent to tainted?.

A convenience method that’s only available when the you require the IRB::XMP standard library.

Creates a new XMP object, using the given expressions as the exps parameter, and optional binding as bind or uses the top-level binding. Then evaluates the given expressions using the :XMP prompt mode.

For example:

require 'irb/xmp'
ctx = binding
xmp 'foo = "bar"', ctx
#=> foo = "bar"
ctx.eval 'foo'
#=> "bar"

See for more information.

Yields self to the block and returns the result of the block. {|x| x**x }.to_s             #=> "256"
"my string".yield_self {|s| s.upcase }   #=> "MY STRING"

Good usage for yield_self is value piping in method chains:

require 'open-uri'
require 'json'

  yield_self {|url| open(url).read }.
  yield_self {|response| JSON.parse(response) }

When called without block, the method returns Enumerator, which can be used, for example, for conditional circuit-breaking:

# meets condition, no-op
1.yield_self.detect(&:odd?)            # => 1
# does not meet condition, drop value
2.yield_self.detect(&:odd?)            # => nil