Object

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?.

Returns true or false.

Like Object#==, if object is an instance of Object (and not an instance of one of its many subclasses).

This method is commonly overridden by those subclasses, to provide meaningful semantics in case statements.

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
  end
end

or:

MyClass = DelegateClass(ClassToDelegateTo) do    # Step 1
  def initialize
    super(obj_of_ClassToDelegateTo)              # Step 2
  end
end

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.open(tmpname, File::RDWR|File::CREAT|File::EXCL, 0600)

    # ...

    super(@tmpfile)

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

  # ...
end

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

require 'digest'

Digest("MD5")
# => Digest::MD5

Digest(:SHA256)
# => Digest::SHA256

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

Defines a public 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. If a block or a method has parameters, they’re used as method parameters.

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

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

chris = "Chris"
chris.define_singleton_method(:greet) {|greeting| "#{greeting}, I'm Chris!" }
chris.greet("Hi") #=> "Hi, I'm Chris!"

Writes self on the given port:

1.display
"cat".display
[ 4, 5, 6 ].display
puts

Output:

1cat[4, 5, 6]

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

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
end

module Foo
  def foo; 'foo'; end
end

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

s2 = s1.clone #=> #<Klass:0x401be280>
s2.foo #=> "foo"

s3 = s1.dup #=> #<Klass:0x401c1084>
s3.foo #=> NoMethodError: undefined method `foo' for #<Klass:0x401c1084>

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 any pair of objects where eql? returns true, the hash value of both objects must be equal. So any subclass that overrides eql? should also override hash appropriately.

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"
  end
end

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

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

Prevents further modifications to obj. A FrozenError 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.freeze
a << "z"

produces:

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.

Generates an Integer hash value for this object. This function must have the property that a.eql?(b) implies a.hash == b.hash.

The hash value is used along with eql? by the Hash class to determine if two objects reference the same hash key. Any hash value that exceeds the capacity of an Integer will be truncated before being used.

The hash value for an object may not be identical across invocations or implementations of Ruby. If you need a stable identifier across Ruby invocations and implementations you will need to generate one with a custom method.

Certain core classes such as Integer use built-in hash calculations and do not call the hash method when used as a hash key.

When implementing your own hash based on multiple values, the best practice is to combine the class and any values using the hash code of an array:

For example:

def hash
  [self.class, a, b, c].hash
end

The reason for this is that the Array#hash method already has logic for safely and efficiently combining multiple hash values.

Returns a string containing a human-readable representation of obj. The default inspect shows the object’s class name, an encoding of its memory address, 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\"]"
Time.new.inspect                 #=> "2008-03-08 19:43:39 +0900"

class Foo
end
Foo.new.inspect                  #=> "#<Foo:0x0300c868>"

class Bar
  def initialize
    @bar = 1
  end
end
Bar.new.inspect                  #=> "#<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.new
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
  end
end
fred = Fred.new('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
  end
end
fred = Fred.new('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. This may circumvent the encapsulation intended by the author of the class, so it should be used with care. 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
  end
end
fred = Fred.new('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
end
Fred.new.instance_variables   #=> [:@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
end
class B < A; end
class C < B; end

b = B.new
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
  end
  def hello()
    "Hello, @iv = #{@iv}"
  end
end

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

l = Demo.new('Fred')
m = l.method("hello")
m.call   #=> "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 = File.open('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’s public and protected singleton methods, the array will not include methods in modules included in obj.

class Klass
  def klass_method()
  end
end
k = Klass.new
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]

Only the object nil responds true to nil?.

Object.new.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.

BasicObject implements __id__, Kernel implements object_id.

Immediate values are not passed by reference but are passed by value: nil, true, false, Fixnums, Symbols, and some Floats.

Object.new.object_id  == Object.new.object_id  # => 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

Removes the named instance variable from obj, returning that variable’s value. The name can be passed as a symbol or as a string.

class Dummy
  attr_reader :var
  def initialize
    @var = 99
  end
  def remove
    remove_instance_variable(:@var)
  end
end
d = Dummy.new
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.

DO NOT USE THIS DIRECTLY.

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. When the method is identified by a string, the string is converted to a symbol.

BasicObject implements __send__, Kernel implements send. __send__ is safer than send when obj has the same method name like Socket. See also public_send.

class Klass
  def hello(*args)
    "Hello " + args.join(' ')
  end
end
k = Klass.new
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.

Object.new.singleton_class  #=> #<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
  end
  def hello()
    "Hello, @iv = #{@iv}"
  end
end

k = Demo.new(99)
def k.hi
  "Hi, @iv = #{@iv}"
end
m = k.singleton_method(:hi)
m.call   #=> "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
end

class Single
  def Single.four() end
end

a = Single.new

def a.one()
end

class << a
  include Other
  def two()
  end
end

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

Creates a new Enumerator which will enumerate by calling method on obj, passing args if any. What was yielded by method becomes values of enumerator.

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).

Examples

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]
some_method(a.to_enum)

# String#split in block form is more memory-effective:
very_large_string.split("|") { |chunk| return chunk if chunk.include?('DATE') }
# This could be rewritten more idiomatically with to_enum:
very_large_string.to_enum(:split, "|").lazy.grep(/DATE/).first

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
      end
    end
    each do |*val|
      n.times { yield *val }
    end
  end
end

%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.

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"
  #==>"bar"
ctx.eval 'foo'
#=> "bar"

See XMP.new for more information.