Class: Class

Relationships & Source Files
Super Chains via Extension / Inclusion / Inheritance
Class Chain: self, `::Module`
Instance Chain: self, `::Module`
Inherits:	Module `::Object` Module Class
Defined in:	object.c, class.c

Overview

Classes in Ruby are first-class objects—each is an instance of class Class.

Typically, you create a new class by using:

class Name
 # some code describing the class behavior
end

When a new class is created, an object of type Class is initialized and assigned to a global constant (Name in this case).

When Name.new is called to create a new object, the #new method in Class is run by default. This can be demonstrated by overriding #new in Class:

class Class
  alias old_new new
  def new(*args)
    print "Creating a new ", self.name, "\n"
    old_new(*args)
  end
end

class Name
end

n = Name.new

produces:

Creating a new Name

Classes, modules, and objects are interrelated. In the diagram that follows, the vertical arrows represent inheritance, and the parentheses metaclasses. All metaclasses are instances of the class ‘Class’.

                         {---------}             +-...
                         |         |             |
         BasicObject-----|-->(BasicObject)-------|-...
             ^           |         ^             |
             |           |         |             |
          Object---------|----->(Object)---------|-...
             ^           |         ^             |
             |           |         |             |
             {-------}   |         {--------}    |
             |       |   |         |        |    |
             |    Module-|---------|--->(Module)-|-...
             |       ^   |         |        ^    |
             |       |   |         |        |    |
             |     Class-|---------|---->(Class)-|-...
             |       ^   |         |        ^    |
             |       {---}         |        {----}
             |                     |
obj--->OtherClass---------->(OtherClass)-----------...

Class Method Summary

.new(super_class = Object) ⇒ Class constructor

Creates a new anonymous (unnamed) class with the given superclass (or ::Object if no parameter is given).

`::Module` - Inherited

.constants	In the first form, returns an array of the names of all constants accessible from the point of call.
.nesting	Returns the list of `Modules` nested at the point of call.
.new	Creates a new anonymous module.
.used_modules	Returns an array of all modules used in the current scope.
.used_refinements	Returns an array of all modules used in the current scope.

Instance Attribute Summary

`::Module` - Inherited

#singleton_class?

Returns true if mod is a singleton class or false if it is an ordinary class or module.

Instance Method Summary

#allocate ⇒ Object

Allocates space for a new object of class’s class and does not call initialize on the new instance.
#attached_object ⇒ Object

Returns the object for which the receiver is the singleton class.
#new(args, ...) ⇒ Object

Calls #allocate to create a new object of class’s class, then invokes that object’s #initialize method, passing it args.
#subclasses ⇒ Array

Returns an array of classes where the receiver is the direct superclass of the class, excluding singleton classes.
#superclass ⇒ Class^?

Returns the superclass of class, or nil.
#inherited(subclass) private

Callback invoked whenever a subclass of the current class is created.

`::Module` - Inherited

#<	Returns true if mod is a subclass of other.
#<=	Returns true if mod is a subclass of other or is the same as other.
#<=>	Comparison—Returns -1, 0, +1 or nil depending on whether `module` includes `other_module`, they are the same, or if `module` is included by `other_module`.
#==	Alias for Object#eql?.
#===	Case Equality—Returns `true` if obj is an instance of mod or an instance of one of mod’s descendants.
#>	Returns true if mod is an ancestor of other.
#>=	Returns true if mod is an ancestor of other, or the two modules are the same.
#alias_method	Makes new_name a new copy of the method old_name.
#ancestors	Returns a list of modules included/prepended in mod (including mod itself).
#attr	The first form is equivalent to `#attr_reader`.
#attr_accessor	Defines a named attribute for this module, where the name is symbol.`id2name`, creating an instance variable (`@name`) and a corresponding access method to read it.
#attr_reader	Creates instance variables and corresponding methods that return the value of each instance variable.
#attr_writer	Creates an accessor method to allow assignment to the attribute symbol`.id2name`.
#autoload	Registers filename to be loaded (using Kernel.require) the first time that const (which may be a `::String` or a symbol) is accessed in the namespace of mod.
#autoload?	Returns filename to be loaded if name is registered as `autoload` in the namespace of mod or one of its ancestors.
#class_eval	Evaluates the string or block in the context of mod, except that when a block is given, constant/class variable lookup is not affected.
#class_exec	Evaluates the given block in the context of the class/module.
#class_variable_defined?	Returns `true` if the given class variable is defined in obj.
#class_variable_get	Returns the value of the given class variable (or throws a `::NameError` exception).
#class_variable_set	Sets the class variable named by symbol to the given object.
#class_variables	Returns an array of the names of class variables in mod.
#const_defined?	Says whether mod or its ancestors have a constant with the given name:
#const_get	Checks for a constant with the given name in mod.
#const_missing	Invoked when a reference is made to an undefined constant in mod.
#const_set	Sets the named constant to the given object, returning that object.
#const_source_location	Returns the Ruby source filename and line number containing the definition of the constant specified.
#constants	Returns an array of the names of the constants accessible in mod.
#define_method	Defines an instance method in the receiver.
#deprecate_constant	Makes a list of existing constants deprecated.
#freeze	Prevents further modifications to mod.
#include	Invokes Module#append_features on each parameter in reverse order.
#include?	Returns `true` if module is included or prepended in mod or one of mod’s ancestors.
#included_modules	Returns the list of modules included or prepended in mod or one of mod’s ancestors.
#inspect	Alias for Module#to_s.
#instance_method	Returns an `::UnboundMethod` representing the given instance method in mod.
#instance_methods	Returns an array containing the names of the public and protected instance methods in the receiver.
#method_defined?	Returns `true` if the named method is defined by mod.
#module_eval	Alias for Module#class_eval.
#module_exec	Alias for Module#class_exec.
#name	Returns the name of the module mod.
#prepend	Invokes Module#prepend_features on each parameter in reverse order.
#private_class_method	Makes existing class methods private.
#private_constant	Makes a list of existing constants private.
#private_instance_methods	Returns a list of the private instance methods defined in mod.
#private_method_defined?	Returns `true` if the named private method is defined by mod.
#protected_instance_methods	Returns a list of the protected instance methods defined in mod.
#protected_method_defined?	Returns `true` if the named protected method is defined mod.
#public_class_method	Makes a list of existing class methods public.
#public_constant	Makes a list of existing constants public.
#public_instance_method	Similar to instance_method, searches public method only.
#public_instance_methods	Returns a list of the public instance methods defined in mod.
#public_method_defined?	Returns `true` if the named public method is defined by mod.
#refinements	Returns an array of `::Refinement` defined within the receiver.
#remove_class_variable	Removes the named class variable from the receiver, returning that variable’s value.
#remove_method	Removes the method identified by symbol from the current class.
#set_temporary_name	Sets the temporary name of the module.
#to_s	Returns a string representing this module or class.
#undef_method	Prevents the current class from responding to calls to the named method.
#undefined_instance_methods	Returns a list of the undefined instance methods defined in mod.
#append_features	When this module is included in another, Ruby calls `#append_features` in this module, passing it the receiving module in mod.
#const_added	Invoked as a callback whenever a constant is assigned on the receiver.
#extend_object	Extends the specified object by adding this module’s constants and methods (which are added as singleton methods).
#extended	The equivalent of `included`, but for extended modules.
#included	Callback invoked whenever the receiver is included in another module or class.
#method_added	Invoked as a callback whenever an instance method is added to the receiver.
#method_removed	Invoked as a callback whenever an instance method is removed from the receiver.
#method_undefined	Invoked as a callback whenever an instance method is undefined from the receiver.
#module_function	Creates module functions for the named methods.
#prepend_features	When this module is prepended in another, Ruby calls `#prepend_features` in this module, passing it the receiving module in mod.
#prepended	The equivalent of `included`, but for prepended modules.
#private	With no arguments, sets the default visibility for subsequently defined methods to private.
#protected	With no arguments, sets the default visibility for subsequently defined methods to protected.
#public	With no arguments, sets the default visibility for subsequently defined methods to public.
#refine	Refine mod in the receiver.
#remove_const	Removes the definition of the given constant, returning that constant’s previous value.
#ruby2_keywords	For the given method names, marks the method as passing keywords through a normal argument splat.
#using	Import class refinements from module into the current class or module definition.
#initialize_clone, #initialize_copy

Constructor Details

.new(super_class = Object) ⇒ `Class` .new(super_class = Object) {|mod| ... } ⇒ `Class`

Creates a new anonymous (unnamed) class with the given superclass (or ::Object if no parameter is given). You can give a class a name by assigning the class object to a constant.

If a block is given, it is passed the class object, and the block is evaluated in the context of this class like #class_eval.

fred = Class.new do
  def meth1
    "hello"
  end
  def meth2
    "bye"
  end
end

a = fred.new     #=> #<#<Class:0x100381890>:0x100376b98>
a.meth1          #=> "hello"
a.meth2          #=> "bye"

Assign the class to a constant (name starting uppercase) if you want to treat it like a regular class.

[ GitHub ]

# File 'object.c', line 2039


static VALUE
rb_class_initialize(int argc, VALUE *argv, VALUE klass)
{
    VALUE super;

    if (RCLASS_SUPER(klass) != 0 || klass == rb_cBasicObject) {
        rb_raise(rb_eTypeError, "already initialized class");
    }
    if (rb_check_arity(argc, 0, 1) == 0) {
        super = rb_cObject;
    }
    else {
        super = argv[0];
        rb_check_inheritable(super);
        if (super != rb_cBasicObject && !RCLASS_SUPER(super)) {
            rb_raise(rb_eTypeError, "can't inherit uninitialized class");
        }
    }
    RCLASS_SET_SUPER(klass, super);
    rb_make_metaclass(klass, RBASIC(super)->klass);
    rb_class_inherited(super, klass);
    rb_mod_initialize_exec(klass);

    return klass;
}

Instance Method Details

#allocate ⇒ Object

Allocates space for a new object of class’s class and does not call initialize on the new instance. The returned object must be an instance of class.

klass = Class.new do
  def initialize(*args)
    @initialized = true
  end

  def initialized?
    @initialized || false
  end
end

klass.allocate.initialized? #=> false

[ GitHub ]

# File 'object.c', line 2098


static VALUE
rb_class_alloc_m(VALUE klass)
{
    rb_alloc_func_t allocator = class_get_alloc_func(klass);
    if (!rb_obj_respond_to(klass, rb_intern("allocate"), 1)) {
        rb_raise(rb_eTypeError, "calling %"PRIsVALUE".allocate is prohibited",
                 klass);
    }
    return class_call_alloc_func(allocator, klass);
}

#attached_object ⇒ Object

Returns the object for which the receiver is the singleton class.

Raises an ::TypeError if the class is not a singleton class.

class Foo; end

Foo.singleton_class.attached_object        #=> Foo
Foo.attached_object                        #=> TypeError: `Foo' is not a singleton class
Foo.new.singleton_class.attached_object    #=> #<Foo:0x000000010491a370>
TrueClass.attached_object                  #=> TypeError: `TrueClass' is not a singleton class
NilClass.attached_object                   #=> TypeError: `NilClass' is not a singleton class

[ GitHub ]

# File 'class.c', line 1716


VALUE
rb_class_attached_object(VALUE klass)
{
    if (!RCLASS_SINGLETON_P(klass)) {
        rb_raise(rb_eTypeError, "'%"PRIsVALUE"' is not a singleton class", klass);
    }

    return RCLASS_ATTACHED_OBJECT(klass);
}

#inherited(subclass) (private)

Callback invoked whenever a subclass of the current class is created.

Example:

class Foo
  def self.inherited(subclass)
    puts "New subclass: #{subclass}"
  end
end

class Bar < Foo
end

class Baz < Bar
end

produces:

New subclass: Bar
New subclass: Baz

[ GitHub ]

#new(args, ...) ⇒ Object

Calls #allocate to create a new object of class’s class, then invokes that object’s #initialize method, passing it args. This is the method that ends up getting called whenever an object is constructed using .new.

[ GitHub ]

# File 'object.c', line 2167


VALUE
rb_class_new_instance_pass_kw(int argc, const VALUE *argv, VALUE klass)
{
    VALUE obj;

    obj = rb_class_alloc(klass);
    rb_obj_call_init_kw(obj, argc, argv, RB_PASS_CALLED_KEYWORDS);

    return obj;
}

#subclasses ⇒ Array

Returns an array of classes where the receiver is the direct superclass of the class, excluding singleton classes. The order of the returned array is not defined.

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

A.subclasses        #=> [D, B]
B.subclasses        #=> [C]
C.subclasses        #=> []

Anonymous subclasses (not associated with a constant) are returned, too:

c = Class.new(A)
A.subclasses        # => [#<Class:0x00007f003c77bd78>, D, B]

Note that the parent does not hold references to subclasses and doesn’t prevent them from being garbage collected. This means that the subclass might disappear when all references to it are dropped:

# drop the reference to subclass, it can be garbage-collected now
c = nil

A.subclasses
# It can be
#  => [#<Class:0x00007f003c77bd78>, D, B]
# ...or just
#  => [D, B]
# ...depending on whether garbage collector was run

[ GitHub ]

# File 'class.c', line 1693


VALUE
rb_class_subclasses(VALUE klass)
{
    return class_descendants(klass, true);
}

#superclass ⇒ `Class`^?

Returns the superclass of class, or nil.

File.superclass          #=> IO
IO.superclass            #=> Object
Object.superclass        #=> BasicObject
class Foo; end
class Bar < Foo; end
Bar.superclass           #=> Foo

Returns nil when the given class does not have a parent class:

BasicObject.superclass   #=> nil

[ GitHub ]

# File 'object.c', line 2223


VALUE
rb_class_superclass(VALUE klass)
{
    RUBY_ASSERT(RB_TYPE_P(klass, T_CLASS));

    VALUE super = RCLASS_SUPER(klass);

    if (!super) {
        if (klass == rb_cBasicObject) return Qnil;
        rb_raise(rb_eTypeError, "uninitialized class");
    }

    if (!RCLASS_SUPERCLASS_DEPTH(klass)) {
        return Qnil;
    }
    else {
        super = RCLASS_SUPERCLASSES(klass)[RCLASS_SUPERCLASS_DEPTH(klass) - 1];
        RUBY_ASSERT(RB_TYPE_P(klass, T_CLASS));
        return super;
    }
}

Class: Class

Overview

Class Method Summary

::Module - Inherited

Instance Attribute Summary

::Module - Inherited

Instance Method Summary

::Module - Inherited

Constructor Details

.new(super_class = Object) ⇒ Class .new(super_class = Object) {|mod| ... } ⇒ Class

Instance Method Details

#allocate ⇒ Object

#attached_object ⇒ Object

#inherited(subclass) (private)

#new(args, ...) ⇒ Object

#subclasses ⇒ Array

#superclass ⇒ Class?

`::Module` - Inherited

`::Module` - Inherited

`::Module` - Inherited

.new(super_class = Object) ⇒ `Class` .new(super_class = Object) {|mod| ... } ⇒ `Class`

#superclass ⇒ `Class`^?