class Class
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)-----------...
Public Class Methods
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.
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; }
Public Instance Methods
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
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); }
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
VALUE rb_class_attached_object(VALUE klass) { if (!FL_TEST(klass, FL_SINGLETON)) { rb_raise(rb_eTypeError, "`%"PRIsVALUE"' is not a singleton class", klass); } return RCLASS_ATTACHED_OBJECT(klass); }
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
.
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; }
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
VALUE rb_class_subclasses(VALUE klass) { return class_descendants(klass, true); }
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
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; } }
Private Instance Methods
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
#define rb_obj_class_inherited rb_obj_dummy1