Skip to Content Skip to Search

Extends any Class to include json_creatable? method.

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)-----------...
Methods
A
J
N
S

Class Public methods

Class.new(super_class=Object) → a_class
Class.new(super_class=Object) { |mod| ... } → a_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.

Source: show 

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 Public methods

class.allocate() → obj

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

Source: show 

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

Source: show 

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);
}

json_creatable?()

Returns true if this class can be used to create an instance from a serialised JSON string. The class has to implement a class method json_create that expects a hash as first parameter. The hash should include the required data.

Source: show | on GitHub 

# File ruby/ext/json/lib/json/common.rb, line 694
def json_creatable?
  respond_to?(:json_create)
end

class.new(args, ...) → obj

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.

Source: show 

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

Source: show 

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

class.superclass → a_super_class or nil

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

Source: show 

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;
    }
}