Chapter 4: Classes and modules

In this chapter, we’ll see the details of the data structures created for classes and modules.

Classes and methods definition

First, I’d like to have a look at how Ruby classes are defined at the C level. This chapter investigates almost only particular cases, so I’d like you to know first the way used most often.

The main API to define classes and modules consists of the following 6 functions:

There are a few other versions of these functions, but the extension libraries and even most of the core library is defined using just this API. I’ll introduce to you these functions one by one.

Class definition

rb_define_class() defines a class at the top-level. Let’s take the Ruby array class, Array, as an example.

Array class definition
  19  VALUE rb_cArray;

1809  void
1810  Init_Array()
1811  {
1812      rb_cArray  = rb_define_class("Array", rb_cObject);


rb_cObject and rb_cArray correspond respectively to Object and Array at the Ruby level. The added prefix rb shows that it belongs to ruby and the c that it is a class object. These naming rules are used everywhere in ruby.

This call to rb_define_class() allows to define Array that inherits from Object. At the same time as rb_define_class() creates the class object, it also defines the constant. That means that after this you can already access Array from a Ruby program. It corresponds to the following Ruby program:

class Array < Object

I’d like you to note the fact that there is no end. It was written like this on purpose. It is because with rb_define_class() the body of the class has not been executed.

Nested class definition

After that, there’s rb_define_class_under(). This function defines a class nested in an other class or module. This time the example is what is returned by stat(2), File::Stat.

▼ Definition of File::Stat
  78  VALUE rb_cFile;
  80  static VALUE rb_cStat;

2581      rb_cFile = rb_define_class("File", rb_cIO);
2674      rb_cStat = rb_define_class_under(rb_cFile, "Stat", rb_cObject);


This code corresponds to the following Ruby program;

class File < IO
  class Stat < Object

This time again I omitted the end on purpose.

Module definition

rb_define_module() is simple so let’s end this quickly.

▼ Definition of Enumerable
  17  VALUE rb_mEnumerable;

 492      rb_mEnumerable = rb_define_module("Enumerable");


The m in the beginning of rb_mEnumerable is similar to the c for classes: it shows that it is a module. The corresponding Ruby program is:

module Enumerable

rb_define_module_under() is not used much so we’ll skip it.

Method definition

This time the function is the one for defining methods, rb_define_method(). It’s used very often. We’ll take once again an example from Array.

▼ Definition of Array#to_s
1818  rb_define_method(rb_cArray, "to_s", rb_ary_to_s, 0);


With this the to_s method is defined in Array. The method body is given by a function pointer (rb_ary_to_s). The fourth parameter is the number of parameters taken by the method. As to_s does not take any parameters, it’s 0. If we write the corresponding Ruby program, we’ll have this:

class Array < Object
  def to_s
    # content of rb_ary_to_s()

Of course the class part is not included in rb_define_method() and only the def part is accurate. But if there is no class part, it will look like the method is defined like a function, so I also wrote the enclosing class part.

One more example, this time taking a parameter:

▼ Definition of Array#concat
1835  rb_define_method(rb_cArray, "concat", rb_ary_concat, 1);


The class for the definition is rb_cArray (Array), the method name is concat, its body is rb_ary_concat() and the number of parameters is 1. It corresponds to writing the corresponding Ruby program:

class Array < Object
  def concat( str )
    # content of rb_ary_concat()

Singleton methods definition

We can define methods that are specific to an instance of an object. They are called singleton methods. As I used File.unlink as an example in chapter 1 “Ruby language minimum”, I first wanted to show it here, but for a particular reason we’ll look at instead.

▼ Definition of
2624  rb_define_singleton_method(rb_cFile, "link", rb_file_s_link, 2);


It’s used like rb_define_method(). The only difference is that here the first parameter is just the object where the method is defined. In this case, it’s defined in rb_cFile.

Entry point

Being able to make definitions like before is great, but where are these functions called from, and by what means are they executed? These definitions are grouped in functions named Init_xxxx(). For instance, for Array a function Init_Array() like this has been made:

1809  void
1810  Init_Array()
1811  {
1812      rb_cArray  = rb_define_class("Array", rb_cObject);
1813      rb_include_module(rb_cArray, rb_mEnumerable);
1815      rb_define_singleton_method(rb_cArray, "allocate",
                                     rb_ary_s_alloc, 0);
1816      rb_define_singleton_method(rb_cArray, "[]", rb_ary_s_create, -1);
1817      rb_define_method(rb_cArray, "initialize", rb_ary_initialize, -1);
1818      rb_define_method(rb_cArray, "to_s", rb_ary_to_s, 0);
1819      rb_define_method(rb_cArray, "inspect", rb_ary_inspect, 0);
1820      rb_define_method(rb_cArray, "to_a", rb_ary_to_a, 0);
1821      rb_define_method(rb_cArray, "to_ary", rb_ary_to_a, 0);
1822      rb_define_method(rb_cArray, "frozen?",  rb_ary_frozen_p, 0);


The Init for the built-in functions are explicitely called during the startup of ruby. This is done in inits.c.

  47  void
  48  rb_call_inits()
  49  {
  50      Init_sym();
  51      Init_var_tables();
  52      Init_Object();
  53      Init_Comparable();
  54      Init_Enumerable();
  55      Init_Precision();
  56      Init_eval();
  57      Init_String();
  58      Init_Exception();
  59      Init_Thread();
  60      Init_Numeric();
  61      Init_Bignum();
  62      Init_Array();


This way, Init_Array() is called properly.

That explains it for built-in libraries, but what about extension libraries? In fact, for extension libraries the convention is the same. Take the following code:

require "myextension" 

With this, if the loaded extension library is, at load time, the (extern) function named Init_myextension() is called. How they are called is beyond the scope of this chapter. For that, you should read the chapter 18 “Load”. Here we’ll just end this with an example of Init.

The following example is from stringio, an extension library provided with ruby, that is to say not from a built-in library.

Init_stringio() (beginning)
 895  void
 896  Init_stringio()
 897  {
 898      VALUE StringIO = rb_define_class("StringIO", rb_cData);
 899      rb_define_singleton_method(StringIO, "allocate",
                                     strio_s_allocate, 0);
 900      rb_define_singleton_method(StringIO, "open", strio_s_open, -1);
 901      rb_define_method(StringIO, "initialize", strio_initialize, -1);
 902      rb_enable_super(StringIO, "initialize");
 903      rb_define_method(StringIO, "become", strio_become, 1);
 904      rb_define_method(StringIO, "reopen", strio_reopen, -1);


Singleton classes


You should now be able to more or less understand how normal methods are defined. Somehow making the body of the method, then registering it in m_tbl will do. But what about singleton methods? We’ll now look into the way singleton methods are defined.

 721  void
 722  rb_define_singleton_method(obj, name, func, argc)
 723      VALUE obj;
 724      const char *name;
 725      VALUE (*func)();
 726      int argc;
 727  {
 728      rb_define_method(rb_singleton_class(obj), name, func, argc);
 729  }


As I explained, rb_define_method() is a function used to define normal methods, so the difference from normal methods is only rb_singleton_class(). But what on earth are singleton classes?

In brief, singleton classes are virtual classes that are only used to execute singleton methods. Singleton methods are functions defined in singleton classes. Classes themselves are in the first place (in a way) the “implementation” to link objects and methods, but singleton classes are even more on the implementation side. In the Ruby language way, they are not formally included, and don’t appear much at the Ruby level.


Well, let’s confirm what the singleton classes are made of. It’s too simple to each time just show you the code of function so this time I’ll use a new weapon, a call graph.


Call graphs are graphs showing calling relationships among functions (or more generally procedures). The call graphs showing all the calls written in the source code are called static call graphs. The ones expressing only the calls done during an execution are called dynamic call graphs.

This diagram is a static call graph and the indentation expresses which function calls which one. For instance, rb_define_singleton_method() calls rb_define_method() and rb_singleton_class(). And this rb_singleton_class() itself calls SPECIAL_SINGLETON() and rb_make_metaclass().

Let’s go back to the code. When looking at the call graph, you can see that the calls made by rb_singleton_class() go very deep. Until now all call levels were shallow, so we could simply look at the functions without getting too lost. But at this depth, I easily forget what is going on in the code. That’s why in those situations I check the call graph to have a better understanding. This time, we’ll decode in parallel what the procedures below rb_singleton_class() do. The two points to look out for are the following ones:

Normal classes and singleton classes

Singleton classes are special classes: they’re basically the same as normal classes, but there are a few differences. We can say that finding these differences is explaining concretely singleton classes.

What should we do to find them? We should find the differences between the function creating normal classes and the one creating singleton classes. For this, we have to find the function for creating normal classes. That is as normal classes can be defined by rb_define_class(), it must call in a way or another a function to create normal classes. For the moment, we’ll not look at the content of rb_define_class() itself. I have some reasons to be interested in something that’s deeper. That’s why we will first look at the call graph of rb_define_class().


I’m interested by rb_class_new(). Doesn’t this name means it creates a new class? Let’s confirm that.

  37  VALUE
  38  rb_class_new(super)
  39      VALUE super;
  40  {
  41      Check_Type(super, T_CLASS);
  42      if (super == rb_cClass) {
  43          rb_raise(rb_eTypeError, "can't make subclass of Class");
  44      }
  45      if (FL_TEST(super, FL_SINGLETON)) {
  46          rb_raise(rb_eTypeError, "can't make subclass of virtual class");
  47      }
  48      return rb_class_boot(super);
  49  }


Check_Type() is checks the type of object structure, so we can ignore it. rb_raise() is error handling so we can ignore it. Only rb_class_boot() remains. So let’s look at it.

  21  VALUE
  22  rb_class_boot(super)
  23      VALUE super;
  24  {
  25      NEWOBJ(klass, struct RClass);        /* allocates struct RClass */
  26      OBJSETUP(klass, rb_cClass, T_CLASS); /* initialization of the RBasic part */
  28      klass->super = super;       /* (A) */
  29      klass->iv_tbl = 0;
  30      klass->m_tbl = 0;
  31      klass->m_tbl = st_init_numtable();
  33      OBJ_INFECT(klass, super);
  34      return (VALUE)klass;
  35  }


NEWOBJ() and OBJSETUP() are fixed expressions used when creating Ruby objects that possess one of the internal structure types (struct Rxxxx). They are both macros. In NEWOBJ(), struct RClass is created and the pointer is put in its first parameter klass. In OBJSETUP(), the struct RBasic member of the RClass (and thus basic.klass and basic.flags) is initialized.

OBJ_INFECT() is a macro related to security. From now on, we’ll ignore it.

At (A), the super member of klassis set to the super parameter. It looks like rb_class_boot() is a function that creates a class inheriting from super.

So, as rb_class_boot() is a function that creates a class, what does rb_class_new() is very similar.

Then, let’s once more look at rb_singleton_class()’s call graph:


Here also rb_class_boot() is called. So up to that point, it’s the same as in normal classes. What’s going on after is what’s different between normal classes and singleton classes, in other words the characteristics of singleton classes. If you everything’s clear so far, we just need to read rb_singleton_class() and rb_make_metaclass().

Compressed rb_singleton_class()

rb_singleton_class() is a little long so we’ll first remove its non-essential parts.

 678  #define SPECIAL_SINGLETON(x,c) do {\
 679      if (obj == (x)) {\
 680          return c;\
 681      }\
 682  } while (0)

 684  VALUE
 685  rb_singleton_class(obj)
 686      VALUE obj;
 687  {
 688      VALUE klass;
 690      if (FIXNUM_P(obj) || SYMBOL_P(obj)) {
 691          rb_raise(rb_eTypeError, "can't define singleton");
 692      }
 693      if (rb_special_const_p(obj)) {
 694          SPECIAL_SINGLETON(Qnil, rb_cNilClass);
 695          SPECIAL_SINGLETON(Qfalse, rb_cFalseClass);
 696          SPECIAL_SINGLETON(Qtrue, rb_cTrueClass);
 697          rb_bug("unknown immediate %ld", obj);
 698      }
 700      DEFER_INTS;
 701      if (FL_TEST(RBASIC(obj)->klass, FL_SINGLETON) &&
 702          (BUILTIN_TYPE(obj) == T_CLASS ||
 703           rb_iv_get(RBASIC(obj)->klass, "__attached__") == obj)) {
 704          klass = RBASIC(obj)->klass;
 705      }
 706      else {
 707          klass = rb_make_metaclass(obj, RBASIC(obj)->klass);
 708      }
 709      if (OBJ_TAINTED(obj)) {
 710          OBJ_TAINT(klass);
 711      }
 712      else {
 713          FL_UNSET(klass, FL_TAINT);
 714      }
 715      if (OBJ_FROZEN(obj)) OBJ_FREEZE(klass);
 716      ALLOW_INTS;
 718      return klass;
 719  }


The first and the second half are separated by a blank line. The first half handles a special case and the second half handles the general case. In other words, the second half is the trunk of the function. That’s why we’ll keep it for later and talk about the first half.

Everything that is handled in the first half are non-pointer VALUEs, in other words objects without an existing C structure. First, Fixnum and Symbol are explicitely picked. Then, rb_special_const_p() is a function that returns true for non-pointer VALUEs, so there only Qtrue, Qfalse and Qnil should get caught. Other than that, there are no valid non-pointer value so a bug is reported with rb_bug().

DEFER_INTS() and ALLOW_INTS() both end with the same INTS so you should see a pair in them. That’s the case, and they are macros related to signals. Because they are defined in rubysig.h, you can guess that INTS is the abbreviation of interrupts. You can ignore them.

Compressed rb_make_metaclass()

 142  VALUE
 143  rb_make_metaclass(obj, super)
 144      VALUE obj, super;
 145  {
 146      VALUE klass = rb_class_boot(super);
 147      FL_SET(klass, FL_SINGLETON);
 148      RBASIC(obj)->klass = klass;
 149      rb_singleton_class_attached(klass, obj);
 150      if (BUILTIN_TYPE(obj) == T_CLASS) {
 151          RBASIC(klass)->klass = klass;
 152          if (FL_TEST(obj, FL_SINGLETON)) {
 153              RCLASS(klass)->super =
 154          }
 155      }
 157      return klass;
 158  }


We already saw rb_class_boot(). It creates a (normal) class using the super parameter as its superclass. After that, the FL_SINGLETON of this class is set. This is clearly suspicious. The name of the function makes us think that it is not the indication of a singleton class.

What are singleton classes?

Continuing the simplification process, furthermore as parameters, return values, local variables are all VALUE, we can throw away the declarations. That makes us able to compress to the following:

rb_singleton_class() rb_make_metaclass() (after compression)
    if (FL_TEST(RBASIC(obj)->klass, FL_SINGLETON) &&
        (BUILTIN_TYPE(obj) == T_CLASS || BUILTIN_TYPE(obj) == T_MODULE) &&
        rb_iv_get(RBASIC(obj)->klass, "__attached__") == obj) {
        klass = RBASIC(obj)->klass;
    else {
        klass = rb_make_metaclass(obj, RBASIC(obj)->klass);
    return klass;

rb_make_metaclass(obj, super)
    klass = create a class with super as superclass;
    FL_SET(klass, FL_SINGLETON);
    RBASIC(obj)->klass = klass;
    rb_singleton_class_attached(klass, obj);
    if (BUILTIN_TYPE(obj) == T_CLASS) {
        RBASIC(klass)->klass = klass;
        if (FL_TEST(obj, FL_SINGLETON)) {
            RCLASS(klass)->super =

    return klass;

The condition of the if statement of rb_singleton_class() seems quite complicated. However, this condition is not connected to the mainstream of rb_make_metaclass() so we’ll see it later. Let’s first think about what happens on the false branch of the if.

The BUILTIN_TYPE() of rb_make_metaclass() is similar to TYPE() as it is a macro to get the structure type flag (T_xxxx). That means this check in rb_make_metaclass means “if obj is a class”. For the moment it’s better not to limit ourselves to obj being a class, so we’ll remove it.

With these simplifications, we get the the following:

rb_singleton_class() rb_make_metaclass() (after recompression)
    klass = create a class with RBASIC(obj)->klass as superclass;
    FL_SET(klass, FL_SINGLETON);
    RBASIC(obj)->klass = klass;
    return klass;

But there is still a quite hard to understand side to it. That’s because klass is used too often. So let’s rename the klass variable to sclass.

rb_singleton_class() rb_make_metaclass() (variable substitution)
    sclass = create a class with RBASIC(obj)->klass as superclass;
    FL_SET(sclass, FL_SINGLETON);
    RBASIC(obj)->klass = sclass;
    return sclass;

Now it should be very easy to understand. To make it even simpler, I’ve represented what is done with a diagram (figure 1). In the horizontal direction is the “instance – class” relation, and in the vertical direction is inheritance (the superclasses are above).

Figure 1: rb_singleton_class

When comparing the first and last part of this diagram, you can understand that sclass is inserted without changing the structure. That’s all there is to singleton classes. In other words the inheritance is increased one step. If a method is defined in a singleton class, this construction allows the other instances of klass to define completely different methods.

Singleton classes and instances

By the way, you must have seen that during the compression process, the call to rb_singleton_class_attached() was stealthily removed. Here:

rb_make_metaclass(obj, super)
    klass = create a class with super as superclass;
    FL_SET(klass, FL_SINGLETON);
    RBASIC(obj)->klass = klass;
    rb_singleton_class_attached(klass, obj);   /* THIS */

Let’s have a look at what it does.

 130  void
 131  rb_singleton_class_attached(klass, obj)
 132      VALUE klass, obj;
 133  {
 134      if (FL_TEST(klass, FL_SINGLETON)) {
 135          if (!RCLASS(klass)->iv_tbl) {
 136              RCLASS(klass)->iv_tbl = st_init_numtable();
 137          }
 138          st_insert(RCLASS(klass)->iv_tbl,
                        rb_intern("__attached__"), obj);
 139      }
 140  }


If the FL_SINGLETON flag of klass is set… in other words if it’s a singleton class, put the __attached__obj relation in the instance variable table of klass (iv_tbl). That’s how it looks like (in our case klass is always a singleton class… in other words its FL_SINGLETON flag is always set).

__attached__ does not have the @ prefix, but it’s stored in the instance variables table so it’s still an instance variable. Such an instance variable can never be read at the Ruby level so it can be used to keep values for the system’s exclusive use.

Let’s now think about the relationship between klass and obj. klass is the singleton class of obj. In other words, this “invisible” instance variable allows the singleton class to remember the instance it was created from. Its value is used when the singleton class is changed, notably to call hook methods on the instance (i.e. obj). For example, when a method is added to a singleton class, the obj’s singleton_method_added method is called. There is no logical necessity to doing it, it was done because that’s how it was defined in the language.

But is it really all right? Storing the instance in __attached__ will force one singleton class to have only one attached instance. For example, by getting (in some way or an other) the singleton class and calling new on it, won’t a singleton class end up having multiple instances?

This cannot be done because the proper checks are done to prevent the creation of an instance of a singleton class.

Singleton classes are in the first place for singleton methods. Singleton methods are methods existing only on a particular object. If singleton classes could have multiple instances, there would the same as normal classes. That’s why they are forced to only have one instance.


We’ve done a lot, maybe made a real mayhem, so let’s finish and put everything in order with a summary.

What are singleton classes? They are classes that have the FL_SINGLETON flag set and that can only have one instance.

What are singleton methods? They are methods defined in the singleton class of an object.


Inheritance of singleton methods

Infinite chain of classes

Even a class has a class, and it’s Class. And the class of Class is again Class. We find ourselves in an infinite loop (figure 2).

Infinite loop of classes
Figure 2: Infinite loop of classes

Up to here it’s something we’ve already gone through. What’s going after that is the theme of this chapter. Why do classes have to make a loop?

First, in Ruby all data are objects. And classes are data so in Ruby they have to be objects.

As they are objects, they must answer to methods. And setting the rule “to answer to methods you must belong to a class” made processing easier. That’s where comes the need for a class to also have a class.

Let’s base ourselves on this and think about the way to implement it. First, we can try first with the most naïve way, Class’s class is ClassClass, ClassClass’s class is ClassClassClass..., chaining classes of classes one by one. But whichever the way you look at it, this can’t be implemented effectively. That’s why it’s common in object oriented languages where classes are objects that Class’s class is to Class itself, creating an endless virtual instance-class relationship.

I’m repeating myself, but the fact that Class’s class is Class is only to make the implementation easier, there’s nothing important in this logic.

“Class is also an object”

“Everything is an object” is often used as advertising statement when speaking about Ruby. And as a part of that, “Classes are also object!” also appears. But these expressions often go too far. When thinking about these sayings, we have to split them in two:

Talking about data or code makes a discussion much harder to understand. That’s why here we’ll restrict the meaning of “data” to “what can be put in variables in programs”.

Being able to manipulate classes from programs gives programs the ability to manipulate themselves. This is called reflection. It fits object oriented languages, and even more Ruby with the classes it has, to be able to directly manipulate classes.

Nevertheless, classes could be made available in a form that is not an object. For example, classes could be manipulated with function-style methods (functions defined at the top-level). However, as inside the interpreter there are data structures to represent the classes, it’s more natural in object oriented languages to make them available directly. And Ruby did this choice.

Furthermore, an objective in Ruby is for all data to be objects. That’s why it’s appropriate to make them objects.

By the way, there is a reason not linked to reflection why in Ruby classes had to be made objects. That is to be able to define methods independently from instances (what is called static methods in Java in C++).

And to implement static methods, another thing was necessary: singleton methods. By chain reaction, that also makes singleton classes necessary. Figure 3 shows these dependency relationships.

Requirements dependencies
Figure 3: Requirements dependencies

Class methods inheritance

In Ruby, singleton methods defined in a class are called class methods. However, their specification is a little strange. Why are class methods inherited?

class A
  def A.test    # defines a singleton method in A

class B < A

B.test()  # calls it

This can’t occur with singleton methods from objects that are not classes. In other words, classes are the only ones handled specially. In the following section we’ll see how class methods are inherited.

Singleton class of a class

Assuming that class methods are inherited, where is this operation done? At class definition (creation)? At singleton method definition? Then let’s look at the code defining classes.

Class definition means of course rb_define_class(). Now let’s take the call graph of this function.


If you’re wondering where you’ve seen it before, we looked at it in the previous section. At that time you did not see it but if you look closely, why does rb_make_metaclass() appear? As we saw before, this function introduces a singleton class. This is very suspicious. Why is this called even if we are not defining a singleton function? Furthermore, why is the lower level rb_make_metaclass() used instead of rb_singleton_class()? It looks like we have to check these surroundings again.


Let’s first start our reading with its caller, rb_define_class_id().

 160  VALUE
 161  rb_define_class_id(id, super)
 162      ID id;
 163      VALUE super;
 164  {
 165      VALUE klass;
 167      if (!super) super = rb_cObject;
 168      klass = rb_class_new(super);
 169      rb_name_class(klass, id);
 170      rb_make_metaclass(klass, RBASIC(super)->klass);
 172      return klass;
 173  }


rb_class_new() was a function that creates a class with super as its superclass. rb_name_class()’s name means it names a class, but for the moment we do note care about names so we’ll skip it. After that there’s the rb_make_metaclass() in question. I’m concerned by the fact that when called from rb_singleton_class(), the parameters were different. Last time was like this:

rb_make_metaclass(obj, RBASIC(obj)->klass);

But this time is like this:

rb_make_metaclass(klass, RBASIC(super)->klass);

So as you can see it’s slightly different. How do the results change depending on that? Let’s have once again a look at a simplified rb_make_metaclass().

rb_make_metaclass (once more)

rb_make_metaclass (after first compression)
rb_make_metaclass(obj, super)
    klass = create a class with super as superclass;
    FL_SET(klass, FL_SINGLETON);
    RBASIC(obj)->klass = klass;
    rb_singleton_class_attached(klass, obj);
    if (BUILTIN_TYPE(obj) == T_CLASS) {
        RBASIC(klass)->klass = klass;
        if (FL_TEST(obj, FL_SINGLETON)) {
            RCLASS(klass)->super =

    return klass;

Last time, the if statement was skillfully skipped, but looking once again, something is done only for T_CLASS, in other words classes. This clearly looks important. In rb_define_class_id(), as it’s called like this:

rb_make_metaclass(klass, RBASIC(super)->klass);

Let’s expand rb_make_metaclass()’s parameter variables with this values.

rb_make_metaclass (recompression)
rb_make_metaclass(klass, super_klass /* == RBASIC(super)->klass */)
    sclass = create a class with super_class as superclass;
    RBASIC(klass)->klass = sclass;
    RBASIC(sclass)->klass = sclass;
    return sclass;

Doing this as a diagram gives something like figure 4. In it, the names between parentheses are singleton classes. This notation is often used in this book so I’d like you to remember it. This means that obj’s singleton class is written as (obj). And (klass) is the singleton class for klass. It looks like the singleton class is caught between a class and this class’s superclass’s class.

Introduction of a class's singleton class
Figure 4: Introduction of a class’s singleton class

From this result, and moreover when thinking more deeply, we can think that the superclass’s class must again be the superclass’s singleton class. You’ll understand with one more inheritance level (figure 5).

Hierarchy of multi-level inheritance
Figure 5: Hierarchy of multi-level inheritance

As the relationship between super and klass is the same as the one between klass and klass2, c must be the singleton class (super). If you continue like this, finally you’ll arrive at the conclusion that Object’s class must be (Object). And that’s the case in practice. For example, by inheriting like in the following program :

class A < Object
class B < A

internally, a structure like figure 6 is created.

Class hierarchy and metaclasses
Figure 6: Class hierarchy and metaclasses

As classes and their metaclasses are linked and inherit like this, class methods are inherited.

Class of a class of a class

You’ve understood the working of class methods inheritance, but by doing that, in the opposite some questions have appeared. What is the class of a class’s singleton class? To do this we can try debugging. I’ve made the figure 7 from the results of this investigation.

Class of a class's singleton class
Figure 7: Class of a class’s singleton class

A class’s singleton class puts itself as its own class. Quite complicated.

The second question: the class of Object must be Class. Didn’t I properly confirm this in chapter 1: Ruby language minimum?

p(Object.class())   # Class

Certainly, that’s the case “at the Ruby level”. But “at the C level”, it’s the singleton class (Object). If (Object) does not appear at the Ruby level, it’s because Object#class skips the singleton classes. Let’s look at the body of the method, rb_obj_class() to confirm that.

  86  VALUE
  87  rb_obj_class(obj)
  88      VALUE obj;
  89  {
  90      return rb_class_real(CLASS_OF(obj));
  91  }

  76  VALUE
  77  rb_class_real(cl)
  78      VALUE cl;
  79  {
  80      while (FL_TEST(cl, FL_SINGLETON) || TYPE(cl) == T_ICLASS) {
  81          cl = RCLASS(cl)->super;
  82      }
  83      return cl;
  84  }


CLASS_OF(obj) returns the basic.klass of obj. While in rb_class_real(), all singleton classes are skipped (advancing towards the superclass). In the first place, singleton class are caught between a class and its superclass, like a proxy. That’s why when a “real” class is necessary, we have to follow the superclass chain (figure 8).

I_CLASS will appear later when we will talk about include.

Singleton class and real class
Figure 8: Singleton class and real class

Singleton class and metaclass

Well, the singleton classes that were introduced in classes is also one type of class, it’s a class’s class. So it can be called metaclass.

However, you should be wary of the fact that singleton class are not metaclasses. It’s the singleton classes introduced in classes that are metaclasses. The important fact is not that they are singleton classes, but that they are the classes of classes. I was stuck on this point when I started learning Ruby. As I may not be the only one, I would like to make this clear.

Thinking about this, the rb_make_metaclass() function name is not very good. When used in classes, it does indeed create a metaclass, but not in the other cases, when using objects.

Then finally, even if you understood that some class are metaclasses, it’s not as if there was any concrete gain. I’d like you not to care too much about it.


We have nearly finished our talk about classes and metaclasses. But there is still one problem left. It’s about the 3 metaobjects Object, Module and Class. These 3 cannot be created with the common use API. To make a class, its metaclass must be built, but like we saw some time ago, the metaclass’s superclass is Class. However, as Class has not been created yet, the metaclass cannot be build. So in ruby, only these 3 classes’s creation is handled specially.

Then let’s look at the code:

Object, Module and Class creation
1243  rb_cObject = boot_defclass("Object", 0);
1244  rb_cModule = boot_defclass("Module", rb_cObject);
1245  rb_cClass =  boot_defclass("Class",  rb_cModule);
1247  metaclass = rb_make_metaclass(rb_cObject, rb_cClass);
1248  metaclass = rb_make_metaclass(rb_cModule, metaclass);
1249  metaclass = rb_make_metaclass(rb_cClass, metaclass);


First, in the first half, boot_defclass() is similar to rb_class_boot(), it just creates a class with its given superclass set. These links give us something like the left part of figure 9.

And in the three lines of the second half, (Object), (Module) and (Class) are created and set (right figure 9). (Object) and (Module)’s classes… that is themselves… is already set in rb_make_metaclass() so there is no problem. With this, the metaobjects’ bootstrap is finished.

Metaobjects creation
Figure 9: Metaobjects creation

After taking everything into account, it gives us a the final shape like figure 10.

Ruby metaobjects
Figure 10: Ruby metaobjects

Class names

In this section, we will analyse how’s formed the reciprocal conversion between class and class names, in other words constants. Concretely, we will target rb_define_class() and rb_define_class_under().

Name → class

First we’ll read rb_defined_class(). After the end of this function, the class can be found from the constant.

 183  VALUE
 184  rb_define_class(name, super)
 185      const char *name;
 186      VALUE super;
 187  {
 188      VALUE klass;
 189      ID id;
 191      id = rb_intern(name);
 192      if (rb_autoload_defined(id)) {             /* (A) autoload */
 193          rb_autoload_load(id);
 194      }
 195      if (rb_const_defined(rb_cObject, id)) {    /* (B) rb_const_defined */
 196          klass = rb_const_get(rb_cObject, id);  /* (C) rb_const_get */
 197          if (TYPE(klass) != T_CLASS) {
 198              rb_raise(rb_eTypeError, "%s is not a class", name);
 199          }                                      /* (D) rb_class_real */
 200          if (rb_class_real(RCLASS(klass)->super) != super) {
 201              rb_name_error(id, "%s is already defined", name);
 202          }
 203          return klass;
 204      }
 205      if (!super) {
 206          rb_warn("no super class for '%s', Object assumed", name);
 207      }
 208      klass = rb_define_class_id(id, super);
 209      rb_class_inherited(super, klass);
 210      st_add_direct(rb_class_tbl, id, klass);
 212      return klass;
 213  }


Many things can be understood with what’s before and after rb_define_class_id()... Before we acquire or create the class. After we set the constant. We will look at it in more detail below.

(A) In Ruby, there is an autoload function that automatically loads libraries when some constants are accessed. This is done in the rb_autoload_xxxx() function. You can ignore it without any problem.

(B) We determine whether the name constant has been defined or not in Object.

(C) Get the value of the name constant. This will be explained in detail in chapter 6.

(D) We’ve seen rb_class_real() some time ago. If the class c is a singleton class or an ICLASS, it climbs the super hierarchy up to a class that is not and returns it. In short, this function skips the virtual classes that should not appear at the Ruby level.

That’s what we can read nearby.

As around constants are involved, it is very troublesome. However, we will talk about class definition in the constants chapter so for the moment we will content ourselves with a partial description.

After rb_define_class_id, we can find the following:

st_add_direct(rb_class_tbl, id, klass);

This part assigns the class to the constant. However, whichever the way you look at it you do not see that. In fact, top-level classes are separated from the other constants and regrouped in rb_class_tbl(). The split is slightly related to the GC. It’s not essential.

Class → name

We understood how the class can be obtained from the class name, but how to do the opposite? By doing things like calling p or Class#name, we can get the name of the class, but how is it implemented?

In fact this was already done a long time ago by rb_name_class(). The call is around the following:


Let’s look at its content:

 269  void
 270  rb_name_class(klass, id)
 271      VALUE klass;
 272      ID id;
 273  {
 274      rb_iv_set(klass, "__classid__", ID2SYM(id));
 275  }


__classid__ is another instance variable that can’t be seen from Ruby. As only VALUEs can be put in the instance variable table, the ID is converted to Symbol using ID2SYM().

That’s how we are able to find the constant name from the class.

Nested classes

So, in the case of classes defined at the top-level, we know how works the reciprocal link between name and class. What’s left is the case of classes defined in modules or other classes, and for that it’s a little more complicated. The function to define these nested classes is rb_define_class_under().

 215  VALUE
 216  rb_define_class_under(outer, name, super)
 217      VALUE outer;
 218      const char *name;
 219      VALUE super;
 220  {
 221      VALUE klass;
 222      ID id;
 224      id = rb_intern(name);
 225      if (rb_const_defined_at(outer, id)) {
 226          klass = rb_const_get(outer, id);
 227          if (TYPE(klass) != T_CLASS) {
 228              rb_raise(rb_eTypeError, "%s is not a class", name);
 229          }
 230          if (rb_class_real(RCLASS(klass)->super) != super) {
 231              rb_name_error(id, "%s is already defined", name);
 232          }
 233          return klass;
 234      }
 235      if (!super) {
 236          rb_warn("no super class for '%s::%s', Object assumed",
 237                  rb_class2name(outer), name);
 238      }
 239      klass = rb_define_class_id(id, super);
 240      rb_set_class_path(klass, outer, name);
 241      rb_class_inherited(super, klass);
 242      rb_const_set(outer, id, klass);
 244      return klass;
 245  }


The structure is like the one of rb_define_class(): before the call to rb_define_class_id() is the redefinition check, after is the creation of the reciprocal link between constant and class. The first half is pretty boringly similar to rb_define_class() so we’ll skip it. In the second half, rb_set_class() is new. We’re going to look at it.


This function gives the name name to the class klass nested in the class under. “class path” means a name including all the nesting information starting from top-level, for example “Net::NetPrivate::Socket”.

 210  void
 211  rb_set_class_path(klass, under, name)
 212      VALUE klass, under;
 213      const char *name;
 214  {
 215      VALUE str;
 217      if (under == rb_cObject) {
              /* defined at top-level */
 218          str = rb_str_new2(name);    /* create a Ruby string from name */
 219      }
 220      else {
              /* nested constant */
 221          str = rb_str_dup(rb_class_path(under));  /* copy the return value */
 222          rb_str_cat2(str, "::");     /* concatenate "::" */
 223          rb_str_cat2(str, name);     /* concatenate name */
 224      }
 225      rb_iv_set(klass, "__classpath__", str);
 226  }


Everything except the last line is the construction of the class path, and the last line makes the class remember its own name. __classpath__ is of course another instance variable that can’t be seen from a Ruby program. In rb_name_class() there was __classid__, but id is different because it does not include nesting information (look at the table below).

__classpath__    Net::NetPrivate::Socket
__classid__                       Socket

It means classes defined for example in rb_defined_class() all have __classid__ or __classpath__ defined. So to find under’s classpath we can look up in these instance variables. This is done by rb_class_path(). We’ll omit its content.

Nameless classes

Contrary to what I have just said, there are in fact cases in which neither __classpath__ nor __classid__ are set. That is because in Ruby you can use a method like the following to create a class.

c =

If you create a class like this, we won’t go through rb_define_class_id() and the classpath won’t be set. In this case, c does not have any name, which is to say we get an unnamed class.

However, if later it’s assigned into a constant, the name of this constant will be attached to the class.

SomeClass = c   # the class name is SomeClass

Strictly speaking, the name is attached after the assignment, the first time it is requested. For instance, when calling p on this SomeClass class or when calling the Class#name method. When doing this, a value equal to the class is searched in rb_class_tbl, and a name has to be chosen. The following case can also happen:

class A
  class B
    C = tmp =
    p(tmp)   # here we search for the name

so in the worst case we have to search for the whole constant space. However, generally, there aren’t many constants so searching all constants does not take too much time.


We only talked about classes so let’s finish this chapter with something else and talk about module inclusion.

rb_include_module (1)

Includes are done by the ordinary method Module#include. Its corresponding function in C is rb_include_module(). In fact, to be precise, its body is rb_mod_include(), and there Module#append_feature is called, and this function’s default implementation finally calls rb_include_module(). Mixing what’s happening in Ruby and C gives us the following call graph.

Module#include (rb_mod_include)
    Module#append_features (rb_mod_append_features)

All usual includes are done by rb_include_module(). This function is a little long so we’ll look at it a half at a time.

rb_include_module (first half)
      /* include module in class */
 347  void
 348  rb_include_module(klass, module)
 349      VALUE klass, module;
 350  {
 351      VALUE p, c;
 352      int changed = 0;
 354      rb_frozen_class_p(klass);
 355      if (!OBJ_TAINTED(klass)) {
 356          rb_secure(4);
 357      }
 359      if (NIL_P(module)) return;
 360      if (klass == module) return;
 362      switch (TYPE(module)) {
 363        case T_MODULE:
 364        case T_CLASS:
 365        case T_ICLASS:
 366          break;
 367        default:
 368          Check_Type(module, T_MODULE);
 369      }


For the moment it’s only security and type checking, therefore we can ignore it. The process itself is below:

rb_include_module (second half)
 371      OBJ_INFECT(klass, module);
 372      c = klass;
 373      while (module) {
 374          int superclass_seen = Qfalse;
 376          if (RCLASS(klass)->m_tbl == RCLASS(module)->m_tbl)
 377              rb_raise(rb_eArgError, "cyclic include detected");
 378          /* (A) skip if the superclass already includes module */
 379          for (p = RCLASS(klass)->super; p; p = RCLASS(p)->super) {
 380              switch (BUILTIN_TYPE(p)) {
 381                case T_ICLASS:
 382                  if (RCLASS(p)->m_tbl == RCLASS(module)->m_tbl) {
 383                      if (!superclass_seen) {
 384                          c = p;  /* move the insertion point */
 385                      }
 386                      goto skip;
 387                  }
 388                  break;
 389                case T_CLASS:
 390                  superclass_seen = Qtrue;
 391                  break;
 392              }
 393          }
 394          c = RCLASS(c)->super =
                          include_class_new(module, RCLASS(c)->super);
 395          changed = 1;
 396        skip:
 397          module = RCLASS(module)->super;
 398      }
 399      if (changed) rb_clear_cache();
 400  }


First, what the (A) block does is written in the comment. It seems to be a special condition so let’s first skip reading it for now. By extracting the important parts from the rest we get the following:

c = klass;
while (module) {
    c = RCLASS(c)->super = include_class_new(module, RCLASS(c)->super);
    module = RCLASS(module)->super;

In other words, it’s a repetition of module’s super. What is in module’s super must be a module included by module (because our intuition tells us so). Then the superclass of the class where the inclusion occurs is replaced with something. We do not understand much what, but at the moment I saw that I felt “Ah, doesn’t this look the addition of elements to a list (like LISP’s cons)?” and it suddenly make the story faster. In other words it’s the following form:

list = new(item, list)

Thinking about this, it seems we can expect that module is inserted between c and c->super. If it’s like this, it fits module’s specification.

But to be sure of this we have to look at include_class_new().


 319  static VALUE
 320  include_class_new(module, super)
 321      VALUE module, super;
 322  {
 323      NEWOBJ(klass, struct RClass);               /* (A) */
 324      OBJSETUP(klass, rb_cClass, T_ICLASS);
 326      if (BUILTIN_TYPE(module) == T_ICLASS) {
 327          module = RBASIC(module)->klass;
 328      }
 329      if (!RCLASS(module)->iv_tbl) {
 330          RCLASS(module)->iv_tbl = st_init_numtable();
 331      }
 332      klass->iv_tbl = RCLASS(module)->iv_tbl;     /* (B) */
 333      klass->m_tbl = RCLASS(module)->m_tbl;
 334      klass->super = super;                       /* (C) */
 335      if (TYPE(module) == T_ICLASS) {             /* (D) */
 336          RBASIC(klass)->klass = RBASIC(module)->klass;   /* (D-1) */
 337      }
 338      else {
 339          RBASIC(klass)->klass = module;                  /* (D-2) */
 340      }
 341      OBJ_INFECT(klass, module);
 342      OBJ_INFECT(klass, super);
 344      return (VALUE)klass;
 345  }


We’re lucky there’s nothing we do not know.

(A) First create a new class.

(B) Transplant module’s instance variable and method tables into this class.

(C) Make the including class’s superclass (super) the super class of this new class.

In other words, this function creates an include class for the module. The important point is that at (B) only the pointer is moved on, without duplicating the table. Later, if a method is added, the module’s body and the include class will still have exactly the same methods (figure 11).

Include class
Figure 11: Include class

If you look closely at (A), the structure type flag is set to T_ICLASS. This seems to be the mark of an include class. This function’s name is include_class_new() so ICLASS’s I must be include.

And if you think about joining what this function and rb_include_module() do, we know that our previous expectations were not wrong. In brief, including is inserting the include class of a module between a class and its superclass (figure 12).

Figure 12: Include

At (D-2) the module is stored in the include class’s klass. At (D-1), the module’s body is taken out… at least that’s what I’d like to say, but in fact this check does not have any use. The T_ICLASS check is already done at the beginning of this function, so when arriving here there can’t still be a T_ICLASS. Modification to ruby piled up at a fast pace during quite a long period of time so there are quite a few small overlooks.

There is one more thing to consider. Somehow the include class’s basic.klass is only used to point to the module’s body, so for example calling a method on the include class would be very bad. So include classes must not be seen from Ruby programs. And in practice all methods skip include classes, with no exception.


It was complicated so let’s look at a concrete example. I’d like you to look at figure 13 (1). We have the c1 class and the m1 module that includes m2. From there, the changes made to include m1 in c1 are (2) and (3). ims are of course include classes.

Figure 13: Include

rb_include_module (2)

Well, now we can explain the part of rb_include_module() we skipped.

rb_include_module (avoiding double inclusion)
 378  /* (A) skip if the superclass already includes module */
 379  for (p = RCLASS(klass)->super; p; p = RCLASS(p)->super) {
 380      switch (BUILTIN_TYPE(p)) {
 381        case T_ICLASS:
 382          if (RCLASS(p)->m_tbl == RCLASS(module)->m_tbl) {
 383              if (!superclass_seen) {
 384                  c = p;  /* the inserting point is moved */
 385              }
 386              goto skip;
 387          }
 388          break;
 389        case T_CLASS:
 390          superclass_seen = Qtrue;
 391          break;
 392      }
 393  }


If one of the T_ICLASSes (include classes) that are in klass’s superclasses (p) has the same table as one of the modules we want to include (module), it’s an include class for module. That’s why we skip the inclusion to not include the module twice. If this module includes an other module (module->super), we check this once more.

However, when we skip an inclusion, p is a module that has been included once, so its included modules must already be included… that’s what I thought for a moment, but we can have the following context:

module M
module M2
class C
  include M   # M2 is not yet included in M
end           # therefore M2 is not in C's superclasses

module M
  include M2  # as there M2 is included in M,
class C
  include M   # I would like here to only add M2

So on the contrary, there are cases for which include does not have real-time repercussions.

For class inheritance, the class’s singleton methods were inherited but in the case of module there is no such thing. Therefore the singleton methods of the module are not inherited by the including class (or module). When you want to also inherit singleton methods, the usual way is to override Module#append_features.

The original work is Copyright © 2002 - 2004 Minero AOKI.
Translated by Vincent ISAMBART
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This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike2.5 License.