Ruby Hacking Guide

Translated by Vincent ISAMBART

Chapter 4: Classes and modules

In this chapter, we’ll see the details of the data structures created by 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:

rb_define_class()
rb_define_class_under()
rb_define_module()
rb_define_module_under()
rb_define_method()
rb_define_singleton_method()

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

VALUE rb_cArray;

void
Init_Array()
{
    rb_cArray  = rb_define_class("Array", rb_cObject);

(array.c)

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() defines a class called Array, which 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

VALUE rb_cFile;
static VALUE rb_cStat;

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

(file.c)

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");

(enum.c)

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);

(array.c)

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()
  end
end

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);

(array.c)

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()
  end
end

Singleton methods definition

We can define methods that are specific to a single object instance. 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 File.link instead.

▼ Definition of File.link

2624  rb_define_singleton_method(rb_cFile, "link", rb_file_s_link, 2);

(file.c)

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:

▼ Init_Array

void
Init_Array()
{
    rb_cArray  = rb_define_class("Array", rb_cObject);
    rb_include_module(rb_cArray, rb_mEnumerable);
1814
    rb_define_singleton_method(rb_cArray, "allocate",
                                     rb_ary_s_alloc, 0);
    rb_define_singleton_method(rb_cArray, "[]", rb_ary_s_create, -1);
    rb_define_method(rb_cArray, "initialize", rb_ary_initialize, -1);
    rb_define_method(rb_cArray, "to_s", rb_ary_to_s, 0);
    rb_define_method(rb_cArray, "inspect", rb_ary_inspect, 0);
    rb_define_method(rb_cArray, "to_a", rb_ary_to_a, 0);
    rb_define_method(rb_cArray, "to_ary", rb_ary_to_a, 0);
    rb_define_method(rb_cArray, "frozen?",  rb_ary_frozen_p, 0);

(array.c)

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

▼ rb_call_inits()

void
rb_call_inits()
{
    Init_sym();
    Init_var_tables();
    Init_Object();
    Init_Comparable();
    Init_Enumerable();
    Init_Precision();
    Init_eval();
    Init_String();
    Init_Exception();
    Init_Thread();
    Init_Numeric();
    Init_Bignum();
    Init_Array();

(inits.c)

This way, Init_Array() is called properly.

That explains it for the 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 myextension.so, 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 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)

void
Init_stringio()
{
    VALUE StringIO = rb_define_class("StringIO", rb_cData);
    rb_define_singleton_method(StringIO, "allocate",
                                     strio_s_allocate, 0);
    rb_define_singleton_method(StringIO, "open", strio_s_open, -1);
    rb_define_method(StringIO, "initialize", strio_initialize, -1);
    rb_enable_super(StringIO, "initialize");
    rb_define_method(StringIO, "become", strio_become, 1);
    rb_define_method(StringIO, "reopen", strio_reopen, -1);

(ext/stringio/stringio.c)

Singleton classes

`rb_define_singleton_method()`

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.

▼ rb_define_singleton_method()

void
rb_define_singleton_method(obj, name, func, argc)
    VALUE obj;
    const char *name;
    VALUE (*func)();
    int argc;
{
    rb_define_method(rb_singleton_class(obj), name, func, argc);
}

(class.c)

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.

`rb_singleton_class()`

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

rb_define_singleton_method
    rb_define_method
    rb_singleton_class
        SPECIAL_SINGLETON
        rb_make_metaclass
            rb_class_boot
            rb_singleton_class_attached

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(). In order to obtain call graphs, you can use cflow and such. {cflow: see also doc/callgraph.html in the attached CD-ROM}

In this book, because I wanted to obtain call graphs that contain only functions, I created a ruby-specific tool by myself. Perhaps it can be generalized by modifying its code analyzing part, thus I’d like to somehow make it until around the publication of this book. These situations are also explained in doc/callgraph.html of the attached CD-ROM.

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 I was doing. In such situation you must bring a call graph to keep aware of where it is when reading. This time, as an example, we’ll decode the procedures below rb_singleton_class() in parallel. We should look out for the following two points:

What exactly are singleton classes?
What is the purpose of singleton classes?

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().

rb_define_class
    rb_class_inherited
    rb_define_class_id
        rb_class_new
            rb_class_boot
        rb_make_metaclass
            rb_class_boot
            rb_singleton_class_attached

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

▼ rb_class_new()

VALUE
rb_class_new(super)
    VALUE super;
{
    Check_Type(super, T_CLASS);
    if (super == rb_cClass) {
        rb_raise(rb_eTypeError, "can't make subclass of Class");
    }
    if (FL_TEST(super, FL_SINGLETON)) {
        rb_raise(rb_eTypeError, "can't make subclass of virtual class");
    }
    return rb_class_boot(super);
}

(class.c)

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.

▼ rb_class_boot()

VALUE
rb_class_boot(super)
    VALUE super;
{
    NEWOBJ(klass, struct RClass);        /* allocates struct RClass */
    OBJSETUP(klass, rb_cClass, T_CLASS); /* initialization of the RBasic part */
  27
    klass->super = super;       /* (A) */
    klass->iv_tbl = 0;
    klass->m_tbl = 0;
    klass->m_tbl = st_init_numtable();
  32
    OBJ_INFECT(klass, super);
    return (VALUE)klass;
}

(class.c)

NEWOBJ() and OBJSETUP() are fixed expressions used when creating Ruby objects that possess one of the built-in 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, and rb_class_new() is almost identical.

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

rb_singleton_class
    SPECIAL_SINGLETON
    rb_make_metaclass
        rb_class_boot
        rb_singleton_class_attached

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 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.

▼ rb_singleton_class()

#define SPECIAL_SINGLETON(x,c) do {\
    if (obj == (x)) {\
        return c;\
    }\
} while (0)

VALUE
rb_singleton_class(obj)
    VALUE obj;
{
    VALUE klass;
 689
    if (FIXNUM_P(obj) || SYMBOL_P(obj)) {
        rb_raise(rb_eTypeError, "can't define singleton");
    }
    if (rb_special_const_p(obj)) {
        SPECIAL_SINGLETON(Qnil, rb_cNilClass);
        SPECIAL_SINGLETON(Qfalse, rb_cFalseClass);
        SPECIAL_SINGLETON(Qtrue, rb_cTrueClass);
        rb_bug("unknown immediate %ld", obj);
    }
 699
    DEFER_INTS;
    if (FL_TEST(RBASIC(obj)->klass, FL_SINGLETON) &&
        (BUILTIN_TYPE(obj) == T_CLASS ||
         rb_iv_get(RBASIC(obj)->klass, "__attached__") == obj)) {
        klass = RBASIC(obj)->klass;
    }
    else {
        klass = rb_make_metaclass(obj, RBASIC(obj)->klass);
    }
    if (OBJ_TAINTED(obj)) {
        OBJ_TAINT(klass);
    }
    else {
        FL_UNSET(klass, FL_TAINT);
    }
    if (OBJ_FROZEN(obj)) OBJ_FREEZE(klass);
    ALLOW_INTS;
 717
    return klass;
}

(class.c)

The first and the second half are separated by a blank line. The first half handles special cases 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, it means their object structs do not exist. First, Fixnum and Symbol are explicitly 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 it would be reported as a bug 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()`

▼ rb_make_metaclass()

VALUE
rb_make_metaclass(obj, super)
    VALUE obj, super;
{
    VALUE klass = rb_class_boot(super);
    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 =
                          RBASIC(rb_class_real(RCLASS(obj)->super))->klass;
        }
    }
 156
    return klass;
}

(class.c)

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 the indication of a singleton class.

What are singleton classes?

Finishing the above process, furthermore, we’ll through away the declarations because parameters, return values and local variables are all VALUE. That makes us able to compress to the following:

▼ rb_singleton_class() rb_make_metaclass() (after compression)

rb_singleton_class(obj)
{
    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 =
                    RBASIC(rb_class_real(RCLASS(obj)->super))->klass;
        }
    }

    return klass;
}

The condition of the if statement of rb_singleton_class() seems quite complicated. However, this condition is not connected to rb_make_metaclass(), which is the mainstream, 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 we assume that obj is a class, so we’ll remove it.

With these simplifications, we get the following:

▼ rb_singleton_class() rb_make_metaclass() (after recompression)

rb_singleton_class(obj)
{
    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)

rb_singleton_class(obj)
{
    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).

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. By defining methods there, we can define methods which have completely nothing to do with other instances of klass.

Singleton classes and instances

By the way, did you notice about, 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.

▼ rb_singleton_class_attached()

void
rb_singleton_class_attached(klass, obj)
    VALUE klass, obj;
{
    if (FL_TEST(klass, FL_SINGLETON)) {
        if (!RCLASS(klass)->iv_tbl) {
            RCLASS(klass)->iv_tbl = st_init_numtable();
        }
        st_insert(RCLASS(klass)->iv_tbl,
                        rb_intern("__attached__"), obj);
    }
}

(class.c)

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, they would be the same as normal classes. Hence, each singleton class has only one instance … or rather, it must be limited to one.

Summary

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.

Metaclasses

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).

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 in Ruby so 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.

((errata:
This structure is implemented efficiently in recent Ruby 1.8, thus it can be implemented efficiently. ))

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 objects!” also appears. But these expressions often go too far. When thinking about these sayings, we have to split them in two:

all data are objects
classes are data

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. In Ruby, which is a object oriented language and furthermore has classes, it is equivalent to be able to directly manipulate classes.

Nevertheless, there’s also a way in which classes are not objects. For example, there’s no problem in providing a feature to manipulate classes as 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 also a reason not linked to reflection why in Ruby classes had to be made objects. That is to prepare the place to define methods which are independent from instances (what are called static methods in Java and 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.

Class methods inheritance

In Ruby, singleton methods defined in a class are called class methods. However, their specification is a little strange. For some reasons, class methods are inheritable.

class A
  def A.test    # defines a singleton method in A
    puts("ok")
  end
end

class B < A
end

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? It must be done either at class definition (creation) or at singleton method definition. Then let’s first look at the code defining classes.

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

rb_define_class
    rb_class_inherited
    rb_define_class_id
        rb_class_new
            rb_class_boot
        rb_make_metaclass
            rb_class_boot
            rb_singleton_class_attached

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, somehow rb_make_metaclass() appeared. 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.

`rb_define_class_id()`

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

▼ rb_define_class_id()

VALUE
rb_define_class_id(id, super)
    ID id;
    VALUE super;
{
    VALUE klass;
 166
    if (!super) super = rb_cObject;
    klass = rb_class_new(super);
    rb_name_class(klass, id);
    rb_make_metaclass(klass, RBASIC(super)->klass);
 171
    return klass;
}

(class.c)

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 not 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 =
                    RBASIC(rb_class_real(RCLASS(obj)->super))->klass;
        }
    }

    return klass;
}

Last time, the if statement was wholly 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 the actual 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.

figure 4: Introduction of a class's singleton class

By expanding our imagination further from this result, we can think that the superclass’s class (the c in figure 4) must again be a singleton class. You’ll understand with one more inheritance level (figure 5).

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
end
class B < A
end

internally, a structure like figure 6 is created.

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? For this, we can check it by using debuggers. I’ve made figure 7 from the results of this investigation.

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 by using class() method?

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.

▼ rb_obj_class()

VALUE
rb_obj_class(obj)
    VALUE obj;
{
    return rb_class_real(CLASS_OF(obj));
}

VALUE
rb_class_real(cl)
    VALUE cl;
{
    while (FL_TEST(cl, FL_SINGLETON) || TYPE(cl) == T_ICLASS) {
        cl = RCLASS(cl)->super;
    }
    return cl;
}

(object.c)

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.

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 being a singleton class does not mean being a metaclass. The singleton classes introduced in classes 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 for a class, it does indeed create a metaclass, but when used for other objects, the created class is not a metaclass.

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

Bootstrap

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

rb_cObject = boot_defclass("Object", 0);
rb_cModule = boot_defclass("Module", rb_cObject);
rb_cClass =  boot_defclass("Class",  rb_cModule);
1246
metaclass = rb_make_metaclass(rb_cObject, rb_cClass);
metaclass = rb_make_metaclass(rb_cModule, metaclass);
metaclass = rb_make_metaclass(rb_cClass, metaclass);

(object.c)

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.

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

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.

▼ rb_define_class()

VALUE
rb_define_class(name, super)
    const char *name;
    VALUE super;
{
    VALUE klass;
    ID id;
 190
    id = rb_intern(name);
    if (rb_autoload_defined(id)) {             /* (A) autoload */
        rb_autoload_load(id);
    }
    if (rb_const_defined(rb_cObject, id)) {    /* (B) rb_const_defined */
        klass = rb_const_get(rb_cObject, id);  /* (C) rb_const_get */
        if (TYPE(klass) != T_CLASS) {
            rb_raise(rb_eTypeError, "%s is not a class", name);
        }                                      /* (D) rb_class_real */
        if (rb_class_real(RCLASS(klass)->super) != super) {
            rb_name_error(id, "%s is already defined", name);
        }
        return klass;
    }
    if (!super) {
        rb_warn("no super class for '%s', Object assumed", name);
    }
    klass = rb_define_class_id(id, super);
    rb_class_inherited(super, klass);
    st_add_direct(rb_class_tbl, id, klass);
 211
    return klass;
}

(class.c)

This can be clearly divided into the two parts: before and after rb_define_class_id(). The former is to acquire or create the class. The latter is to assign it to the constant. We will look at it in more detail below.

(A) In Ruby, there is a feature named autoload that automatically loads libraries when certain constants are accessed. These functions named rb_autoload_xxxx() are for its checks. 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 constants are involved around this, it is very troublesome. But I feel like the chapter about constants is probably not so right place to talk about class definition, that’s the reason of such halfway description around here.

Moreover, about this coming after rb_define_class_id(),

st_add_direct(rb_class_tbl, id, klass);

This part assigns the class to the constant. However, whichever way you look at it you do not see that. In fact, top-level classes and modules that are defined in C 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 is done by rb_name_class() which already appeared a long time ago. The call is around the following:

rb_define_class
    rb_define_class_id
        rb_name_class

Let’s look at its content:

▼ rb_name_class()

void
rb_name_class(klass, id)
    VALUE klass;
    ID id;
{
    rb_iv_set(klass, "__classid__", ID2SYM(id));
}

(variable.c)

__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().

▼ rb_define_class_under()

VALUE
rb_define_class_under(outer, name, super)
    VALUE outer;
    const char *name;
    VALUE super;
{
    VALUE klass;
    ID id;
 223
    id = rb_intern(name);
    if (rb_const_defined_at(outer, id)) {
        klass = rb_const_get(outer, id);
        if (TYPE(klass) != T_CLASS) {
            rb_raise(rb_eTypeError, "%s is not a class", name);
        }
        if (rb_class_real(RCLASS(klass)->super) != super) {
            rb_name_error(id, "%s is already defined", name);
        }
        return klass;
    }
    if (!super) {
        rb_warn("no super class for '%s::%s', Object assumed",
                rb_class2name(outer), name);
    }
    klass = rb_define_class_id(id, super);
    rb_set_class_path(klass, outer, name);
    rb_class_inherited(super, klass);
    rb_const_set(outer, id, klass);
 243
    return klass;
}

(class.c)

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_path() is new. We’re going to look at it.

`rb_set_class_path()`

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

▼ rb_set_class_path()

void
rb_set_class_path(klass, under, name)
    VALUE klass, under;
    const char *name;
{
    VALUE str;
 216
    if (under == rb_cObject) {
              /* defined at top-level */
        str = rb_str_new2(name);    /* create a Ruby string from name */
    }
    else {
              /* nested constant */
        str = rb_str_dup(rb_class_path(under));  /* copy the return value */
        rb_str_cat2(str, "::");     /* concatenate "::" */
        rb_str_cat2(str, name);     /* concatenate name */
    }
    rb_iv_set(klass, "__classpath__", str);
}

(variable.c)

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 = Class.new()

If a class is created like this, it 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 to a constant, a name will be attached to the class at that moment.

SomeClass = c   # the class name is SomeClass

Strictly speaking, at the first time requesting the name after assigning it to a constant, the name will be attached to the class. 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 = Class.new()
    p(tmp)   # here we search for the name
  end
end

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

Include

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)
        rb_include_module

Anyway, the manipulations that are usually regarded as inclusions 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 */
void
rb_include_module(klass, module)
    VALUE klass, module;
{
    VALUE p, c;
    int changed = 0;
 353
    rb_frozen_class_p(klass);
    if (!OBJ_TAINTED(klass)) {
        rb_secure(4);
    }
 358
    if (NIL_P(module)) return;
    if (klass == module) return;
 361
    switch (TYPE(module)) {
      case T_MODULE:
      case T_CLASS:
      case T_ICLASS:
        break;
      default:
        Check_Type(module, T_MODULE);
    }

(class.c)

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)

    OBJ_INFECT(klass, module);
    c = klass;
    while (module) {
        int superclass_seen = Qfalse;
 375
        if (RCLASS(klass)->m_tbl == RCLASS(module)->m_tbl)
            rb_raise(rb_eArgError, "cyclic include detected");
        /* (A) skip if the superclass already includes module */
        for (p = RCLASS(klass)->super; p; p = RCLASS(p)->super) {
            switch (BUILTIN_TYPE(p)) {
              case T_ICLASS:
                if (RCLASS(p)->m_tbl == RCLASS(module)->m_tbl) {
                    if (!superclass_seen) {
                        c = p;  /* move the insertion point */
                    }
                    goto skip;
                }
                break;
              case T_CLASS:
                superclass_seen = Qtrue;
                break;
            }
        }
        c = RCLASS(c)->super =
                          include_class_new(module, RCLASS(c)->super);
        changed = 1;
      skip:
        module = RCLASS(module)->super;
    }
    if (changed) rb_clear_cache();
}

(class.c)

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().

`include_class_new()`

▼ include_class_new()

static VALUE
include_class_new(module, super)
    VALUE module, super;
{
    NEWOBJ(klass, struct RClass);               /* (A) */
    OBJSETUP(klass, rb_cClass, T_ICLASS);
 325
    if (BUILTIN_TYPE(module) == T_ICLASS) {
        module = RBASIC(module)->klass;
    }
    if (!RCLASS(module)->iv_tbl) {
        RCLASS(module)->iv_tbl = st_init_numtable();
    }
    klass->iv_tbl = RCLASS(module)->iv_tbl;     /* (B) */
    klass->m_tbl = RCLASS(module)->m_tbl;
    klass->super = super;                       /* (C) */
    if (TYPE(module) == T_ICLASS) {             /* (D) */
        RBASIC(klass)->klass = RBASIC(module)->klass;   /* (D-1) */
    }
    else {
        RBASIC(klass)->klass = module;                  /* (D-2) */
    }
    OBJ_INFECT(klass, module);
    OBJ_INFECT(klass, super);
 343
    return (VALUE)klass;
}

(class.c)

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, it looks like this function creates an include class which we can regard it as something like an “avatar” of 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).

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).

At (D-2) the module is stored in the include class’s klass. At (D-1), the module’s body is taken out… I’d like to say so if possible, 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 piece by piece 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.

Simulation

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.

`rb_include_module` (2)

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

▼ rb_include_module (avoiding double inclusion)

/* (A) skip if the superclass already includes module */
for (p = RCLASS(klass)->super; p; p = RCLASS(p)->super) {
    switch (BUILTIN_TYPE(p)) {
      case T_ICLASS:
        if (RCLASS(p)->m_tbl == RCLASS(module)->m_tbl) {
            if (!superclass_seen) {
                c = p;  /* the inserting point is moved */
            }
            goto skip;
        }
        break;
      case T_CLASS:
        superclass_seen = Qtrue;
        break;
    }
}

(class.c)

Among the superclasses of the +klass+ (p), if a p is T_ICLASS (an include class) and has the same method table as the one of the module we want to include (module), it means that the p is an include class of the module. Therefore, it would be skipped to not include the module twice. However, if this module includes another module (module->super), It would be checked once more.

But, because p is a module that has been included once, the modules included by it must also already be included… that’s what I thought for a moment, but we can have the following context:

module M
end
module M2
end
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,
end
class C
  include M   # I would like here to only add M2
end

To say this conversely, there are cases that a result of include is not propagated soon.

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.