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    <p>Looking at MRI code, every time you create a <code>Class</code> which in Ruby is object of type <code>Class</code>, automatically, ruby creates "metaclass" class for that new class, which is another <code>Class</code> object of singleton type.</p> <p>The C function calls (<code>class.c</code>) are: </p> <pre><code>rb_define_class rb_define_class_id rb_class_new(super); rb_make_metaclass(klass, RBASIC(super)-&gt;klass); </code></pre> <p>So, every time that you define a new class, Ruby will define another class with meta information.</p> <p>When you define a class method, I mean, <code>def self.method</code>, internally, ruby calls <code>rb_define_singleton_method</code>. You can check it doing the follow step:</p> <p>Create a ruby file <code>test.rb</code>:</p> <pre><code>class A def self.foo end end </code></pre> <p>And run the following command:</p> <pre><code>ruby --dump insns test.rb </code></pre> <p>You will have the following output:</p> <pre><code>== disasm: &lt;RubyVM::InstructionSequence:&lt;main&gt;@kcount.rb&gt;=============== 0000 trace 1 ( 70) 0002 putspecialobject 3 0004 putnil 0005 defineclass :A, &lt;class:A&gt;, 0 0009 leave == disasm: &lt;RubyVM::InstructionSequence:&lt;class:A&gt;@kcount.rb&gt;============ 0000 trace 2 ( 70) 0002 trace 1 ( 71) 0004 putspecialobject 1 0006 putself 0007 putobject :foo 0009 putiseq foo 0011 opt_send_simple &lt;callinfo!mid:core#define_singleton_method, argc:3, ARGS_SKIP&gt; 0013 trace 4 ( 73) 0015 leave ( 71) == disasm: &lt;RubyVM::InstructionSequence:foo@kcount.rb&gt;================== 0000 trace 8 ( 71) 0002 putnil 0003 trace 16 ( 72) 0005 leave </code></pre> <p>The <code>define_singleton_method</code> is mapped to the <code>rb_obj_define_method</code> C function (<code>object.c</code>), which do following calls:</p> <pre><code> rb_obj_define_method rb_singleton_class(obj) rb_mod_define_method </code></pre> <p>The function <code>rb_singleton_class</code> exposes the metaclass created when the class was defined, but it also creates a new metaclass for this metaclass. </p> <p>According the Ruby documentation for this function: "if a obj is a class, the returned singleton class also has its own singleton class in order to keep consistency of the inheritance structure of metaclasses".</p> <p>That is the reason why the number of class increases by 1 when you define a class method.</p> <p>The same effect happens if you change your code by:</p> <pre><code>class A end A.singleton_class </code></pre> <p>The <code>singleton_class</code> is mapped to <code>rb_obj_singleton_class</code> C function, which calls the <code>rb_singleton_class</code>.</p> <p>Even if you create a class method and call the <code>singleton_class</code> method, the number of created classes will not change, because all classes necessary to handle meta information is already created. Example:</p> <pre><code>class A def self.foo nil end end A.singleton_class </code></pre> <p>The code above will keep returning 3.</p>
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    1. PORuby metaclasses: why three when defined singleton methods?
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    1. PORuby metaclasses: why three when defined singleton methods?
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    1. COwow. All I thought about ruby's object model is wrong. What is the difference between the metaclass and the singleton class ? I always thought it was the same thing.
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    2. COThank you, Tlewin! But in the literature, always write that metaclass and sinlgeton class (and eigen class) is the same. It will be necessary to open source rb_define_singleton_method C function.
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    3. CO@m_x I updated the post content to elucidate this question of metaclass and singleton class.
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