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    <p>It's a top-level constant. You can access it through another module, but it doesn't actually live there. <code>Class::RbConfig</code>, <code>Fixnum::RbConfig</code>, etc, all work, but the constant itself only lives at the top level.</p> <p>You can find it in <code>Object.constants</code>. <code>Object::RbConfig</code> is the only fully-qualified way of referencing it that doesn't result in a 'toplevel constant referenced by Module::' warning.</p> <p><strong>EDIT:</strong> Normally, inherited constants <em>do</em> show up in the value of <code>.constants</code>, but the ones from <code>Object</code> seem to be excluded:</p> <pre><code>irb(main):001:0&gt; Object.constants =&gt; [:Object, :Module, :Class, :BasicObject, :Kernel, :NilClass, :NIL, :Data, :TrueClass, :TRUE, :FalseClass, :FALSE, :Encoding, :Comparable, :Enumerable, :String, :Symbol, :Exception, :SystemExit, :SignalException, :Interrupt, :StandardError, :TypeError, :ArgumentError, :IndexError, :KeyError, :RangeError, :ScriptError, :SyntaxError, :LoadError, :NotImplementedError, :NameError, :NoMethodError, :RuntimeError, :SecurityError, :NoMemoryError, :EncodingError, :SystemCallError, :Errno, :ZeroDivisionError, :FloatDomainError, :Numeric, :Integer, :Fixnum, :Float, :Bignum, :Array, :Hash, :ENV, :Struct, :RegexpError, :Regexp, :MatchData, :Marshal, :Range, :IOError, :EOFError, :IO, :STDIN, :STDOUT, :STDERR, :ARGF, :FileTest, :File, :Dir, :Time, :Random, :Signal, :Process, :Proc, :LocalJumpError, :SystemStackError, :Method, :UnboundMethod, :Binding, :Math, :GC, :ObjectSpace, :Enumerator, :StopIteration, :RubyVM, :Thread, :TOPLEVEL_BINDING, :ThreadGroup, :Mutex, :ThreadError, :Fiber, :FiberError, :Rational, :Complex, :RUBY_VERSION, :RUBY_RELEASE_DATE, :RUBY_PLATFORM, :RUBY_PATCHLEVEL, :RUBY_REVISION, :RUBY_DESCRIPTION, :RUBY_COPYRIGHT, :RUBY_ENGINE, :ARGV, :Gem, :RbConfig, :Config, :CROSS_COMPILING, :Exception2MessageMapper, :IRB, :RubyToken, :RubyLex, :Readline, :Date] irb(main):002:0&gt; class Parent &lt; Object; Answer = 42; end =&gt; 42 irb(main):003:0&gt; class Child &lt; Parent; end =&gt; nil irb(main):004:0&gt; Child.constants =&gt; [:Answer] </code></pre> <p>You may wish to read <a href="http://cirw.in/blog/constant-lookup.html" rel="nofollow">this article</a> about constant lookups in Ruby. This passage seems relevant:</p> <blockquote> <p>Ruby assumes that you will mix modules into something that inherits from Object. So if the currently open module is a module, it will also add Object.ancestors to the lookup chain so that top-level constants work as expected.</p> </blockquote>
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    1. POWhere does Config comes from?
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    1. USMark Reed
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    1. POWhere does Config comes from?
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    1. COThanks for the answer. However how gets the inheritance applied here ? Why I can access such constant at all with a subclass (NilClass as a subclass of Object in this case), but cann't get its symbol by sending `constants` method or other equivalent ?
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    2. COIt's not inheritance, it's just a namespacing shortcut; Ruby allows you to access top-level names as if they were part of any other namespace. Otherwise, any time you were coding inside a class or module definition you'd have to qualify all your top-level class names - `::File.exists?` and so on.
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    3. COThe unqualified access to outer modules namespace seems to be a bit confusing. Also thanks for the link, I'm reading it now. If it would satisfy my questions I'm going to mark your answer as accepted.
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