# Instance methods can't call protected class methods? **URL:** \ **Category:** Ruby\ **Created:** [June 6, 2006, 8:31pm UTC](https://www.ruby-forum.com/t/instance-methods-cant-call-protected-class-methods/62632 "2006-06-06T20:31:43Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![seanhussey](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/a88e57/32.png) [@seanhussey](https://www.ruby-forum.com/u/seanhussey)\ **Post date:** [June 6, 2006, 8:31pm UTC](https://www.ruby-forum.com/t/instance-methods-cant-call-protected-class-methods/62632/1 "2006-06-06T20:31:43Z")
I am trying to understand this behavior. From page 356 of the Pickaxe (2nd ed): Protected. Can be invoked only by objects of the defining class and its subclasses. If I have class Foo … end f = Foo.new Is f not an “object of the defining class”? If so, why should an instance method of Foo not be able to call a protected class method? I’m not looking for an answer of the form “because in class scope, self.class.type.object.class # =\> Object, however in _instance_ scope” etc. Logically, why should it be this way? In C++ it makes sense that a static class method cannot call an non-static method, since static methods exist even when there are no objects of the class in existence. However, they do not limit you on the reverse and prevent a non-static method from calling a static method. ---
**Author:** ![seanhussey](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/a88e57/32.png) [@seanhussey](https://www.ruby-forum.com/u/seanhussey)\ **Post date:** [June 6, 2006, 9:51pm UTC](https://www.ruby-forum.com/t/instance-methods-cant-call-protected-class-methods/62632/2 "2006-06-06T21:51:36Z")
where’s the problem? class Test protected def self.call\_me puts “static and protected” end public def invoke\_me puts “try to call” Test.call\_me # this will _not_ work: self.call\_me puts “done” end end t = Test.new t.invoke\_me > try to call > static and protected > done Sean Hermany schrieb: ---
**Author:** ![seanhussey](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/a88e57/32.png) [@seanhussey](https://www.ruby-forum.com/u/seanhussey)\ **Post date:** [June 6, 2006, 10:06pm UTC](https://www.ruby-forum.com/t/instance-methods-cant-call-protected-class-methods/62632/3 "2006-06-06T22:06:42Z")
The problem is that self.call\_me is not protected. class Test protected def self.call\_me puts “static and protected” end public def invoke\_me puts “try to call” Test.call\_me # this will _not_ work: self.call\_me puts “done” end end t = Test.new print “Output of p Test.protected\_methods.sort:\n\n” p Test.protected\_methods.sort print “\nOutput of p Test.public\_methods.sort:\n\n” p Test.public\_methods.sort print “\n” t.invoke\_me ## OUTPUT: sean@jefferson:~$ ruby protected\_test.rb Output of p Test.protected\_methods.sort: [] Output of p Test.public\_methods.sort: ["\<", “\<=”, “\<=\>”, “==”, “===”, “=~”, “\>”, “\>=”, “ **id** ”, “ **send** ”, “allocate”, “ancestors”, “autoload”, “autoload?”, “call\_me”, “class”, “class\_eval”, “class\_variables”, “clone”, “const\_defined?”, “const\_get”, “const\_missing”, “const\_set”, “constants”, “display”, “dup”, “eql?”, “equal?”, “extend”, “freeze”, “frozen?”, “hash”, “id”, “include?”, “included\_modules”, “inspect”, “instance\_eval”, “instance\_method”, “instance\_methods”, “instance\_of?”, “instance\_variable\_get”, “instance\_variable\_set”, “instance\_variables”, “is\_a?”, “kind\_of?”, “method”, “method\_defined?”, “methods”, “module\_eval”, “name”, “new”, “nil?”, “object\_id”, “private\_class\_method”, “private\_instance\_methods”, “private\_method\_defined?”, “private\_methods”, “protected\_instance\_methods”, “protected\_method\_defined?”, “protected\_methods”, “public\_class\_method”, “public\_instance\_methods”, “public\_method\_defined?”, “public\_methods”, “respond\_to?”, “send”, “singleton\_methods”, “superclass”, “taint”, “tainted?”, “to\_a”, “to\_s”, “type”, “untaint”] try to call static and protected done - Sean Pete wrote: > where’s the problem? > > class Test > > protected > def self.call\_me > puts “static and protected” > end > > public > def invoke\_me > puts “try to call” > Test.call\_me > # this will _not_ work: self.call\_me > puts “done” > end > > end > > t = Test.new > t.invoke\_me > > > try to call > > static and protected > > done > > Sean Hermany schrieb: ---
**Author:** ![seanhussey](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/a88e57/32.png) [@seanhussey](https://www.ruby-forum.com/u/seanhussey)\ **Post date:** [June 6, 2006, 10:17pm UTC](https://www.ruby-forum.com/t/instance-methods-cant-call-protected-class-methods/62632/4 "2006-06-06T22:17:16Z")
Pete wrote: […] > protected > def self.call\_me > puts “static and protected” > end > […] Unfortunately, this method is neither “static” nor “protected”. Although class methods are often used for the same purposes as static methods in Java/C++, they are quite a different animal. And as Sean pointed out, the protected only applies to the methods instance methods of Test, not instance methods of Test’s singleton class. – Jim W. ---
**Author:** ![seanhussey](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/a88e57/32.png) [@seanhussey](https://www.ruby-forum.com/u/seanhussey)\ **Post date:** [June 6, 2006, 10:13pm UTC](https://www.ruby-forum.com/t/instance-methods-cant-call-protected-class-methods/62632/5 "2006-06-06T22:13:14Z")
Sean Hermany wrote: [… example elided …] > Is f not an “object of the defining class”? If so, why should an > instance method of Foo not be able to call a protected class method? > > I’m not looking for an answer of the form “because in class scope, > self.class.type.object.class # =\> Object, however in _instance_ scope” > etc. So, I’m guessing that saying “Because f and Foo have different defining classes” isn’t the answer you are looking for, because you go on to say: > Logically, why should it be this way? So you are looking for the reason the protected rule was chosen, rather than an explaination of the rule itself. Hmm, only Matz himself can only answer that question definitively, however, I can make some guesses. The first reason is a complexity/benefit tradeoff. The rule currently is that only instances of the same class (or related subclass) can call protected methods. Your suggestion would add additional cases to the rule. Is the benefit gained from the extra conditions worth the extra complexity of the rule? Perhaps not. There may also be a runtime cost to allowing additional classes to call protected methods. Since determining if a protected method call is valid involves some runtime checking, making the list of possible calling classes larger might impact performance. Note the heavy use of “may” and “might” in this paragraph, I’m no expert on Ruby internals. – Jim W.