Given a class see if instance has method Ruby

Question

I know in Ruby that I can use respond to  to check if an object has a certain method   But  given the class  how can I check if the instance has a certain method   i e  something like  Foo new respond to   bar    But I feel like there s gotta be a better way than instantiating a new instance

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You can use method defined  as follows   String method defined   upcase     gt  true   Much easier  portable and efficient than the instance methods include  everyone else seems to be suggesting   Keep in mind that you won t know if a class responds dynamically to some calls with method missing  for example by redefining respond to   or since Ruby 1 9 2 by defining respond to missing

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Try Foo instance methods include   bar

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If you re checking to see if an object can respond to a series of methods  you could do something like   methods     valid    chase   test   def has methods  something  methods    methods  amp  something methods    methods end   the methods  amp  something methods will join the two arrays on their common matching elements  something methods includes all of the methods you re checking for  it ll equal methods  For example    1 2   amp   1 2 3 4 5     gt   1 2    so    1 2   amp   1 2 3 4 5      1 2     gt  true   In this situation  you d want to use symbols  because when you call  methods  it returns an array of symbols and if you used   my    methods    it d return false

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I think there is something wrong with method defined  in Rails  It may be inconsistent or something  so if you use Rails  it s better to use something from attribute method  attribute     testing for method defined  on ActiveRecord classes doesn t work until an instantiation  is a question about the inconsistency

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While respond to  will return true only for public methods  checking for  method definition  on a class may also pertain to private methods   On Ruby v2 0  checking both public and private sets can be achieved with  Foo private instance methods include   bar     Foo instance methods include   bar

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I don t know why everyone is suggesting you should be using instance methods and include  when method defined  does the job   class Test   def hello  end end  Test method defined   hello    gt  true   NOTE  In case you are coming to Ruby from another OO language OR you think that method defined means ONLY methods that you defined explicitly with   def my method end   then read this   In Ruby  a property  attribute  on your model is basically a method also  So method defined  will also return true for properties  not just methods   For example   Given an instance of a class that has a String attribute first name    lt instance gt  first name class    gt  String   lt instance gt  class method defined   first name     gt  true   since first name is both an attribute and a method  and a string of type String

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class Foo  def self fclass method  end  def finstance method  end end  foo obj   Foo new foo obj class methods false    gt    fclass method    foo obj class instance methods false    gt    fclass method     Hope this helps you

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Not sure if this is the best way  but you could always do this   Foo instance methods include   bar

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Actually this doesn t work for both Objects and Classes   This does   class TestClass   def methodName   end end   So with the given answer  this works   TestClass method defined   methodName     gt  TRUE   But this does NOT work   t   TestClass new t method defined    methodName      gt  ERROR    So I use this for both classes and objects   Classes   TestClass methods include   methodName       gt  TRUE   Objects   t   TestClass new t methods include   methodName       gt  TRUE

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klass instance methods include  method name or  method name   depending on the Ruby version I think

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On my case working with ruby 2 5 3 the following sentences have worked perfectly    value    hello world   value methods include   upcase   It will return a boolean value true or false

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The answer to  Given a class  see if instance has method  Ruby   is better  Apparently Ruby has this built-in  and I somehow missed it  My answer is left for reference  regardless   Ruby classes respond to the methods instance methods and public instance methods  In Ruby 1 8  the first lists all instance method names in an array of strings  and the second restricts it to public methods  The second behavior is what you d most likely want  since respond to  restricts itself to public methods by default  as well   Foo public instance methods include   bar     In Ruby 1 9  though  those methods return arrays of symbols   Foo public instance methods include   bar    If you re planning on doing this often  you might want to extend Module to include a shortcut method   It may seem odd to assign this to Module instead of Class  but since that s where the instance methods methods live  it s best to keep in line with that pattern    class Module   def instance respond to  method name      public instance methods include  method name    end end   If you want to support both Ruby 1 8 and Ruby 1 9  that would be a convenient place to add the logic to search for both strings and symbols  as well

[ruby] Given a class, see if instance has method (Ruby)

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