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 certai

Answer 1

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.

Answer 2

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 & something.methods == methods
end

the methods & 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] & [1,2,3,4,5]
==> [1,2]

so

[1,2] & [1,2,3,4,5] == [1,2]
==> 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.

Answer 3

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)

Answer 4

klass.instance_methods.include :method_name or "method_name", depending on the Ruby version I think.

Answer 5

class Foo
 def self.fclass_method
 end
 def finstance_method
 end
end

foo_obj = Foo.new
foo_obj.class.methods(false)
=> [:fclass_method] 

foo_obj.class.instance_methods(false)
=> [:fclass_method] 

Hope this helps you!

Answer 6

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 #=> 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:

<instance>.first_name.class #=> String

<instance>.class.method_defined?(:first_name) #=> true

since first_name is both an attribute and a method (and a string of type String).