Tutorial :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.


Solution:1

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  


Solution:2

You can use method_defined? as follows:

String.method_defined? :upcase # => 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?.


Solution:3

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 # => TRUE  

But this does NOT work:

t = TestClass.new  t.method_defined? : methodName  # => ERROR!  

So I use this for both classes and objects:

Classes:

TestClass.methods.include? 'methodName'  # => TRUE  

Objects:

t = TestClass.new  t.methods.include? 'methodName'  # => TRUE  


Solution:4

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.


Solution:5

Not sure if this is the best way, but you could always do this:

Foo.instance_methods.include? 'bar'  


Solution:6

Try Foo.instance_methods.include? :bar


Solution:7

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.


Solution:8

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.


Solution:9

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


Solution:10

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!


Solution:11

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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