Check whether a variable is a string in Ruby

I think you are looking for instance_of?. is_a? and kind_of? will return true for instances from derived classes.

class X < String
end

foo = X.new

foo.is_a? String         # true
foo.kind_of? String      # true
foo.instance_of? String  # false
foo.instance_of? X       # true

A more duck-typing approach would be to say

foo.respond_to?(:to_str)

to_str indicates that an object's class may not be an actual descendant of the String, but the object itself is very much string-like (stringy?).

You can do:

foo.instance_of?(String)

And the more general:

foo.kind_of?(String)

foo.instance_of? String

or

foo.kind_of? String 

if you you only care if it is derrived from String somewhere up its inheritance chain

In addition to the other answers, Class defines the method === to test whether an object is an instance of that class.

o.class class of o.

o.instance_of? c determines whether o.class == c

o.is_a? c Is o an instance of c or any of it's subclasses?

o.kind_of? c synonym for *is_a?*

c === o for a class or module, determine if *o.is_a? c* (String === "s" returns true)

I think a better way is to create some predicate methods. This will also save your "Single Point of Control".

class Object
 def is_string?
   false
 end
end

class String
 def is_string?
   true
 end
end

print "test".is_string? #=> true
print 1.is_string?      #=> false

The more duck typing way ;)