CODEWITHSUNDEEP
rubyidiomstypechecking
davidchambers
davidchambers

Reputation: 24806

Check whether a variable is a string in Ruby

Is there anything more idiomatic than the following?

foo.class == String

Upvotes: 166

Views: 176821

Answers (6)

steenslag
steenslag

Reputation: 80065

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
  • c === o for a class or module, determine if o.is_a? c (String === "s" returns true)
  • o.is_a? c Is o an instance of c or any of it's subclasses?
  • o.kind_of? c synonym for is_a?

Upvotes: 5

Federico Builes
Federico Builes

Reputation: 5097

You can do:

foo.instance_of?(String)

And the more general:

foo.kind_of?(String)

Upvotes: 35

schlegel11
schlegel11

Reputation: 383

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 ;)

Upvotes: -1

Andrew Grimm
Andrew Grimm

Reputation: 81510

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?).

Upvotes: 33

Roland Mai
Roland Mai

Reputation: 31077

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

Upvotes: 260

Matthew
Matthew

Reputation: 13332

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

Upvotes: 8

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