String(1.1) == (1.1).to_s => true
String(1.1) === (1.1).to_s => true
Is there a difference between these two coercion methods? If so, can you demonstrate?
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3 Answers
16
The docs for the String method say:
Converts arg to a String by calling its to_s method.
So generally they are the same, but there are some differences – although you’re unlikely to see them for real. String() checks the class of its parameter, and if it’s not already a String then calls to_s on it. Calling to_s directly means the method is called regardless.
Consider the class:
class MyString < String
def to_s
"Overridden to_s method"
end
end
An instance of MyString is already a String object, so passing it as a parameter to String() won’t do anything. Calling to_s on it will however return Overridden to_s method.
1.9.3p286 :010 > m = MyString.new 'my string'
=> "my string"
1.9.3p286 :011 > o = String.new 'orig string'
=> "orig string"
1.9.3p286 :012 > String o
=> "orig string"
1.9.3p286 :013 > String m
=> "my string"
1.9.3p286 :014 > o.to_s
=> "orig string"
1.9.3p286 :015 > m.to_s
=> "Overridden to_s method"
You’re unlikely ever to need to override to_s on a String subclass like this, in general you can treat String() and to_s as the same, but it might be useful to know what’s going on.
matt
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RE overriding #to_s: As of Ruby 2.0.0 the docs clarify the behavior: "First tries to call its to_str method, then its to_s method.". This is why overriding #to_s has no effect -- because Ruby first invokes .to_str on the MyString object, which hasn't been overridden in the example above. – Jordan Brough Feb 18 '20 at 17:21
5
They raise different exceptions when they fail:
bo = BasicObject.new
String(bo)
TypeError: can't convert BasicObject into String
bo.to_s
NoMethodError: undefined method `to_s' for #<BasicObject:0x0003efbfd79c10>
Clint Pachl
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