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Ruby: to_s vs. to_str

Concept

(The blockquote style does not look so well so I just pasted directly, but these are all quoted from the links in the bottom of this page)

You should not implement to_str unless your object acts like a string, rather than just having a string representation. The only core class that implements to_str is String itself.

[to_i and to_s] are not particularly strict: if an object has some kind of decent representation as a string, for example, it will probably have a to_s method… [to_int and to_str] are strict conversion functions: you implement them only if you object can naturally be used every place a string or an integer could be used.

to_str is used by methods such as String#concat to convert their arguments to a string. Unlike to_s, which is supported by almost all classes, to_str is normally implemented only by those classes that act like strings. Of the built-in classes, only Exception and String implement to_str

print, puts and string interpolation(#{} syntax) use to_s. Mostly everything else (glob, split, match, string concatenation) uses to_str.

Simple Summary

to_s is defined on every object and will always return something.

to_str is only defined on objects that are string-like. For example, Symbol has to_str but Array does not.

Thus, you can use obj.respond_to?(:to_str) instead of something like obj.is_a?(String) if you want to take advantage of duck typing without worrying about whether the class you're working with is a subclass of String or not.

The documentation for Exception#to_str reads: "By supplying a to_str method, exceptions are agreeing to be used where Strings are expected." So it's all about expectations.

to_s: Give me a String no matter what! to_str: I am pretty sure you are String-like. So sure, that I'd prefer to get a NoMethodException in the case where you're not.

Use case?

Borrowed from Confident Ruby.

class ArticleTitle
  def initialize(text)
    @text = text
  end

  def slug
    @text.strip.tr_s("^A-Za-z0-9","-").downcase
  end

  def to_str
    puts 'to_str called'
    @text
  end
end

title = ArticleTitle.new("A Modest Proposal")
# By defining #to_str, ArticleTitle can be used in places Ruby expects
# a String:
puts "Today’s Feature: " + title 
# => "to_str called"
# => "Today’s Feature: A Modest Proposal"

References

@soulcutter

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soulcutter commented Sep 29, 2015

Neither Symbol nor Exception implement #to_s - I know the pickaxe book suggests that Exception does, however that changed long, long ago.

@pinguinjkeke

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pinguinjkeke commented Jan 19, 2017

Thanks for nice explanation!

@xiangzhuyuan

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xiangzhuyuan commented Apr 25, 2017

awesome`

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