Just a quick reference post.

There are several pairs of type coercions methods in Ruby:

  • to_i and to_int;
  • to_s and to_str;
  • to_a and to_ary;
  • to_h and to_hash.

What’s the difference and when to use/implement each of pair?

Shorter version is explicit conversion:

  • your type should implement if it can be somehow converted into target type (integer, string, array and so on);
  • you should call those method on incoming data, if you expect it to have some means of converting to target type.

Longer version is implicit conversion:

  • your type should implement it only if it sees itself as “kind of” target type (“better” or “specialized” number, string, array, hash);
  • you should call those methods on incoming data, if you have strict requirement for it being of target type, but want to check it in Ruby way, with duck typing, not class comparison;
  • you should not implement and use this methods in other cases!

Most of Ruby operators and core methods use implicit conversions on data:

  • "string" + other will call #to_str on other, [1,2,3] + other will call #to_ary and so on (and will raise TypeError: no implicit conversion if object not responds to this methods);
  • "string" * other and [1,2,3] * other will call #to_int on other;
  • a, b, c = *other will call other#to_ary (and, therefore, can be used to “unpack” custom collection objects—but also can cause unintended behavior, if other implements to_ary but was not thought as a collection);
  • some_method(*other)—same as above, uses other#to_ary;
  • some_method(**other)—new in Ruby 2, uses other#to_hash.

I should repeat: never implement implicit conversion methods unless you sure know what you are doing! It is widely seen, for example, the #to_hash method being implemented (maybe because of “prettier name” than #to_h?) and causing strangest effects.

Practical example:

class Dog
  attr_reader :name, :age
  def initialize(name, age)
    @name, @age = name, age

  # erroneously defined for serialization instead of to_h
  def to_hash
    {species: 'Dog', name: name, age: age}

def set_options(**some_options)
  p some_options

dog = Dog.new('Rex', 5)

set_options(foo: 'bar')
# ok, prints {foo: 'bar'}

# can't be called this way, raises helpful ArgumentError

# ooops, treats our dog as an options hash
# and prints {species: 'Dog', name: name, age: age}

require 'awesome_print' # pretty popular pretty printing gem
ap dog
# ooops, uses Hash pretty-printing method
# instead of Dog#inspect

See also: