Implicit Conversions

Some Ruby methods accept one or more objects that can be either:

Of a given class, and so accepted as is.
Implicitly convertible to that class, in which case the called method converts the object.

For each of the relevant classes, the conversion is done by calling a specific conversion method:

Array: to_ary
Hash: to_hash
Integer: to_int
String: to_str

Array-Convertible Objects

An Array-convertible object is an object that:

Has instance method to_ary.
The method accepts no arguments.
The method returns an object obj for which obj.kind_of?(Array) returns true.

The Ruby core class that satisfies these requirements is:

Array

The examples in this section use method Array#replace, which accepts an Array-convertible argument.

This class is Array-convertible:

class ArrayConvertible
  def to_ary
    [:foo, 'bar', 2]
  end
end
a = []
a.replace(ArrayConvertible.new) # => [:foo, "bar", 2]

This class is not Array-convertible (no to_ary method):

class NotArrayConvertible; end
a = []
# Raises TypeError (no implicit conversion of NotArrayConvertible into Array)
a.replace(NotArrayConvertible.new)

This class is not Array-convertible (method to_ary takes arguments):

class NotArrayConvertible
  def to_ary(x)
    [:foo, 'bar', 2]
  end
end
a = []
# Raises ArgumentError (wrong number of arguments (given 0, expected 1))
a.replace(NotArrayConvertible.new)

This class is not Array-convertible (method to_ary returns non-Array):

class NotArrayConvertible
  def to_ary
    :foo
  end
end
a = []
# Raises TypeError (can't convert NotArrayConvertible to Array (NotArrayConvertible#to_ary gives Symbol))
a.replace(NotArrayConvertible.new)

Hash-Convertible Objects

A Hash-convertible object is an object that:

Has instance method to_hash.
The method accepts no arguments.
The method returns an object obj for which obj.kind_of?(Hash) returns true.

The Ruby core class that satisfies these requirements is:

Hash

The examples in this section use method Hash#merge, which accepts a Hash-convertible argument.

This class is Hash-convertible:

class HashConvertible
  def to_hash
    {foo: 0, bar: 1, baz: 2}
  end
end
h = {}
h.merge(HashConvertible.new) # => {:foo=>0, :bar=>1, :baz=>2}

This class is not Hash-convertible (no to_hash method):

class NotHashConvertible; end
h = {}
# Raises TypeError (no implicit conversion of NotHashConvertible into Hash)
h.merge(NotHashConvertible.new)

This class is not Hash-convertible (method to_hash takes arguments):

class NotHashConvertible
  def to_hash(x)
    {foo: 0, bar: 1, baz: 2}
  end
end
h = {}
# Raises ArgumentError (wrong number of arguments (given 0, expected 1))
h.merge(NotHashConvertible.new)

This class is not Hash-convertible (method to_hash returns non-Hash):

class NotHashConvertible
  def to_hash
    :foo
  end
end
h = {}
# Raises TypeError (can't convert NotHashConvertible to Hash (ToHashReturnsNonHash#to_hash gives Symbol))
h.merge(NotHashConvertible.new)

Integer-Convertible Objects

An Integer-convertible object is an object that:

Has instance method to_int.
The method accepts no arguments.
The method returns an object obj for which obj.kind_of?(Integer) returns true.

The Ruby core classes that satisfy these requirements are:

Integer
Float
Complex
Rational

The examples in this section use method Array.new, which accepts an Integer-convertible argument.

This user-defined class is Integer-convertible:

class IntegerConvertible
  def to_int
    3
  end
end
a = Array.new(IntegerConvertible.new).size
a # => 3

This class is not Integer-convertible (method to_int takes arguments):

class NotIntegerConvertible
  def to_int(x)
    3
  end
end
# Raises ArgumentError (wrong number of arguments (given 0, expected 1))
Array.new(NotIntegerConvertible.new)

This class is not Integer-convertible (method to_int returns non-Integer):

class NotIntegerConvertible
  def to_int
    :foo
  end
end
# Raises TypeError (can't convert NotIntegerConvertible to Integer (NotIntegerConvertible#to_int gives Symbol))
Array.new(NotIntegerConvertible.new)

String-Convertible Objects

A String-convertible object is an object that:

Has instance method to_str.
The method accepts no arguments.
The method returns an object obj for which obj.kind_of?(String) returns true.

The Ruby core class that satisfies these requirements is:

String

The examples in this section use method String::new, which accepts a String-convertible argument.

This class is String-convertible:

class StringConvertible
  def to_str
    'foo'
  end
end
String.new(StringConvertible.new) # => "foo"

This class is not String-convertible (no to_str method):

class NotStringConvertible; end
# Raises TypeError (no implicit conversion of NotStringConvertible into String)
String.new(NotStringConvertible.new)

This class is not String-convertible (method to_str takes arguments):

class NotStringConvertible
  def to_str(x)
    'foo'
  end
end
# Raises ArgumentError (wrong number of arguments (given 0, expected 1))
String.new(NotStringConvertible.new)

This class is not String-convertible (method to_str returns non-String):

class NotStringConvertible
  def to_str
    :foo
  end
end
# Raises TypeError (can't convert NotStringConvertible to String (NotStringConvertible#to_str gives Symbol))
String.new(NotStringConvertible.new)