The Ruby Programming Language

Types, Classes, and Modules

The most commonly used reflective methods are those for determining the type of an object—what class it is an instance of and what methods it responds to. We introduced most of these important methods early in this book in Object Class and Object Type. To review:

o.class

Returns the class of an object o.

c.superclass

Returns the superclass of a class c.

o.instance_of? c

Determines whether the object o.class == c.

o.is_a? c

Determines whether o is an instance of c, or of any of its subclasses. If c is a module, this method tests whether o.class (or any of its ancestors) includes the module.

o.kind_of? c

kind_of? is a synonym for is_a?.

c === o

For any class or module c, determines if o.is_a?(c).

o.respond_to? name

Determines whether the object o has a public or protected method with the specified name. Pass true as the second argument to check private methods as well.

Ancestry and Modules

In addition to these methods that you’ve already seen, there are a few related reflective methods for determining the ancestors of a class or module and for determining which modules are included by a class or module. These methods are easy to understand when demonstrated:

module A; end # Empty module module B; include A; end; # Module B includes A class C; include B; end; # Class C includes module B C < B # => true: C includes B B < A # => true: B includes A C < A # => true Fixnum < Integer # => true: all fixnums are integers Integer < Comparable # => true: integers are comparable Integer ...