Type matchers

rspec-expectations includes two matchers to specify types of objects:

expect(obj).to be_kind_of(type): calls obj.kind_of?(type), which returns true if type is in obj’s class hierarchy or is a module and is included in a class in obj’s class hierarchy.
expect(obj).to be_instance_of(type): calls obj.instance_of?(type), which returns true if and only if type if obj’s class.

Both of these matchers have aliases:

  expect(obj).to be_a_kind_of(type)      # same as expect(obj).to be_kind_of(type)
  expect(obj).to be_a(type)              # same as expect(obj).to be_kind_of(type)
  expect(obj).to be_an(type)             # same as expect(obj).to be_kind_of(type)
  expect(obj).to be_an_instance_of(type) # same as expect(obj).to be_instance_of(type)

With `be_(a_)kind_of` matcher

Given a file named “bekindofmatcherspec.rb” with:

module MyModule; end

class Float
  include MyModule
end

RSpec.describe 17.0 do
  # the actual class
  it { is_expected.to be_kind_of(Float) }
  it { is_expected.to be_a_kind_of(Float) }
  it { is_expected.to be_a(Float) }

  # the superclass
  it { is_expected.to be_kind_of(Numeric) }
  it { is_expected.to be_a_kind_of(Numeric) }
  it { is_expected.to be_an(Numeric) }

  # an included module
  it { is_expected.to be_kind_of(MyModule) }
  it { is_expected.to be_a_kind_of(MyModule) }
  it { is_expected.to be_a(MyModule) }

  # negative passing case
  it { is_expected.not_to be_kind_of(String) }
  it { is_expected.not_to be_a_kind_of(String) }
  it { is_expected.not_to be_a(String) }

  # deliberate failures
  it { is_expected.not_to be_kind_of(Float) }
  it { is_expected.not_to be_a_kind_of(Float) }
  it { is_expected.not_to be_a(Float) }
  it { is_expected.not_to be_kind_of(Numeric) }
  it { is_expected.not_to be_a_kind_of(Numeric) }
  it { is_expected.not_to be_an(Numeric) }
  it { is_expected.not_to be_kind_of(MyModule) }
  it { is_expected.not_to be_a_kind_of(MyModule) }
  it { is_expected.not_to be_a(MyModule) }
  it { is_expected.to be_kind_of(String) }
  it { is_expected.to be_a_kind_of(String) }
  it { is_expected.to be_a(String) }
end

When I run rspec be_kind_of_matcher_spec.rb

Then the output should contain all of these:


24 examples, 12 failures
expected 17.0 not to be a kind of Float
expected 17.0 not to be a kind of Numeric
expected 17.0 not to be a kind of MyModule
expected 17.0 to be a kind of String

With `be_(an_)instance_of` matcher

Given a file named “beinstanceofmatcherspec.rb” with:

module MyModule; end

class Float
  include MyModule
end

RSpec.describe 17.0 do
  # the actual class
  it { is_expected.to be_instance_of(Float) }
  it { is_expected.to be_an_instance_of(Float) }

  # the superclass
  it { is_expected.not_to be_instance_of(Numeric) }
  it { is_expected.not_to be_an_instance_of(Numeric) }

  # an included module
  it { is_expected.not_to be_instance_of(MyModule) }
  it { is_expected.not_to be_an_instance_of(MyModule) }

  # another class with no relation to the subject's hierarchy
  it { is_expected.not_to be_instance_of(String) }
  it { is_expected.not_to be_an_instance_of(String) }

  # deliberate failures
  it { is_expected.not_to be_instance_of(Float) }
  it { is_expected.not_to be_an_instance_of(Float) }
  it { is_expected.to be_instance_of(Numeric) }
  it { is_expected.to be_an_instance_of(Numeric) }
  it { is_expected.to be_instance_of(MyModule) }
  it { is_expected.to be_an_instance_of(MyModule) }
  it { is_expected.to be_instance_of(String) }
  it { is_expected.to be_an_instance_of(String) }
end

When I run rspec be_instance_of_matcher_spec.rb

Then the output should contain all of these:


16 examples, 8 failures
expected 17.0 not to be an instance of Float
expected 17.0 to be an instance of Numeric
expected 17.0 to be an instance of MyModule
expected 17.0 to be an instance of String

Type matchers

With be_(a_)kind_of matcher

With be_(an_)instance_of matcher

With `be_(a_)kind_of` matcher

With `be_(an_)instance_of` matcher