Project: RSpec Expectations 2.6

Publisher: RSpec

specify types of objects

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

obj.should 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.
obj.should 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:

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

Scenarios

be_(a_)kind_of matcher

Given

a file named "be_kind_of_matcher_spec.rb" with:

module MyModule; end

class Fixnum
  include MyModule
end

describe 17 do
  # the actual class
  it { should be_kind_of(Fixnum) }
  it { should be_a_kind_of(Fixnum) }
  it { should be_a(Fixnum) }

  # the superclass
  it { should be_kind_of(Integer) }
  it { should be_a_kind_of(Integer) }
  it { should be_an(Integer) }

  # an included module
  it { should be_kind_of(MyModule) }
  it { should be_a_kind_of(MyModule) }
  it { should be_a(MyModule) }

  # negative passing case
  it { should_not be_kind_of(String) }
  it { should_not be_a_kind_of(String) }
  it { should_not be_a(String) }

  # deliberate failures
  it { should_not be_kind_of(Fixnum) }
  it { should_not be_a_kind_of(Fixnum) }
  it { should_not be_a(Fixnum) }
  it { should_not be_kind_of(Integer) }
  it { should_not be_a_kind_of(Integer) }
  it { should_not be_an(Integer) }
  it { should_not be_kind_of(MyModule) }
  it { should_not be_a_kind_of(MyModule) }
  it { should_not be_a(MyModule) }
  it { should be_kind_of(String) }
  it { should be_a_kind_of(String) }
  it { should 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 not to be a kind of Fixnum

expected 17 not to be a kind of Integer

expected 17 not to be a kind of MyModule

expected 17 to be a kind of String

be_(an_)instance_of matcher

Given

a file named "be_instance_of_matcher_spec.rb" with:

module MyModule; end

class Fixnum
  include MyModule
end

describe 17 do
  # the actual class
  it { should be_instance_of(Fixnum) }
  it { should be_an_instance_of(Fixnum) }

  # the superclass
  it { should_not be_instance_of(Integer) }
  it { should_not be_an_instance_of(Integer) }

  # an included module
  it { should_not be_instance_of(MyModule) }
  it { should_not be_an_instance_of(MyModule) }

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

  # deliberate failures
  it { should_not be_instance_of(Fixnum) }
  it { should_not be_an_instance_of(Fixnum) }
  it { should be_instance_of(Integer) }
  it { should be_an_instance_of(Integer) }
  it { should be_instance_of(MyModule) }
  it { should be_an_instance_of(MyModule) }
  it { should be_instance_of(String) }
  it { should 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 not to be an instance of Fixnum

expected 17 to be an instance of Integer

expected 17 to be an instance of MyModule

expected 17 to be an instance of String

Last published over 6 years ago by dchelimsky.

specify types of objects

RSpec Expectations settings