How can I check if my subject raises an exception?

Question

I'm currently creating an object in subject and need to test if this raises an exception. The following code illustrates what I'm trying to achieve:

describe MyClass do
  describe '#initialize' do
    subject { MyClass.new }

    it { is_expected.not_to raise_error(Some::Error) }
  end
end

I have a feeling I'm going about this the wrong way. What is the preferred way to set the subject to a new object, without creating the object twice?

---

Update

My problem was two-fold. Firstly, this syntax does not work:

it { is_expected.not_to raise_error }

Using expect inside an it block does, however (as pointed out by Jimmy Cuadra):

it 'does not raise an error' do
  expect { subject }.not_to raise_error
end

I am not well enough acquainted with RSpec to tell you why this is.

Secondly, since RSpec 3.0.0.beta1, it is longer possible to use raise_error with a specific error class. The following, therefore, is invalid:

expect { subject }.to raise_error(Some::Error)

For more information, see

• Rspec 3.0.0.beta1 changelog
• Consider deprecating `expect { }.not_to raise_error(SpecificErrorClass)` #231
• Remove expect {}.not_to raise_error(SomeSpecificClass) #294

Answer 1

If I'm understanding correctly, you're trying to test if instantiating a class causes an exception. You would just do this:

describe MyClass do
  it "doesn't raise an exception when instantiated" do
    expect { subject }.not_to raise_error
  end
end