instance_double_usage

🧩 Use instance_double for Verifying Real Interfaces

An instance_double creates a test double that only allows methods defined on the real class, preventing typos and ensuring your stubs match the actual API. Combine it with allow to stub methods safely.

class PaymentGateway
  def charge(amount); end
end

RSpec.describe OrderProcessor do
  it 'charges the correct amount' do
    gateway = instance_double(PaymentGateway)
    allow(gateway).to receive(:charge).with(100).and_return('OK')

    processor = OrderProcessor.new(gateway)
    expect(processor.process(100)).to eq('OK')
  end
end