Cover the patient with tests

Part 2 of 4 learning hours on this topic. All are specific for the C language.

Learning Goals

• define a Stub
• recognize link-time substitution
• use stubs in unit tests

Session Outline

• 5 min connect: When do you have enough tests?
• 10 min concept: stubs and link-time substitution
• 35 min concrete: cover the patient
• 5 min conclusions: confidence in these tests?

Connect - when do you have enough tests?

How do you know you have enough unit tests? Put a mark next to any you agree with.

• When the Architect is happy
• When coverage is 80%
• When I get fed up of writing tests
• When I feel confident the code works
• When I’ve written the tests on my test list
• When Jupiter is in Ares and Saturn is in Scorpio
• When I no longer have the feeling there are bugs in the code I’m looking at
• When I can run the code and it seems to work
• When I’m confident the tests will survive a refactoring of the code

Concept: Break dependencies using stubs

Last time we identified one problem with adding tests to existing code is to break dependencies. In the exercise repo Grep with Marketing there is a branch ‘with_stubs_no_tests’. It has the patient in the surgery, and each of the three dependencies is replaced by a stub implementation. Review the stub code.

Explain what a stub is and what problem it solves. Show how the link-time substitution works.

Concrete: add tests

Write tests for the patient, using the stubs. Aim for 100% code coverage.

Conclusions

In your own words, write down what a stub is and when you might use one.