Clare Macrae presents... Quickly Testing Legacy C++ Code with Approval Tests

ACCU Cambridge
ACCU Cambridge
Public group
Location image of event venue


You've inherited some legacy code: it's valuable, but it doesn't have tests, and it wasn't designed to be testable, so you need to start refactoring. But you can't refactor safely until the code has tests, and you can't add tests without refactoring! How can you ever break out of this loop?

Whether Legacy code for you means "old code", "code without tests", or "code you wish to redesign for new features or unit-tests", this talk will enable you to become productive and work safely, quickly.

The simplicity, convenience, ease-of-use, power and flexibility of Llewellyn Falco's "Approval Tests" approach has long been proven in a dozen programming languages. And now all this is now available to C++ developers too!

Clare will present a small but surprisingly effective C++11 library for applying "Approval Tests" to cross-platform C++ code - for both legacy and green-field systems, and with a range of testing frameworks.

She will describe its use in some real-world situations, including how to quickly lock down the behaviour of legacy code. She will show how to quickly achieve good test coverage, even for very large sets of inputs. Finally, she will describe some general techniques she learned along the way.

Attendees will discover some quick, practical techniques to use for common challenges, such as testing outputs containing dates and times, that can be applied very easily using Approval Tests.


Clare is an independent consultant, helping teams streamline their work with legacy and hard-to-test C++ and Qt code.

She has worked in software development for over 30 years, and in C++ for 20 years.

Since 2017, she has used her spare time to work remotely with Llewellyn Falco on [ApprovalTests.cpp](, to radically simplify testing of legacy code. She has enjoyed this so much that she recently went independent, to focus even more on helping others to work more easily with legacy code.

Clare was until recently a Principal Scientific Software Engineer at Cambridge Crystallographic Data Centre. She is the original author of their popular 3D crystal structure visualisation program [Mercury](


7.00pm: Arrive for networking and chat, with pizza & beverages courtesy of DisplayLink
7.30pm: Clare will deliver her talk. There will be time for Q&A.
~9.00pm: Close, with option to continue conversation at the pub