How do you introduce unit testing into a large, legacy (C/C++) codebase?

Question

We have a large, multi-platform application written in C. (with a small but growing amount of C++) It has evolved over the years with many features you would expect in a lar

Answer 1

Its much easier to make it more modular first. You can't really unittest something with a whole lot of dependencies. When to refactor is a tricky calculation. You really have to weigh the costs and risks vs the benefits. Is this code something that will be reused extensively? Or is this code really not going to change. If you plan to continue to get use out of it, then you probably want to refactor.

Sounds like though, you want to refactor. You need to start by breaking out the simplest utilities and build on them. You have your C module that does a gazillion things. Maybe, for example, there's some code in there that is always formatting strings a certain way. Maybe this can be brought out to be a stand-alone utility module. You've got your new string formatting module, you've made the code more readable. Its already an improvement. You are asserting that you are in a catch 22 situation. You really aren't. Just by moving things around, you've made the code more readable and maintainable.

Now you can create a unittest for this broken out module. You can do that a couple of ways. You can make a separate app that just includes your code and runs a bunch of cases in a main routine on your PC or maybe define a static function called "UnitTest" that will execute all the test cases and return "1" if they pass. This could be run on the target.

Maybe you can't go 100% with this approach, but its a start, and it may make you see other things that can be easily broken out into testable utilities.