Unit testing is easy for beginners. Unit testing done well is much less so. This results in many inexperienced people writing poor unit tests.
In my last job, I worked on a legacy C++ code base. 500K lines of thorough testing, but none was a unit test. It took 10 hours to run the full test suite. I set about the task of converting portions for unit testability.
I was surprised how much of a challenge it was (and I learned a lot). There were few resources on proper unit testing in C++. Mostly learned from Stack Overflow. Lessons learned:
1. If your code did not have unit tests, then likely your code will need a lot of rearchitecting just to enable a single unit test. The good news is that the rearchitecting made the code better.
As a corollary: You'll never know how much coupling there is in your code until you write unit tests for it. Our code looked fairly good, but when I wanted to test one part, I found too many dependencies I needed to bring in just to test it. In the end, to test one part, I was involving code from all over the code base. Not good. Most of my effort was writing interface classes to separate the parts so that I could unit test them.
2. For C++, this means your code will look very enterprisey. For once, this was a good thing.
3. Mocking is an art. There's no ideal rule/guideline for it. Overmock and you are just encoding bugs into your tests. Undermock and you are testing too many things at once.
4. For the love of God, don't do a 1:1 mapping between functions/methods and tests. It's OK if your test involves more than one method. Even Robert Martin (Uncle Bob) says so. I know I go against much dogma, but make your unit tests test features, not functions.
5. If your unit tests keep breaking due to trivial refactors, then architect your unit tests to be less sensitive to refactors.
6. For classes, don't test private methods directly. Test them through your public interface. If you cannot reach some code in a private method via the public interface, throw the code away!
7. Perhaps the most important: Assume a hostile management (which is what I had). For every code change you make so that you can write a unit test, can you justify that code change assuming your project never will have unit tests? There are multiple ways to write unit tests - many of them are bad. This guideline will keep you from taking convenient shortcuts.
This advice is all about unit tests, and not TDD. With TDD, it is not hard to test yourself into a corner where you then throw everything away and restart. If you insist on TDD, then at least follow Uncle Bob's heuristic. For your function/feature, think of the most complicated result/boundary input, and make that your first unit test. This way you're less likely to develop the function into the wrong corner.
When I completed my proof of work, the team rejected it. The feedback was it required too much skill for some of the people in the team (40+ developers across 4 sites), and the likelihood that all of them will get it was miniscule. And too much of the code would need to change to add unit tests.
I later read this book:
And it contains quite a bit of what I learned from unit testing.
A Pattern Language, Alexander and Ishikawa and Silverstein http://amzn.to/2s9aSSc
Advanced Programming in the Unix Environment , Stevens http://amzn.to/2qPOMjN
Algorithmics: the Spirit of Computing, Harel http://amzn.to/2rW5FNS
Applied Crytography, Wiley http://amzn.to/2rsULxS
Clean Code, Martin http://amzn.to/2sIOWtQ
Clean Coder, Martin http://amzn.to/2rWgbEP
Code Complete, McConnel http://amzn.to/2qSUIwE
Code: The Hidden Language of Computer Hardware and Software, Petzold http://amzn.to/2rWfR9d
Coders at Work, Seibel http://amzn.to/2qPCasZ
Compilers: Principles, Techniques, & Tools, Aho http://amzn.to/2rCSUVA
Computer Systems: A Programmer's Perspective, O'Hallaron and Bryant http://amzn.to/2qPY5jH
Data Flow Analysis: Theory and Practice, Khedker http://amzn.to/2qTnSvr
Dependency Injection in .NET, Seemann http://amzn.to/2rCz0tV
Domain Driven Design, Evans http://amzn.to/2sIGM4N
Fundamentals of Wireless Communication, Tse and Viswanath http://amzn.to/2rCTmTM
Genetic Programming: An Intrduction, Banzhaf http://amzn.to/2s9sdut
Head First Design Patterns, O'Reilly http://amzn.to/2rCISUB
Implementing Domain-Driven Design, Vernon http://amzn.to/2qQ2G5u
Intrduction to Algorithms, CLRS http://amzn.to/2qXmSBU
Introduction to General Systems Thinking, Weinberg http://amzn.to/2qTuGJw
Joy of Clojure, Fogus and Houser http://amzn.to/2qPL4qr
Let over Lambda, Hoyte http://amzn.to/2rWljcp
Operating Systems: Design and Implementation, Tanenbaum http://amzn.to/2rKudsw
Parsing Techniques, Grune and Jacobs http://amzn.to/2rKNXfn
Peopleware: Productive Projects and Teams, DeMarco and Lister http://amzn.to/2qTu86F
Programming Pearls, Bentley http://amzn.to/2sIRPe9
Software Process Design: Out of the Tar Pit, McGraw-Hill http://amzn.to/2rVX0v0
Software Runaways, Glass http://amzn.to/2qT2mHn
Sorting and Searching, Knuth http://amzn.to/2qQ4NWQ
Structure and Interpretation of Computer Programs, Abelson and Sussman http://amzn.to/2qTflsk
The Art of Unit Testing, Manning http://amzn.to/2rsERDu
The Art of Unix Programming, ESR http://amzn.to/2sIAXUZ
The Design of Design: Essays from a Computer Scientist, Brooks http://amzn.to/2rsPjev
The Effective Engineer, Lau http://amzn.to/2s9fY0X
The Elements of Style, Strunk and White http://amzn.to/2svB3Qz
The Healthy Programmer, Kutner http://amzn.to/2qQ2MtQ
The Linux Programming Interface, Kerrisk http://amzn.to/2rsF8Xi
The Mythical Man-Month, Brooks http://amzn.to/2rt0dAR
The Practice of Programming, Kernighan and Pike http://amzn.to/2qTje0C
The Pragmatic Programmer, Hunt and Thomas http://amzn.to/2s9dlvS
The Psychology of Computer Programming, Weinberg http://amzn.to/2rsPypy
Transaction Processing: Concepts and Techniques, Gray and Reuter http://amzn.to/
Types and Programming Languages, Pierce http://amzn.to/2qT2d6G
Understanding MySQL Internals, Pachev http://amzn.to/2svXuFo
Working Effectively with Legacy Code, Feathers http://amzn.to/2sIr09R
Zen of graphics programming, Abrash http://amzn.to/2rKIW6Q
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