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Published on 2024-02-29

You've just inherited a legacy C++ codebase, now what?

🏷️ C++, C, Legacy, CI, Git, Rewrite
Table of contents

This article was discussed on Hacker News, Lobster.rs and Reddit. I've got great suggestions from the comments, see the addendum at the end!

You were minding your own business, and out of nowhere something fell on your lap. Maybe you started a new job, or perhaps changed teams, or someone experienced just left.

And now you are responsible for a C++ codebase. It's big, complex, idiosyncratic; you stare too long at it and it breaks in various interesting ways. In a word, legacy.

But somehow bugs still need to be fixed, the odd feature to be added. In short, you can't just ignore it or better yet nuke it out of existence. It matters. At least to someone who's paying your salary. So, it matters to you.

What do you do now?

Well, fear not, because I have experience this many times in numerous places (the snarky folks in the back will mutter: what C++ codebase isn't exactly like I described above), and there is a way out, that's not overly painful and will make you able to actually fix the bugs, add features, and, one can dream, even rewrite it some day.

So join me on a recollection of what worked for me and what one should absolutely avoid.

And to be fair to C++, I do not hate it (per se), it just happens to be one of these languages that people abuse and invariably leads to a horrifying mess and poor C++ is just the victim here and the C++ committee will fix it in C++45, worry not, by adding std::cmake to the standard library and you'll see how it's absolutely a game changer, and - Ahem, ok let's go back to the topic at hand.

So here's an overview of the steps to take:

  1. Get it to work locally, by only doing the minimal changes required in the code and build system, ideally none. No big refactorings yet, even if itches really bad!
  2. Get out the chainsaw and rip out everything that's not absolutely required to provide the features your company/open source project is advertising and selling
  3. Make the project enter the 21st century by adding CI, linters, fuzzing, auto-formatting, etc
  4. Finally we get to make small, incremental changes to the code, Rinse and repeat until you're not awaken every night by nightmares of Russian hackers p@wning your application after a few seconds of poking at it
  5. If you can, contemplate rewrite some parts in a memory safe language

The overarching goal is exerting the least amount of effort to get the project in an acceptable state in terms of security, developer experience, correctness, and performance. It's crucial to always keep that in mind. It's not about 'clean code', using the new hotness language features, etc.

Ok, let's dive in!

By the way, everything here applies to a pure C codebase or a mixed C and C++ codebase, so if that's you, keep reading!

Get buy-in

You thought I was going to compare the different sanitizers, compile flags, or build systems? No sir, before we do any work, we talk to people. Crazy, right?

Software engineering needs to be a sustainable practice, not something you burn out of after a few months or years. We cannot do this after hours, on a death march, or even, alone! We need to convince people to support this effort, have them understand what we are doing, and why. And that encompasses everyone: your boss, your coworkers, even non-technical folks. And who knows, maybe you'll go on vacation and return to see that people are continuing this effort when you're out of office.

All of this only means: explain in layman terms the problem with a few simple facts, the proposed solution, and a timebox. Simple right? For example (to quote South Park: All characters and events in this show—even those based on real people—are entirely fictional):

And here's what to avoid, again totally, super duper fictional, never-really-happened-to-me examples:

Ok, let's say that now you have buy-in from everyone that matters, let's go over the process:

In my experience, with this approach, you keep everyone happy and can do the improvements that you really need to do.

Alright, let's get down to business now!

Write down the platforms you support

This is so important and not many projects do it. Write in the README (you do have a README, right?). It's just a list of <architecture>-<operating-system> pair, e.g. x86_64-linux or aarch64-darwin, that your codebase officially supports. This is crucial for getting the build working on every one of them but also and we'll see later, removing cruft for platforms you do not support.

If you want to get fancy, you can even write down which version of the architecture such as ARMV6 vs ARMv7, etc.

That helps answer important questions such as:

And an important point: This list should absolutely include the developers workstations. Which leads me to my next point:

Get the build working on your machine

You'd be amazed at how many C++ codebase in the wild that are a core part of a successful product earning millions and they basically do not compile. Well, if all the stars are aligned they do. But that's not what I'm talking about. I'm talking about reliably, consistently building on all platforms you support. No fuss, no 'I finally got it building after 3 weeks of hair-pulling' (this brings back some memories). It just works(tm).

A small aparte here. I used to be really into Karate. We are talking 3, 4 training sessions a week, etc. And I distinctly remember one of my teachers telling me (picture a wise Asian sifu - hmm actually my teacher was a bald white guy... picture Steve Ballmer then):

You do not yet master this move. Sometimes you do and sometimes you don't, so you don't. When eating with a spoon, do you miss your mouth one out of five times?

And I carried that with me as a Software Engineer. 'The new feature works' means it works every time. Not four out of five times. And so the build is the same.

Experience has shown me that the best way to produce software in a fast and efficient way is to be able to build on your machine, and ideally even run it on your machine.

Now if your project is humongous that may be a problem, your system might not even have enough RAM to complete the build. A fallback is to rent a big server somewhere and run your builds here. It's not ideal but better than nothing.

Another hurdle is the code requiring some platform specific API, for example io_uring on Linux. What can help here is to implement a shim, or build inside a virtual machine on your workstation. Again, not ideal but better than nothing.

I have done all of the above in the past and that works but building directly on your machine is still the best option.

Get the tests passing on your machine

First, if there are no tests, I am sorry. This is going to be really difficult to do any change at all. So go write some tests before doing any change to the code, make them pass, and come back. The easiest way is to capture inputs and outputs of the program running in the real world and write end-to-end tests based on that, the more varied the better. It will ensure there are no regressions when making changes, not that the behavior was correct in the first place, but again, better than nothing.

So, now you have a test suite. If some tests fail, disable them for now. Make them pass, even if the whole test suite takes hours to run. We'll worry about that later.

Write down in the README how to build and test the application

Ideally it's one command to build and one for testing. At first it's fine if it's more involved, in that case the respective commands can be put in a build.sh and test.sh that encapsulate the madness.

The goal is to have a non C++ expert be able to build the code and run the tests without having to ask you anything.

Here some folks would recommend documenting the project layout, the architecture, etc. Since the next step is going to rip out most of it, I'd say don't waste your time now, do that at the end.

Find low hanging fruits to speed up the build and tests

Emphasis on 'low hanging'. No change of the build system, no heroic efforts (I keep repeating that in this article but this is so important).

Again, in a typical C++ project, you'd be amazed at how much work the build system is doing without having to do it at all. Try these ideas below and measure if that helps or not:

Once that's done, here are a few things to additionally try, although the gains are typically much smaller or sometimes negative:

Once the iteration cycle feels ok, the code gets to go under the microscope. If the build takes ages, it's not realistic to want to modify the code.

Remove all unnecessary code

Dad, I see dead lines of code.

(Get the reference? Well, ok then.)

I have seen 30%, sometimes more, of a codebase, being completely dead code. That's lines of code you pay for every time you compile, you want to make a refactoring, etc. So let's rip them out.

Here are some ways to go about it:

And the bonus for doing all of this, is not only that you sped up the build time by a factor of 5 with zero downside, is that, if your boss is a tiny bit technical, they'll love seeing PRs deleting thousands of lines of code. And your coworkers as well.


Don't go overboard with linter rules, add a few basic ones, incorporate them in the development life cycle, incrementally tweak the rules and fix the issues that pop up, and move on. Don't try to enable all the rules, it's just a rabbit hole of diminishing returns. I have used clang-tidy and cppcheck in the past, they can be helpful, but also incredibly slow and noisy, so be warned. Having no linter is not an option though. The first time you run the linter, it'll catch so many real issues that you'll wonder why the compiler is not detecting anything even with all the warnings on.

Code formatting

Wait for the appropriate moment where no branches are active (otherwise people will have horrendous merge conflicts), pick a code style at random, do a one time formatting of the entire codebase (no exceptions), typically with clang-format, commit the configuration, done. Don't waste any bit of saliva arguing about the actual code formatting. It only exists to make diffs smaller and avoid arguments, so do not argue about it!


Same as linters, it can be a rabbit hole, unfortunately it's absolutely required to spot real, production affecting, hard to detect, bugs and to be able to fix them. -fsanitize=address,undefined is a good baseline. They usually do not have false positives so if something gets detected, go fix it. Run the tests with it so that issues get detected there as well. I even heard of people running the production code with some sanitizers enabled, so if your performance budget can allow it, it could be a good idea.

If the compiler you (have to) use to ship the production code does not support sanitizers, you can at least use clang or such when developing and running tests. That's when the work you did on the build system comes in handy, it should be relatively easy to use different compilers.

One thing is for sure: even in the best codebase in the world, with the best coding practices and developers, the second you enable the sanitizers, you absolutely will uncover horrifying bugs and memory leaks that went undetected for years. So do it. Be warned that fixing these can require a lot of work and refactorings. Each sanitizer also has options so it could be useful to inspect them if your project is a special snowflake.

One last thing: ideally, all third-party dependencies should also be compiled with the sanitizers enabled when running tests, to spot issues in them as well.

Add a CI pipeline

As Bryan Cantrill once said (quoting from memory), 'I am convinced most firmware just comes out of the home directory of a developer's laptop'. Setting up a CI is quick, free, and automates all the good things we have set up so far (linters, code formatting, tests, etc). And that way we can produce in a pristine environment the production binaries, on every change. If you're not doing this already as a developer, I don't think you really have entered the 21st century yet.

Cherry on the cake: most CI systems allow for running the steps on a matrix of different platforms! So you can demonstrably check that the list of supported platforms is not just theory, it is real.

Typically the pipeline just looks like make all test lint fmt so it's not rocket science. Just make sure that issues that get reported by the tools (linters, sanitizers, etc) actually fail the pipeline, otherwise no one will notice and fix them.

Incremental code improvements

Well that's known territory so I won't say much here. Just that lots of code can often be dramatically simplified.

I remember iteratively simplifying a complicated class that manually allocated and (sometimes) deallocated memory, was meant to handle generic things, and so on. All the class did, as it turned out, was allocate a pointer, later check whether the pointer was null or not, and...that's it. Yeah that's a boolean in my book. True/false, nothing more to it.

I feel that's the step that's the hardest to timebox because each round of simplification opens new avenues to simplify further. Use your best judgment here and stay on the conservative side. Focus on tangible goals such as security, correctness and performance, and stray away from subjective criteria such as 'clean code'.

In my experience, upgrading the C++ standard in use in the project can at times help with code simplifications, for example to replace code that manually increments iterators by a for (auto x : items) loop, but remember it's just a means to an end, not an end in itself. If all you need is std::clamp, just write it yourself.

Rewrite in a memory safe language?

I am doing this right now at work, and that deserves an article of its own. Lots of gotchas there as well. Only do this with a compelling reason.


Well, there you have it. A tangible, step-by-step plan to get out of the finicky situation that's a complex legacy C++ codebase. I have just finished going through that at work on a project, and it's become much more bearable to work on it now. I have seen coworkers, who previously would not have come within a 10 mile radius of the codebase, now make meaningful contributions. So it feels great.

There are important topics that I wanted to mention but in the end did not, such as the absolute necessity of being able to run the code in a debugger locally, fuzzing, dependency scanning for vulnerabilities, etc. Maybe for the next article!

If you go through this on a project, and you found this article helpful, shoot me an email! It's nice to know that it helped someone.

Addendum: Dependency management

This section is very subjective, it's just my strong, biased opinion.

There's a hotly debated topic that I have so far carefully avoided and that's dependency management. So in short, in C++ there's none. Most people resort to using the system package manager, it's easy to notice because their README looks like this:

On Ubuntu 20.04: `sudo apt install [100 lines of packages]`

On macOS: `brew install [100 lines of packages named slightly differently]`

Any other: well you're out of luck buddy. I guess you'll have to pick a mainstream OS and reinstall ¯\_(ツ)_/¯

Etc. I have done it myself. And I think this is a terrible idea. Here's why:

So you're thinking, I know, I will use those fancy new package managers for C++, Conan, vcpkg and the like! Well, not so fast:

I mean, if you have a situation where they work for you, that's great, it's definitely an improvement over using system packages in my mind. It's just that I never encountered (so far) a project where I could make use of them - there was always some blocker.

So what do I recommend? Well, the good old git submodules and compiling from source approach. It's cumbersome, yes, but also:

Compiling each dependency in each submodule can be as simple as add_subdirectory with CMake, or git submodule foreach make by hand.

If submodules are really not an option, an alternative is to still compile from source but do it by hand, with one script, that fetches each dependency and builds it. Example in the wild: Neovim.

Of course, if your dependency graph visualized in Graphviz looks like a Rorschach test and has to build thousands of dependencies, it is not easily doable, but it might be still possible, using a build system like Buck2, which does hybrid local-remote builds, and reuses build artifacts between builds from different users.

If you look at the landscape of package managers for compiled languages (Go, Rust, etc), all of them that I know of compile from source. It's the same approach, minus git, plus the automation.

Addendum: suggestions from readers

I've gathered here some great ideas and feedback from readers (sometimes it's the almagamation of multiple comments from different people, and I am paraphrasing from memory so sorry if it's not completely accurate):

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This blog is open-source! The content of this blog as well as the code snippets are under the BSD-3 License which I also usually use for all my personal projects. It's basically free for every use but you have to mention me as the original author.