Maintaining and Migrating Working Code Changing Legacy Code with Confidence: Brian Ruth (he/him) brian.ruth@garmin.comPart 0: Legacy Code• It works* • It may not have tests, but has lifetime • It is currently making the company money • One person's bug is another's critical feature Legacy code has value *well enough, for some definition of "works"• Best practices and languages change developers may not have a firm understanding of project structure* • Supporting tools change The Legacy of Code *for some definition of "structure"• Preserve working code • Introduce testing • Gradual

v=90I0hH5-r5A A Case-study in Rewriting a Legacy GUI Library for Real-time Audio Software in Modern CLegacy Code …make sure new code is betterWhat is legacy code?What is legacy code? ● No tests ● Lotʼs of code TSANhttps://www.youtube.com/watch?v=vEtGtphI3lc Purging Undefined Behavior & Intel Assumptions in a Legacy C Codebase23 xkcd.com/2030/Improving with static analysis Success ● Incremental rollout of ASAN Unit Tests Integration E2EMoving up the testing pyramid Unit Tests DSP Regression VST3Changing legacy APIs should you?Breaking changes ● class template checked_value ● class template

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real-time audio plug-ins | music, film, television, and radio iZotope Inc.Glass Properties Making a legacy run-time system compile-time safe iZotope Inc.What do I mean by property system? iZotope Inc.What

pattern: a way to incrementally modernize a legacy system as defined by Martin Fowler.  Instead of building an entirely new system, we take a small piece of the legacy system and rebuild it in a way that lets the rest of the legacy system. We launch that piece into production and have users use it seamlessly as if it is part of the legacy system.  Then we take another piece out of the legacy system and do do the same thing. And another. And another. Until eventually there is nothing left of the legacy system—it has vanished, piece by piece, like Alice in Wonderland’s Cheshire Cat.  The strangler pattern

TODAY TOMORROW Quantity construction in mp-units always requires both a number and a unit. 69void legacy_func(std::int64_t seconds) { std::cout << seconds << " s\n"; } CppCon 2024: Improving our safety std::vector vec; // ... }; X x; x.vec.emplace_back(42s); legacy_func(x.vec[0].count()); void legacy_func(std::int64_t seconds) { std::cout << seconds << " s\n"; } CppCon 2024: std::vector vec; // ... }; X x; x.vec.emplace_back(42s); legacy_func(x.vec[0].count()); 42000000 s void legacy_func(std::int64_t seconds) { std::cout << seconds << " s\n"; } CppCon

before the first import! import std; int main () { std:: string const s = f(); }42/50 Modularizing legacy libraries - Include standard library early — File: lib.hpp #include std:: string f(); f(); — File: libm.cpp export module lib; export { #include }42/50 Modularizing legacy libraries - Include standard library early — File: lib.hpp #include std:: string f(); — File: Modularizing legacy libraries - Export names with using — File: lib.hpp class AwesomeType; — File: libm.cpp module; #include export module lib; using :: AwesomeType;44/50 Modularizing legacy libraries

so no runtime virtual calls overhead Drawbacks : • Templates all the way down • Hard to add in legacy code • Increased compilation times • More hieroglypical 28 Bloomberg Dependency Injection via are • Riskier • Take substantial time • Preference for Phased/localized changes 86 Bloomberg Legacy Code DI Need Tools & Tricks to implement Dependency InjectionProblem : API is used far and wide a constructed object 96 Bloomberg DI for lazy object constructionBloomberg 97 Phasing DI into Legacy Codebase class LazyObj { :public LazyObj(…, const std::string& index_name) : index_name_(index_name){};

you to to put the company logo on every screen saver. ● Your colleague is working on the actual legacy text rendering interface ○ text rendering involves: ■ loading font files, a custom string class are solving (1/3) ● We have some some text that must always be uniform ○ This is based off of a legacy or third-party ‘Text’ class ● There are potentially a lot of steps needed to “fit” our specifications are solving (2/3) ● We have some some text that must always be uniform ○ This is based off of a legacy or third-party ‘Text’ class ● There are potentially a lot of steps needed to “fit” our specifications

. 208 9.9.3 Access iSCSI targets via Internal Networking . . . . . . . . . . . . . . . 208 9.10 Legacy commands for using serial ports . . . . . . . . . . . . . . . . . . . . . . . 209 9.11 Fine-tuning Configuring automatic mouse capturing . . . . . . . . . . . . . . . . . . 226 9.20.13 Requesting legacy full-screen mode . . . . . . . . . . . . . . . . . . . . 226 9.21 Starting the VirtualBox web service driver and this cannot be changed. For Windows Vista and later the user can force to install the (legacy) NDIS5 host network filter driver using NETWORKTYPE=NDIS5. For example, to install the NDIS5 driver

. 207 9.9.3 Access iSCSI targets via Internal Networking . . . . . . . . . . . . . . . 207 9.10 Legacy commands for using serial ports . . . . . . . . . . . . . . . . . . . . . . . 208 9.11 Fine-tuning automatic mouse capturing . . . . . . . . . . . . . . . . . . 225 6 Contents 9.20.13 Requesting legacy full-screen mode . . . . . . . . . . . . . . . . . . . . 225 9.21 Starting the VirtualBox web service driver and this cannot be changed. For Windows Vista and later the user can force to install the (legacy) NDIS5 host network filter driver using NETWORKTYPE=NDIS5. For example, to install the NDIS5 driver