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

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

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 14.3 Python requires (legacy) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 14.4 Creating Release 1.46.2 your modified recipe from the one in the Conan Center or any other remote. These are legacy packages, and the ones without user and channel are the ones strongly recommended to use from ConanCenter uploaded to ConanCenter before July 1st (new packages are no longer uploaded to this remote), as well as legacy packages with full reference (zlib/1.2.11@conan/stable). These package binaries were created by users

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 14.3 Python requires (legacy) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 14.4 Creating test_requires() to define test requirements instead of the legacy self.build_requires(..., force_host_context). • Use self.tool_requires() to define the legacy build_requires. Listing 3: From: from conans import “coordinates” of the current URL and commit. The new conan.tools.scm.Git can be used for this (do not use the legacy Git helper but this one) • The export() method, after capturing the coordinates, can store them

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 14.3 Python requires (legacy) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 14.4 Creating test_requires() to define test requirements instead of the legacy self.build_requires(..., force_host_context). • Use self.tool_requires() to define the legacy build_requires. Listing 3: From: from conans import “coordinates” of the current URL and commit. The new conan.tools.scm.Git can be used for this (do not use the legacy Git helper but this one) • The export() method, after capturing the coordinates, can store them