Bloomberg 1© 2024 Bloomberg Finance L.P. All rights reserved. Dependency Injection in C++: A Practical Guide CppCon 2024 September 18, 2024 Pete Muldoon Senior Engineering LeadBloomberg 4 Questions to use it8 Bloomberg Where will we be going ? • Talk will be about inserting meaningful Dependency Injection in applications • Using various DI methods to achieve functionality swapping / instrumentation Code (or just giving up) • Talk is rooted in a real-world system not theory9 Bloomberg Dependency Injection : 1. Decreases coupling between functionality blocks 2. Used to make a class/function independent

1October 3, 2023 2 Libraries: A First Step Toward Standard C++ Dependency Management Bret Brown, C++ Infrastructure Lead, Bloomberg Bill Hoffman, CTO, KitwareHello! Welcome! Bret Brown C++ Infrastructure Projects as portable as the code they contain! ● ✓ Projects should be “cattle,” not “pets”! 7Why dependency management? 8Consensus: Managing dependencies == way too hard Q: Which of these do you find frustrating What’s a dependency bug? Missing interfaces: fangorn.cxx:2:10: fatal error: jsonlog/core.hxx: No such file or directory 10 This could be caused by other root problems!Pain: What’s a dependency bug

com | 16 Global Variables Design Patterns Singleton Meyers Singleton Monostate Dependency Injection (DI) Timelineatomosspace.com | 17 • Hold one copy of global data/interface and allow others global object • Injects the dependency into each of the using objects New Trend – Dependency Injectionatomosspace.com | 23 Aside: Dependency Injection (DI) Vs Dependency Inversion Principle (DIP)atomosspace (DIP)atomosspace.com | 24 • Eliminates the dependency by inverting and adding an interface class • Reduces volatility due to implementation • Allows for testing and mocking Dependency Inversion Principle (DIP) PlotterInterface

assignment)Drawbacks of a Singleton • Acts as hidden dependencies in functions that use it • No dependency injection for testing • Initialization is out of your control • Multiple runs can yield different must be - at least - source compatible • Express the involvement of outside agencies • Allow dependency injection for testing purposes Bloomberg 12Bloomberg 13 // processor.cpp Response sendData(const Data& function must be source compatible  • Express the involvement of an outside agency  • Allow dependency injection for testing purposes  // processor.h Response sendData(const Data& data, CommSingleton*

or_else(return_empty_response) .value(); }DI and Functional Programming ● With Dependency Injection (DI)! template using ServiceCallback = std::function mw_; Map map_; };DI and Functional Programming ● With Dependency Injection (DI)! ○ DI is used to move or “inject” objects into another object template mw); private: std::unique_ptr mw_; Map map_; };● With Dependency Injection (DI)! ○ DI is used to move or “inject” objects into another object ● There still needs

== 6 ); REQUIRE( Factorial(10) == 3628800 ); }MOCKING / DEPENDENCY INJECTIONMOCKING / DEPENDENCY INJECTIONMOCKING / DEPENDENCY INJECTION #include class MockFS : public FS { public: MOCK_METHOD(int, write, (int fd, const void *buf, size_t len), (override)); };MOCKING / DEPENDENCY INJECTION using ::testing::Return; ... EXPECT_CALL(filesystem, read()) .Times(4) .WillOnce(Return(100))

Agenda Build systems and build system generators Dependency management Debugging TestingAgenda Build systems and build system generators Dependency management Demo Debugging Testing DemoBUILD SYSTEMS Total Managing libraries my application depends on 46.54% 478 38.56% 396 14.90% 153 1,027Dependency management: the problems Source: ISO C++ 2020 Developer Survey Major paint point Minor paint application depends on 46.54% 478 38.56% 396 14.90% 153 1,027 Automatic and reproducible dependency installation Single source of truth Consistency across platformsDEPENDENCY MANAGEMENT The

Driven Conversion Decision Coverage Dependency Destructive Domain Dynamic End-to-end Endurance Equivalence Partitioning Error-Handling Exploratory Fault injection Formal verification Functional REQUIRE( list.empty() == false ); REQUIRE( list.size() == 1 ); } Multiple asserts Dependency76 Challenges & Pitfalls Testing of The77 Challenges & Pitfalls Testing of The ‣ Flakey tests Principle Open-Closed Principle Liskov Substitution Principle Interface Segregation Principle Dependency Inversion Principle SRP OCP LSP ISP DIPSimplicity seekingBack To Basics PHIL NASH Testing

2. Code Safety 3. Cross-platform development 4. Developer and Team ProductivitySimplify C++ dependency management with vcpkg Open source library manager for Windows, Linux, and macOS 1400+ popular GitHub – Stephan T. Lavavej Wednesday 16th • Dynamic Polymorphism with Metaclasses and Code Injection – Sy Brand • Cross-Platform Pitfalls and How to Avoid Them – Erika Sweet • Effective Remote C++

test the SW Simulation testing SQA testing Performance testing HW testing Fuzzing Fault injection testing Stress testing Reliability testing Regression testing Testing with Sanitizers debt reviews, Security/Compliance tracking, Established testing framework for upgrades, Proper dependency mgmt. (e.g., Conan), solid documentation, ...Safety Critical Path vs Non-Real Time Domain Domain