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))

should be implemented manually before calling any of these functions. Note Due to potential circular dependency issues, it’s recommended to import the api in the functions that uses its function. e.g. use: instead helps prevent a range of security vulnerabilities stemming from HTML, JavaScript, and CSS Code Injection. To change this behaviour, the project administrator should carefully review each use of this template helps prevent a range of security vulnerabilities stemming from HTML, JavaScript, and CSS Code Injection. To change this behaviour, the project administrator should carefully review each use of this template

be implemented manually before calling any of these functions. Warning Due to potential circular dependency issues, it’s recommended to import the api in the functions that uses its function. e.g. use: def helps prevent a range of security vulnerabilities stemming from HTML, JavaScript, and CSS Code Injection. To change this behaviour, the project administrator should carefully review each use of this template helps prevent a range of security vulnerabilities stemming from HTML, JavaScript, and CSS Code Injection. To change this behaviour, the project administrator should carefully review each use of this template

be implemented manually before calling any of these functions. Warning Due to potential circular dependency issues, it’s recommended to import the api in the functions that uses its function. e.g. use: def helps prevent a range of security vulnerabilities stemming from HTML, JavaScript, and CSS Code Injection. To change this behaviour, the project administrator should carefully review each use of this template helps prevent a range of security vulnerabilities stemming from HTML, JavaScript, and CSS Code Injection. To change this behaviour, the project administrator should carefully review each use of this template

implemented manually before calling any of these functions. Warning: Due to potential circular dependency issues, it’s recommended to import the api in the functions that uses its function. e.g. use: def helps prevent a range of security vulnerabilities stemming from HTML, JavaScript, and CSS Code Injection. To change this behaviour, the project administrator should carefully review each use of this template helps prevent a range of security vulnerabilities stemming from HTML, JavaScript, and CSS Code Injection. To change this behaviour, the project administrator should carefully review each use of this template