David Olsen, CppCon, 17 Sep 2020 BACK TO BASICS: MOVE SEMANTICS2 David Olsen — Back to Basics: Move Semantics — CppCon 2020 #include The Situation It’s 2003… You need to write code that creates Olsen — Back to Basics: Move Semantics — CppCon 2020 #include typedef std::map dictionary_t;4 David Olsen — Back to Basics: Move Semantics — CppCon 2020 #include typedef std::map dictionary_t;5 David Olsen — Back to Basics: Move Semantics — CppCon 2020 #include typedef std::map dictionary_t; dictionary_t

C++ Move Semantics Back to Basics: Move Semantics NICOLAI JOSUTTIS Cppcon 罗 AN Nicolai M. Josuttis C++ Move Semantics C++ Move Semantics Motivation BE josuttis | | eckstein Crpmaae Tcommunication Copy Semantics std: :vector coll1 coll.reserve(3) ; Staclc ，Heap: stad: :string sfgetData() }

Back to Basics: RValue and Move Semantics, Amir Kirsh @ CppCon, 2024About me 2 Back to Basics: RValue and Move Semantics, Amir Kirsh @ CppCon, 2024 Lecturer Academic College of Tel-Aviv-Yaffo Tel-Aviv (C++, but not only)The motivation for Move Semantics 3 Back to Basics: RValue and Move Semantics, Amir Kirsh @ CppCon, 2024The motivation for Move Semantics Godzilla g1 = factory.createFrighteningGodzilla(); RValue and Move Semantics, Amir Kirsh @ CppCon, 2024The motivation for Move Semantics Avoid redundant copying when we can steal from a dead object! 5 Back to Basics: RValue and Move Semantics, Amir Kirsh

know Highly Regarded Principle: std::variant always has a value Rule through Desired Semantics meeting Implementation Reality Exception: valueless_by_exception • std::variant::val b0 } b0 b1 b2 Stack-based objects have outstanding lifecycle symmetry (Guaranteed!) Value Semantics is preferred in Modern C++, due to superior symmetry in object lifecycle Use whenever possible leaked instances • Allocation amortization • Whole-system reset • Resource prioritization and recovery • Resource metrics and runtime monitoring Composable!Charley Bay - charleyb123 at gmail dot com

Goals ▶ Read optimized ▶ Persistence needs no flush or fence after first read ▶ Runtime over recovery ▶ Persist as little as possible ▶ Compact representation and few cache misses ▶ Arrays ▶ Open 25Introduction Persistent Hash Map Persistent Transactional Data Structures Live Demonstration References Recovery ▶ Only persist keys and values ▶ Filesystem data (name, size) infers contents ▶ DAX (Direct Access) mode ▶ Link-and-persist ensures data is consistent ▶ Orphans are possible but discarded during recovery 1024 bits 512 bits 256 bits A Persistent Hash Map for Graph Processing Workloads and a Methodology

30SAFETY ▪ It is a tradeoff… ▪ Performance ▪ Correctness ▪ This is a bitter pill to swallow ▪ Recovery ▪ Getting the system back into a known state ▪ Halting the system not always appropriate 31UNDEFINED environments will have different semantics P2759 DG OPINION ON SAFETY FOR ISO C++P2759 DG OPINION ON SAFETY FOR ISO C++ ▪ Collection of restrictions, requirements, related semantics that define a safety property ban unchecked subscripting, but instead provides runtime checked alternative ▪ Do not change semantics of a valid program ▪ With the exception of UB <==> well-defined behavior ▪ Open (hard) questions:

string_view, span, optional, expected etc., raise new questions regarding values types, whole-part semantics, copies, composite objects, ordering and equality. Designing and implementing regular types Regular types Values Objects Concepts Ordering Relations Requirements Equality Whole-part semantics2023 Victor Ciura | @ciura_victor - Regular, Revisited 17 Slide Title This talk is not just Regular types Values Objects Concepts Ordering Relations Requirements Equality Whole-part semantics Lifetimes2023 Victor Ciura | @ciura_victor - Regular, Revisited 17 Slide Title This talk

– Compositional semantics of independently developed libraries or program parts – Explicit expression of boundaries and dependencies in code 2. Isolation – Self-contained semantics of a component – Generally obtained as result of (1) + (2) 4. Modern semantics-aware developer tools – Bring C++ development experience to next level – Semantics-based browsing, transformation, completion, etc. 3 (C) implementation in Visual C++ – Increased safety, better ODR violation mitigation – Focus on high level semantics; see GDR’s talk on C++ representations ▪ What a C++ programmer can do to benefit from these achievements

... } // ODR violation :( // 1.cpp -fcontract-semantics=enforce void f(int i) pre (i >= 0) { // ... } // 2.cpp -fcontract-semantics=ignore void f(int i) pre (i >= 0) { // ... assert(doesNotExist); // compiles // ... } // ...until you turn off NDEBUG :( // 1.cpp -fcontract-semantics=ignore void g() { contract_assert(doesNotExist); // ... ~~~~~~~~~~~~~~~~~~~ https://timur.audio A contract assertion can be evaluated with one of the following evaluation semantics: • ignore: do not check the predicate (but still parse it)119 Copyright (c) Timur Doumler |

secure. The bytecode can’t be trusted, and invariants must be checked on top of interpretation of semantics. Classes can be loaded at runtime, making dynamic dispatch near-impossible to determine statically the interpreter can jump into the middle of the function while it executes, while preserving the semantics.Design of the Java HotSpot Client Compiler for Java 6 The compiler creates debugging information just the Web. WebAssembly has been designed with a formal semantics from the start. We describe the motivation, design and formal semantics of WebAssembly and provide some preliminary experience with