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Summary of C++17 features

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The presentation shows major features of the new C++ standard (language and the library). The full list of new things is very broad, so I've categorized them to be easier to understand.

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Summary of C++17 features

  1. 1. Summary of C++17 features Is this release good, great, amazing or meh? Bartłomiej Filipek http://www.bfilipek.com/ Blog version, annotated Last update: 25th Nov 2017
  2. 2. About me www.bfilipek.com 10y+ experience Currently @Xara.com Text related features Somehow addicted to Cpp :D
  3. 3. The plan C++17 timeline What we get Fixes and deprecation Language clarification Templates Attributes Simplification Filesystem Parallel STL Utils Summary
  4. 4. Timeline The new standard for C++17 was approved at the beginning of September. But what was the journey to that point? How it all started?
  5. 5. https://isocpp.org/std/status Here’s the up-to-date version of the timeline for the C++ standard. Around 13 years we need to wait for C++11 (since 98), but then the work speeded up! It’s clear from that picture how much work is going on. There’s the current trunk, plus several major branches with larger features (TS). At some point those branches are merged into the trunk. The committee meets several times per year and votes on the proposals.
  6. 6. https://isocpp.org/std/the-committee Here’s the overview of all the groups that work on new features. Each larger feature has it’s own group!
  7. 7. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4700.pdf https://github.com/cplusplus/draft If you have problem with sleep just download the draft for the standard (for free) and start reading!
  8. 8. Ship every 3 years As I see here’s the major changed that happened in the standardization process: C++11 was so late (we expected it to be before 2010!), but the committee waited until all the features were ready. Now we focus on the deadline. If a feature is not ready, then it’s simply moved to the next standard.
  9. 9. Merged: The Library Fundamentals 1 TS (most parts) Removal of some deprecated types and functions, including std::auto_ptr, std::random_shuffle, and old function adaptors Merged: The Parallelism TS, a.k.a. “Parallel STL.”, Merged: File System TS, Merged: The Mathematical Special Functions IS, Improving std::pair and std::tuple std::shared_mutex (untimed) Variant, Optional, Any Splicing Maps and Sets New auto rules for direct-list-initialization static_assert with no message typename in a template template parameter Removing trigraphs Nested namespace definition Attributes for namespaces and enumerators u8 character literals Allow constant evaluation for all non-type template arguments Fold Expressions Unary fold expressions and empty parameter packs Remove Deprecated Use of the register Keyword Remove Deprecated operator++(bool) Removing Deprecated Exception Specifications from C++17 Make exception specifications part of the type system Aggregate initialization of classes with base classes Lambda capture of *this Using attribute namespaces without repetition Dynamic memory allocation for over-aligned data __has_include in preprocessor conditionals Template argument deduction for class templates Non-type template parameters with auto type Guaranteed copy elision New specification for inheriting constructors (DR1941 et al) Direct-list-initialization of enumerations Stricter expression evaluation order constexpr lambda expressions Different begin and end types in range-based for [[fallthrough]] attribute [[nodiscard]] attribute [[maybe_unused]] attribute Ignore unknown attributes Pack expansions in using-declarations Structured Binding Declarations Hexadecimal floating-point literals init-statements for if and switch Inline variables DR: Matching of template template-arguments excludes compatible templates std::uncaught_exceptions() constexpr if-statements 1. Fixes and deprecation 2. Language clarification 3. Templates 4. Attributes 5. Simplification 6. Filesystem 7. Parallel STL 8. Utils
  10. 10. Deprecation & Removal Removing trigraphs Removing register keyword Remove Deprecated operator++(bool) Cannot inherit from std::iterator
  11. 11. auto_ptr is gone! MSVC 2017 using /std:c++latest: error C2039: 'auto_ptr': is not a member of 'std'
  12. 12. Removing Deprecated Exception Specifications from C++17 void fooThrowsInt(int a) throw(int) { printf_s("can throw intsn"); if (a == 0) throw 1; } Use “noexcept”
  13. 13. Fixes static_assert with no message New auto rules for direct-list-initialization auto x5{ 3 }; // decltype(x5) is int, not initializer_list Different begin and end types in range-based for { auto && __range = for - range - initializer; for (auto __begin = begin - expr, __end = end - expr; __begin != __end; ++__begin) { for_range_declaration = *__begin; statement } } { auto && __range = for - range - initializer; auto __begin = begin - expr; auto __end = end - expr; for (; __begin != __end; ++__begin) { for_range_declaration = *__begin; statement } }
  14. 14. Language clarifications Guaranteed copy elision (only for temporary objects, not for Named RVO) Exception specifications part of the type system void foo() noexcept; void foo(); Dynamic memory allocation for over-aligned data new is now aware of the alignment of the object.
  15. 15. Stricter expression evaluation order Why do we need make_unique? foo(make_unique<T>(), otherFunction()); foo(unique_ptr<T>(new T), otherFunction());
  16. 16. Templates
  17. 17. Template argument deduction for class templates std::pair<int, double> p(10, 0.0); // same as std::pair p(10, 0.0); // deduced automatically! std::lock_guard<std::shared_timed_mutex, std::shared_lock<std::shared_timed_mutex>> lck(mut_, r1); // Can now become : std::lock_guard lck(mut_, r1); custom class template deduction guides
  18. 18. Fold expressions auto SumCpp11() { return 0; } template<typename T1, typename... T> auto SumCpp11(T1 s, T... ts) { return s + SumCpp11(ts...); } template<typename ...Args> auto sum(Args ...args) { return (args + ... + 0); } template<typename ...Args> void FoldPrint(Args&&... args) { (cout << ... << forward<Args>(args)) << 'n'; }
  19. 19. // SFINAE template <typename T, std::enable_if_t < std::is_pointer<T>{} > * = nullptr > auto get_value(T t) { return *t; } template <typename T, std::enable_if_t < !std::is_pointer<T>{} > * = nullptr > auto get_value(T t) { return t; } // Tag dispatching template <typename T> auto get_value(T t, std::true_type) { return *t; } template <typename T> auto get_value(T t, std::false_type) { return t; } template <typename T> auto get_value(T t) { return get_value(t, std::is_pointer<T>{}); }https://blog.tartanllama.xyz/if-constexpr/ template <typename T> auto get_value(T t)
  20. 20. constexpr if template <typename T> auto get_value(T t) { if constexpr (std::is_pointer_v<T>) return *t; else return t; }
  21. 21. Attributes [[fallthrough]] [[nodiscard]] [[maybe_unused]] @cppreference.com Attributes available in C++17 [[noreturn]] [[carries_dependency]] [[deprecated]] [[deprecated("msg")]] [[fallthrough]] [[nodiscard]] [[maybe_unused]] enum class[[nodiscard]] ErrorCode{ OK, Fatal, System, FileIssue }; ErrorCode Compute() { ... } ... Compute(); // warning!
  22. 22. How will it simplify the code? Inline variables Great for header-only libs! constexpr if
  23. 23. Structured Binding Declarations // works with arrays: double myArray[3] = { 1.0, 2.0, 3.0 }; auto[a, b, c] = myArray; auto[a, b] = myPair; // binds myPair.first/second (must implement get<N>) // non static, public members struct S { int x1; double y1; }; S f(); const auto[x, y] = f(); std::map myMap; for (const auto &[key, val] : myMap) { }
  24. 24. Init-statement for if/switch { auto val = GetValue(); if (condition(val)) // on success else // on false... } if (auto val = GetValue(); condition(val)) // on success else // on false... if (const auto it = myString.find("Hello"); it != std::string::npos) std::cout << it << " Hellon"; if (const auto it = myString.find("World"); it != std::string::npos) std::cout << it << " Worldn";
  25. 25. Structured bindings & init in if statements // better together: structured bindings + if initializer if (auto[iter, succeeded] = mymap.insert(value); succeeded) { use(iter); // ok // ... } // iter and succeeded are destroyed here
  26. 26. Filesystem namespace fs = std::experimental::filesystem; void DisplayDirTree(const fs::path& pathToShow, int level) { if (fs::exists(pathToShow) && fs::is_directory(pathToShow)) { auto lead = std::string(level * 3, ' '); for (const auto& entry : fs::directory_iterator(pathToShow)) { auto filename = entry.path().filename(); if (fs::is_directory(entry.status())) { cout << lead << "[+] " << filename << "n"; DisplayDirTree(entry, level + 1); cout << "n"; } else if (fs::is_regular_file(entry.status())) DisplayFileInfo(entry, lead, filename); else cout << lead << " [?]" << filename << "n"; } } }
  27. 27. Parallel STL std::vector<int> v = genLargeVector(); // standard sequential sort std::sort(v.begin(), v.end()); // explicitly sequential sort std::sort(std::seq, v.begin(), v.end()); // permitting parallel execution std::sort(std::par, v.begin(), v.end()); // permitting vectorization as well std::sort(std::par_unseq, v.begin(), v.end());
  28. 28. Utils std::any std::variant std::optional string_view Searchers http://www.bfilipek.com/2017/09/cpp17-details-utils.html auto a = std::any(12); a = std::string("hello world"); a = 10.0f; cout << std::any_cast<float>(a); std::variant<int, float, std::string> abc; // get<> // get_if<> // visit() std::optional<std::string> ostr = GetUserResponse(); default_searcher boyer_moore_searcher boyer_moore_horspool_searcher
  29. 29. Compiler support GCC 7.0 is feature complete Clang 5.0 MSVC 2017.4 has most of the features The separate part is the standard library for each compiler. They might be delayed, especially regarding parallel versions of the algorithms.
  30. 30. Summary Is C++17 great? Amazing? Or just meh… ? I think it’s a very decent version of the standard! Maybe people would like to see more, but it was not possible due to the deadline. Without this we would have to wait a year or two to have all the “desired” features to be accepted.
  31. 31. Bonus – seemyfreePDFwithdescriptionofalloftheC++17 features! http://www.bfilipek.com/2017/09/c17-in-detail-summary-bonus.html
  32. 32. https://isocpp.org/

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