C++20

C++20 is the informal name for the revision of the ISO/IEC standard for the C++ programming language expected to follow C++17.[1] The standard became technically finalized[2] by WG21 at the meeting in Prague in February 2020.[3] The standard was expected to be officially published after the end of the DIS ballot in May 2020, but as of July 2020, it is still underway.[4]

C++20 adds more new major features than C++14 or C++17.[5] Below is a partial list of changes that have been accepted into or have been discussed for inclusion into C++20.[6]

New Features

C++20 introduced many new features. The following lists may be incomplete.

Language
  • concepts,[7] with terse syntax[8]
  • modules[9]
  • designated initializers[10] (based on the C99 feature, and common G++ extension)
  • [=, this] as a lambda capture[11]
  • template parameter lists on lambdas[12]
  • three-way comparison using the "spaceship operator", operator <=>
  • initialization of an additional variable within a range-based for statement[13]
  • lambdas in unevaluated contexts[14][15]
  • default constructible and assignable stateless lambdas[14][16]
  • allow pack expansions in lambda init-capture[14][17]
  • string literals as template parameters[14][18]
  • removing the need for typename in certain circumstances[19]
  • new standard attributes [[no_unique_address]],[20] [[likely]] and [[unlikely]][21]
  • conditional explicit, allowing the explicit modifier to be contingent on a boolean expression[22]
  • expanded constexpr: virtual functions,[23] union,[24] try and catch,[25] dynamic_cast and typeid,[26] std::pointer_traits[27]
  • immediate functions using the new consteval keyword[28]
  • signed integers are now defined to be represented using two's complement (signed integer overflow remains undefined behavior)[29]
  • a revised memory model[30]
  • various improvements to structured bindings (interaction with lambda captures, static and thread_local storage duration)[31][32]
  • coroutines[33]
  • using on scoped enums[34]
  • constinit keyword[35]

Library
  • ranges (The One Ranges Proposal)[36]
  • std::make_shared and std::allocate_shared for arrays[37]
  • atomic smart pointers (such as std::atomic<shared_ptr<T>> and std::atomic<weak_ptr<T>>)[38]
  • std::to_address to convert a pointer to a raw pointer[39]
  • calendar and time-zone additions to <chrono>[40]
  • std::span, providing a view to a contiguous array (analogous to std::string_view but span can mutate the referenced sequence)[41]
  • <version> header[42]
  • std::bit_cast<> for type casting of object representations, with less verbosity than memcpy() and more ability to exploit compiler internals[43]
  • feature test macros[44]
  • various constexpr library bits[45]
  • smart pointer creation with default initialization[46]
  • std::map::contains method[47]
  • bit operations, such as leading/trailing zero/one count[48], and log2 operations[49][50][51]

Deprecation
  • Use of comma operator in subscript expressions has been deprecated[52]
  • (most of) volatile has been deprecated[53]

New (and changed) keywords

Many new keywords added (and the new "spaceship operator", operator <=>), such as concept, constinit,[35] consteval, co_await, co_return, co_yield, requires (plus changed meaning for export), and char8_t.[54] And explicit can take an expression since C++20.[55] Most of the uses of the volatile keyword have been deprecated.[53]

In addition to keywords, there are identifiers with special meaning, including new import and module.

New attributes in C++20: [[likely]], [[unlikely]], and [[no_unique_address]][56]

History of changes

Changes applied to the C++20 working draft in July 2017 (Toronto) include:[57]

  • concepts
  • designated initializers
  • [=, this] as a lambda capture
  • template parameter lists on lambdas
  • std::make_shared and std::allocate_shared for arrays

Changes applied to the C++20 working draft in the fall meeting in November 2017 (Albuquerque) include:[58][59]

  • three-way comparison using the "spaceship operator", operator <=>
  • initialization of an additional variable within a range-based for statement
  • lambdas in unevaluated contexts
  • default constructible and assignable stateless lambdas
  • allow pack expansions in lambda init-capture
  • string literals as template parameters
  • atomic smart pointers (such as std::atomic<shared_ptr<T>> and std::atomic<weak_ptr<T>>)
  • std::to_address to convert a pointer to a raw pointer

Changes applied to the C++20 working draft in March 2018 (Jacksonville) include:[60]

  • removing the need for typename in certain circumstances[61]
  • new standard attributes [[no_unique_address]],[62] [[likely]] and [[unlikely]][63]
  • calendar and time-zone additions to <chrono>[64]
  • std::span, providing a view to a contiguous array (analogous to std::string_view but span can mutate the referenced sequence)[65]
  • <version> header[66]

Changes applied to the C++20 working draft in the summer meeting in June 2018 (Rapperswil) include:[67]

  • contracts deferred to a later standard[68]
  • feature test macros[69]
  • bit-casting of object representations, with less verbosity than memcpy() and more ability to exploit compiler internals[70]
  • conditional explicit, allowing the explicit modifier to be contingent on a boolean expression[71]
  • constexpr virtual functions[72]

Changes applied to the C++20 working draft in the fall meeting in November 2018 (San Diego) include:

  • ranges (The One Ranges Proposal)
  • concept terse syntax
  • constexpr union, try and catch, dynamic_cast, typeid and std::pointer_traits.
  • various constexpr library bits[73]
  • immediate functions using the new consteval keyword[74]
  • signed integers are now defined to be represented using two's complement (signed integer overflow remains undefined behavior)[75]
  • refinements of the contracts facility (access control in contract conditions)[76] (see list of features deferred to a later standard)
  • a revised memory model[77]
  • Smart pointer creation with default initialization[78]

Changes applied to the C++20 working draft in the winter meeting in February 2019 (Kona) include:[79][80]

  • coroutines[81] – already experimentally supported in Clang 5[82]
  • modules[83] – experimentally supported in Clang 5[84] and Visual Studio 2015 Update 1[85] as well as GCC[86]
  • various improvements to structured bindings (interaction with lambda captures, static and thread_local storage duration)[87][88]

Changes applied to the C++20 working draft in the summer meeting in July 2019 (Cologne) include:[89][90][91]

  • contracts have been removed (see list of features deferred to a later standard)[92]
  • use of comma operator in subscript expressions has been deprecated[93]
  • constexpr additions (trivial default initialization,[94] unevaluated inline-assembly[95])
  • using scoped enums[96]
  • various changes to the spaceship operator[97][98]
  • DR: minor changes to modules[99]
  • constinit keyword
  • changes to concepts (removal of -> Type return-type-requirements[100])
  • (most of) volatile has been deprecated[53]
  • DR: [[nodiscard]] effects on constructors[101]
  • The new standard library concepts will not use PascalCase (rather standard_case, as the rest of the standard library)[102]
  • text formatting[103][104] (chrono integration,[105] corner case fixes[106])
  • bit operations[107]
  • constexpr INVOKE[108]
  • math constants[109]
  • consistency additions to atomics (std::atomic_ref<T>,[110] std::atomic<std::shared_ptr<T>>[111])
  • add the <=> operator to the standard library[112]
  • header units for the standard library[113]
  • synchronization facilities[114] (merged from: Efficient atomic waiting and semaphores,[115] latches and barriers,[116] Improving atomic_flag,[117] Don't Make C++ Unimplementable On Small CPUs[118])
  • std::source_location[119]
  • constexpr containers (std::string,[120] std::vector[121])
  • std::stop_token and joining thread (std::jthread)[122]

Changes applied during the NB comment resolution in the fall meeting in November 2019 (Belfast) include:[123][124][125]

  • Class Types in Non-Type Template Parameters (NTTP): The restriction of no user-defined operator== allowed has been removed as the meaning of template argument equality has been divorced from operator==.[126] This allows also for array members in class-type NTTP.
  • Floating-point types,[127] pointers and references and unions and union-like classes (class types containing anonymous unions) are now allowed as NTTP.
  • Function identity now also includes trailing requires-clauses (P1971)
  • Constrained non-template functions have been removed
  • <compare> is now available in freestanding implementations[128]
  • std::spans typedef was changed from index_type to size_type to be consistent with the rest of the standard library[129]
  • Concept traits have been renamed to follow the renaming of the concepts as a result from the Cologne meeting
  • Several fixes and additions to ranges (P1456R1: Move-only views,[130] P1391R4: Range constructor for std::string_view (constructor from iterator-pair of characters),[131] P1394R4: Range constructor for std::span<ref>,[132] P1870R1: forwarding-range<T> is too subtle[133])
  • Initialization for std::atomic<T> has been changed to make it work with default and list initialization,[134] std::latch and std::barrier can now report the maximum number of threads that the implementation supports through the new member function max()
  • std::weak_equality and std::strong_equality have been removed as they are not used anymore
  • Algorithms in <numeric> have been made constexpr
  • Missing feature-test macros for new or changed features of C++20 have been added[135]

Features published as Technical Specifications

Features deferred to a later standard
  • Contracts – a new study group (SG21) has been formed to work on a new proposal[140]
  • Reflection[141][142]
  • Metaclasses[143]
  • Executors[144]
  • Networking extensions,[145][146] including async, basic I/O services, timers, buffers and buffer-oriented streams, sockets, and Internet protocols (blocked by executors)
  • Properties[147]
  • Extended futures[148]

See also

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  • Ranges (range-v3) github repository, by Eric Niebler
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