C++20 (original) (raw)

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2020 edition of the C++ programming language standard

C++20 is a version of the ISO/IEC 14882 standard for the C++ programming language. C++20 replaced the prior version of the C++ standard, called C++17, and was later replaced by C++23.[1] The standard was technically finalized[2] by WG21 at the meeting in Prague in February 2020,[3] had its final draft version announced in March 2020,[4] was approved on 4 September 2020,[5][6] and published in December 2020.[7]

C++20 adds more new major features than C++14 or C++17.[8] Changes that have been accepted into C++20 include:[9]

• concepts,[10] with terse syntax[11]
• modules[12]
• designated initializers[13] (based on the C99 feature, and common g++ extension)
• [=, this] as a lambda capture[14]
• template parameter lists on lambdas[15]
• three-way comparison using the "spaceship operator", operator <=>
• initialization of an additional variable within a range-based for statement[16]
• lambdas in unevaluated contexts[17][18]
• default constructible and assignable stateless lambdas[17][19]
• allow pack expansions in lambda init-capture[17][20]
• class types in non-type template parameters, also allowing string literals as template parameters[21]
• removing the need for typename in certain circumstances[22]
• new standard attributes [[no_unique_address]],[23] [[likely]] and [[unlikely]][24]
• conditional explicit, allowing the explicit modifier to be contingent on a boolean expression[25]
• expanded constexpr: virtual functions,[26] union,[27] try and catch,[28] dynamic_cast and typeid,[29] std::pointer_traits[30]
• immediate functions using the new [consteval](/wiki/Consteval "Consteval") keyword[31]
• signed integers are now defined to be represented using two's complement (signed integer overflow remains undefined behavior)[32]
• a revised memory model[33]
• various improvements to structured bindings (interaction with lambda captures, static and thread_local storage duration)[34][35]
• coroutines[36]
• using on scoped enums[37]
• constinit keyword[38]
• ranges (The One Ranges Proposal)[39]
• std::make_shared and std::allocate_shared for arrays[40]
• atomic smart pointers (such as std::atomic<shared_ptr<T>> and std::atomic<weak_ptr<T>>)[41]
• std::to_address to convert a pointer to a raw pointer[42]
• calendar and time-zone additions to <chrono>[43]
• std::span, providing a view to a contiguous array (analogous to std::string_view but span can mutate the referenced sequence)[44]
• std::erase and std::erase_if, simplifying element erasure for most standard containers[45]
• <version> header[46]
• std::bit_cast<> for type casting of object representations, with less verbosity than memcpy() and more ability to exploit compiler internals[47]
• feature test macros[48]
• various constexpr library bits[49]
• smart pointer creation with default initialization[50]
• contains-method for associative containers[51]
• bit operations, such as leading/trailing zero/one count,[52] and log2 operations[53][54][55]
• std::bind_front[56]

New and changed keywords

[edit]

Many new keywords added (and the new "spaceship operator", operator <=>), such as concept, constinit,[38] consteval, co_await, co_return, co_yield, requires (plus changed meaning for export), and char8_t (for UTF-8 support).[57] And explicit can take an expression since C++20.[58] Most of the uses of the [volatile](/wiki/Volatile%5F%28computer%5Fprogramming%29 "Volatile (computer programming)") keyword have been deprecated.[59]

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]][60]

Removed and deprecated

[edit]

Removed features:[61]

• The C-derived headers <ccomplex>, <ciso646>, <cstdalign>, <cstdbool> and <ctgmath> were removed, as they serve no purpose in C++. (The corresponding <*.h> headers remain, for compatibility with C.)
• The use of throw() as an exception specification was removed.
• Some previously deprecated library features were removed, including std::uncaught_exception, std::raw_storage_iterator, std::is_literal_type, std::is_literal_type_v, std::result_of and std::result_of_t.

Deprecated features:

• Use of comma operator in subscript expressions has been deprecated[62]
• (most of) [volatile](/wiki/Volatile%5F%28computer%5Fprogramming%29 "Volatile (computer programming)") has been deprecated[59]

Published as Technical Specifications

[edit]

• Parallelism TS v2[63] (including task blocks)
• Reflection TS v1[64]
• Networking TS v1[65]

Deferred to a later standard

[edit]

• Contracts – a new study group (SG21) has been formed to work on a new proposal[66]
• Reflection[67][68]
• Metaclasses[69]
• Executors[70]
• Networking extensions,[71][72] including async, basic I/O services, timers, buffers and buffer-oriented streams, sockets, and Internet protocols (blocked by executors)
• Properties[73]
• Extended futures[74]

Full support[75]

• Visual Studio 2019 supports all C++20 features through its /std:c++latest option, as of version 16.10.0.[76] An option /std:c++20 to enable C++20 mode is added in version 16.11.0.[77][78]

Microsoft's compiler supports not only Windows but also Linux, Android, and iOS. However, for Linux development, it requires the "Visual C++ for Linux Development" extension. [79]

Partial

• Clang has partial C++20 support that can be enabled with the option -std=c++20 (version 10 and later) or -std=c++2a (version 9 and earlier).[80]
• EDG started implementing C++20 features in version 5.0 and as of version 6.1 supports most C++20 core language features.[81]
• GCC added partial, experimental C++20 support in 2017[82] in version 8 through the option -std=c++2a. Like Clang, GCC replaced this option with -std=c++20 in version 10. It also has an option to enable GNU extensions in addition to the experimental C++20 support, -std=gnu++20.[83]

Changes applied to the C++20 working draft in July 2017 (Toronto) include:[84]

• concepts (what made it into the standard is a cut-down version; also described as "Concepts Lite"[85])
• 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:[86][87]

• 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:[88]

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

Changes applied to the C++20 working draft in the summer meeting in June 2018 (Rapperswil) include:[89]

• contracts (later deferred to a later standard)[90]
• feature test macros
• bit-casting of object representations, with less verbosity than memcpy() and more ability to exploit compiler internals
• conditional explicit, allowing the explicit modifier to be contingent on a boolean expression
• constexpr virtual functions

Changes applied to the C++20 working draft in the fall meeting in November 2018 (San Diego) include:[91]

• ranges (The One Ranges Proposal)
• concept terse syntax
• constexpr union, try and catch, dynamic_cast, typeid and std::pointer_traits.
• various constexpr library bits
• immediate functions using the new consteval keyword
• signed integers are now defined to be represented using two's complement (signed integer overflow remains undefined behavior)
• refinements of the contracts facility (access control in contract conditions)[92] (see list of features deferred to a later standard)
• a revised memory model
• smart pointer creation with default initialization

Changes applied to the C++20 working draft in the winter meeting in February 2019 (Kona) include:[93][94][95]

• coroutines
• modules
• various improvements to structured bindings (interaction with lambda captures, static and thread_local storage duration)

Changes applied to the C++20 working draft in the summer meeting in July 2019 (Cologne) include:[96][97][98][99]

• contracts were removed (see list of features deferred to a later standard)[100]
• use of comma operator in subscript expressions has been deprecated[62]
• constexpr additions (trivial default initialization,[101] unevaluated inline-assembly[102])
• using scoped enums[37]
• various changes to the spaceship operator[103][104]
• DR: minor changes to modules[105]
• constinit keyword
• changes to concepts (removal of -> Type return-type-requirements[106])
• (most of) [volatile](/wiki/Volatile%5F%28computer%5Fprogramming%29 "Volatile (computer programming)") has been deprecated[59]
• DR: [[nodiscard]] effects on constructors[107]
• The new standard library concepts will not use PascalCase (rather standard_case, as the rest of the standard library)[108]
• text formatting (std::format, [109][110] chrono integration,[111] corner case fixes[112])
• bit operations[52]
• constexpr INVOKE[113]
• math constants[114]
• consistency additions to atomics (std::atomic_ref<T>,[115] std::atomic<std::shared_ptr<T>>[116])
• add the <=> operator to the standard library[117]
• header units for the standard library[118]
• synchronization facilities[119] (merged from: Efficient atomic waiting and semaphores,[120] latches and barriers,[121] Improving atomic_flag,[122] Don't Make C++ Unimplementable On Small CPUs[123])
• std::source_location[124]
• constexpr containers (std::string,[125] std::vector[126])
• std::stop_token and joining thread (std::jthread)[127]

Changes applied during the NB comment resolution in the fall meeting in November 2019 (Belfast) include:[128][129][130][131]

• 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==.[132] This allows also for array members in class-type NTTP.
• Floating-point types,[133] 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[134]
• std::spans typedef was changed from index_type to size_type to be consistent with the rest of the standard library[135]
• 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,[136] P1391R4: Range constructor for std::string_view (constructor from iterator-pair of characters),[137] P1394R4: Range constructor for std::span<ref>,[138] P1870R1: forwarding-range is too subtle[139])
• Initialization for std::atomic<T> has been changed to make it work with default and list initialization,[140] 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[141]

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• Link to purchase ISO/IEC 14882:2020 from the ISO online store.
• N4860, the final draft version of the standard.
• JTC1/SC22/WG21 – the ISO/IEC C++ Standard Working Group (a.k.a. the C++ Standards Committee)
• Ranges (range-v3) GitHub repository, by Eric Niebler