C++20

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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.^[1] The standard was technically finalized^[2] by WG21 at the meeting in Prague in February 2020,^[3] approved on 4 September 2020,^[4][5] and published in December 2020.^[6]

Features[]

C++20 adds more new major features than C++14 or C++17.^[7] Changes that have been accepted into C++20 include:^[8]

Language[]

• concepts,^[9] with terse syntax^[10]
• modules^[11]
• designated initializers^[12] (based on the C99 feature, and common g++ extension)
• [=, this] as a lambda capture^[13]
• template parameter lists on lambdas^[14]
• three-way comparison using the "spaceship operator", operator <=>
• initialization of an additional variable within a range-based for statement^[15]
• lambdas in unevaluated contexts^[16][17]
• default constructible and assignable stateless lambdas^[16][18]
• allow pack expansions in lambda init-capture^[16][19]
• string literals as template parameters^[16][20]
• removing the need for typename in certain circumstances^[21]
• new standard attributes [[no_unique_address]],^[22] [[likely]] and [[unlikely]]^[23]
• conditional explicit, allowing the explicit modifier to be contingent on a boolean expression^[24]
• expanded constexpr: virtual functions,^[25] union,^[26] try and catch,^[27] dynamic_cast and typeid,^[28] std::pointer_traits^[29]
• immediate functions using the new consteval keyword^[30]
• signed integers are now defined to be represented using two's complement (signed integer overflow remains undefined behavior)^[31]
• a revised memory model^[32]
• various improvements to structured bindings (interaction with lambda captures, static and thread_local storage duration)^[33][34]
• coroutines^[35]
• using on scoped enums^[36]
• constinit keyword^[37]

Library[]

• ranges (The One Ranges Proposal)^[38]
• std::make_shared and std::allocate_shared for arrays^[39]
• atomic smart pointers (such as std::atomic<shared_ptr<T>> and std::atomic<weak_ptr<T>>)^[40]
• std::to_address to convert a pointer to a raw pointer^[41]
• calendar and time-zone additions to <chrono>^[42]
• std::span, providing a view to a contiguous array (analogous to std::string_view but span can mutate the referenced sequence)^[43]
• std::erase and std::erase_if, simplifying element erasure for most standard containers^[44]
• <version> header^[45]
• std::bit_cast<> for type casting of object representations, with less verbosity than memcpy() and more ability to exploit compiler internals^[46]
• feature test macros^[47]
• various constexpr library bits^[48]
• smart pointer creation with default initialization^[49]
• std::map::contains method^[50]
• bit operations, such as leading/trailing zero/one count,^[51] and log2 operations^[52][53][54]
• std::bind_front^[55]

New and changed keywords[]

Many new keywords added (and the new "spaceship operator", operator <=>), such as concept, constinit,^[37] consteval, co_await, co_return, co_yield, requires (plus changed meaning for export), and char8_t (for UTF-8 support).^[56] And explicit can take an expression since C++20.^[57] Most of the uses of the volatile keyword have been deprecated.^[58]

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]]^[59]

Removed and deprecated[]

Removed features:^[60]

• 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^[61]
• (most of) volatile has been deprecated^[58]

Published as Technical Specifications[]

• Parallelism TS v2^[62] (including task blocks^[63])
• Reflection TS v1^[64]
• Networking TS v1^[65]

Deferred to a later standard[]

• 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]

Compiler support[]

• 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).^[75]
• EDG eccp started implementing C++20 features in version 5.0 and as of version 6.1 supports most C++20 core language features.^[76]
• GCC added partial, experimental C++20 support in 2017^[77] 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.^[78]
• Visual Studio 2019 supports all C++20 features through its /std:c++latest option, as of version 16.10.0.^[79] An option /std:c++20 to enable C++20 mode is added in version 16.11.0.^{[80] [81]}

History[]

Changes applied to the C++20 working draft in July 2017 (Toronto) include:^[82]

• concepts (what made it into the standard is a cut-down version; also described as "Concepts Lite"^[83])
• 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:^[84][85]

• 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:^[86]

• 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:^[87]

• contracts (later deferred to a later standard)^[88]
• 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:

• 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)^[89] (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:^[90][91]

• 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:^[92][93][94]

• contracts were removed (see list of features deferred to a later standard)^[95]
• use of comma operator in subscript expressions has been deprecated^[61]
• constexpr additions (trivial default initialization,^[96] unevaluated inline-assembly^[97])
• using scoped enums^[36]
• various changes to the spaceship operator^[98][99]
• DR: minor changes to modules^[100]
• constinit keyword
• changes to concepts (removal of -> Type return-type-requirements^[101])
• (most of) volatile has been deprecated^[58]
• DR: [[nodiscard]] effects on constructors^[102]
• The new standard library concepts will not use PascalCase (rather standard_case, as the rest of the standard library)^[103]
• text formatting^[104][105] (chrono integration,^[106] corner case fixes^[107])
• bit operations^[51]
• 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]

References[]