C++20

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]

Features

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

Language

• 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 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]

Library

• 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]
• prefix and suffix checking for strings^[52]
• bit operations, such as leading/trailing zero/one count,^[53] and log2 operations^[54][55][56]
• std::bind_front^[57]
• std::atomic::wait, std::atomic::notify_one, andstd::atomic::notify_all were added, giving the standard library futex-like capabilities^[58]

New and changed keywords

Eight new keywords added (plus the new "spaceship operator", operator <=>), such as concept, constinit,^[38] consteval, requires (plus changed meaning for export), and char8_t (for UTF-8 support); of the eight new these three co_await, co_return, co_yield, are only for the new coroutine support^[59] (additionally six previously existing keywords have changed or new meaning added); explicit can take an expression since C++20.^[60] Most of the uses of the volatile keyword have been deprecated.^[61]

In addition to keywords, there are identifiers with special meaning (effectively keywords in certain contexts), including new import and module.

New attributes in C++20: [[likely]], [[unlikely]], and [[no_unique_address]]^[62]

Removed and deprecated

Removed features:^[63]

• 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 clauses 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^[64]
• (most of) volatile has been deprecated^[61]

Published as Technical Specifications

• Parallelism TS v2^[65] (including task blocks)
• Reflection TS v1^[66]
• Networking TS v1^[67]

Deferred to a later standard

• Contracts – a new study group (SG21) has been formed to work on a new proposal^[68]
• Reflection^[69][70]
• Metaclasses^[71]
• Executors^[72]
• Networking extensions,^[73][74] including async, basic I/O services, timers, buffers and buffer-oriented streams, sockets, and Internet protocols (blocked by executors)
• Properties^[75]
• Extended futures^[76]

Compiler support

Full support^[77]

• Visual Studio 2019 supports all C++20 features through its /std:c++latest option, as of version 16.10.0.^[78] An option /std:c++20 to enable C++20 mode is added in version 16.11.0.^[79][80]

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.^[81]

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).^[82]
• EDG started implementing C++20 features in version 5.0 and as of version 6.1 supports most C++20 core language features.^[83]
• GCC added partial, experimental C++20 support in 2017^[84] 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.^[85]

History

Changes applied to the C++20 working draft in July 2017 (Toronto) include:^[86]

• concepts (what made it into the standard is a cut-down version; also described as "Concepts Lite"^[87])
• 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:^[88][89]

• 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:^[90]

• 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:^[91]

• contracts (later deferred to a later standard)^[92]
• 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:^[93]

• 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)^[94] (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:^[95][96][97]

• 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:^{[98][99][100][101]}

• contracts were removed (see list of features deferred to a later standard)^[102]
• use of comma operator in subscript expressions has been deprecated^[64]
• constexpr additions (trivial default initialization,^[103] unevaluated inline-assembly^[104])
• using scoped enums^[37]
• various changes to the spaceship operator^[105][106]
• DR: minor changes to modules^[107]
• constinit keyword
• changes to concepts (removal of -> Type return-type-requirements^[108])
• (most of) volatile has been deprecated^[61]
• DR: [[nodiscard]] effects on constructors^[109]
• The new standard library concepts will not use PascalCase (rather standard_case, as the rest of the standard library)^[110]
• text formatting (std::format,^[111][112] chrono integration,^[113] corner case fixes^[114])
• bit operations^[53]
• constexpr INVOKE^[115]
• math constants^[116]
• consistency additions to atomics (std::atomic_ref<T>,^[117] std::atomic<std::shared_ptr<T>>^[118])
• add the <=> operator to the standard library^[119]
• header units for the standard library^[120]
• synchronization facilities^[121] (merged from: Efficient atomic waiting and semaphores,^[122] latches and barriers,^[123] Improving atomic_flag,^[124] Don't Make C++ Unimplementable On Small CPUs^[125])
• std::source_location^[126]
• constexpr containers (std::string,^[127] std::vector^[128])
• std::stop_token and joining thread (std::jthread)^[129]

Changes applied during the NB comment resolution in the fall meeting in November 2019 (Belfast) include:^{[130][131][132][133]}

• 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==.^[134] This allows also for array members in class-type NTTP.
• Floating-point types,^[135] 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^[136]
• std::spans typedef was changed from index_type to size_type to be consistent with the rest of the standard library^[137]
• 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,^[138] P1391R4: Range constructor for std::string_view (constructor from iterator-pair of characters),^[139] P1394R4: Range constructor for std::span<ref>,^[140] P1870R1: forwarding-range<T> is too subtle^[141])
• Initialization for std::atomic<T> has been changed to make it work with default and list initialization,^[142] 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^[143]