Notes

C++20
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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 C++ Standards Committee began planning C++20 in July 2017.[2] The current draft is N4835.[3]

C++20 is a major update, like C++11 was and unlike the more limited C++14 and C++17 revisions,[4] with about as many new keywords added.

Below is a partial list of changes that have been accepted into or have been discussed for inclusion into C++20.[5]


Contents
1 New features
2 New (and changed) keywords
3 Features published as Technical Specifications
4 Features deferred to a later standard
5 See also
6 References
7 External links
New features
Changes applied to the C++20 working draft in July 2017 (Toronto) include:[6]

concepts[7]
designated initializers[8] (based on the C99 feature)
[=, this] as a lambda capture[9]
template parameter lists on lambdas[10]
Changes applied to the C++20 working draft in the fall meeting in November 2017 (Albuquerque) include:[11][12]

three-way comparison using the "spaceship operator", operator <=>
initialization of an additional variable within a range-based for statement
lambdas in unevaluated contexts[13][14]
default constructible and assignable stateless lambdas[13][15]
allow pack expansions in lambda init-capture[13][16]
string literals as template parameters[13][17]
atomic smart pointers (such as std::atomic<shared_ptr<T>> and std::atomic<weak_ptr<T>>)[18]
Changes applied to the C++20 working draft in March 2018 (Jacksonville) include:[19]

removing the need for typename in certain circumstances[20]
new standard attributes [[no_unique_address]],[21] [[likely]] and [[unlikely]][22]
calendar and time-zone additions to <chrono>[23]
std::span, providing a view to a contiguous array (analogous to std::string_view but span can mutate the referenced sequence)[24]
<version> header[25]
Changes applied to the C++20 working draft in the summer meeting in June 2018 (Rapperswil) include:[26]

contracts[27] (see list of features deferred to a later standard)
feature test macros[28]
bit-casting of object representations, with less verbosity than memcpy() and more ability to exploit compiler internals[29]
conditional explicit, allowing the explicit modifier to be contingent on a boolean expression[30]
constexpr virtual functions[31]
Changes applied to the C++20 working draft in the fall meeting in November 2018 (San Diego) include:

ranges (The One Ranges Proposal)[32]
concept terse syntax[33]
constexpr union,[34] try and catch[35] dynamic_cast and typeid,[36] std::pointer_traits[37]
various constexpr library bits[38]
immediate functions using the new consteval keyword[39]
signed integers are now defined to be represented using two's complement (signed integer overflow remains undefined behavior)[40]
refinements of the contracts facility (access control in contract conditions)[41] (see list of features deferred to a later standard)
a revised memory model[42]
Changes applied to the C++20 working draft in the winter meeting in February 2019 (Kona) include: [43] [44]

coroutines[45] – already experimentally supported in Clang 5[46]
modules[47] – experimentally supported in Clang 5[48] and Visual Studio 2015 Update 1[49] as well as GCC[50]
various improvements to structured bindings (interaction with lambda captures, static and thread_local storage duration)[51][52]
Changes applied to the C++20 working draft in the summer meeting in July 2019 (Cologne) include: [53] [54] [55]

contracts have been removed (see list of features deferred to a later standard)[56]
use of comma operator in subscript expressions has been deprecated[57]
constexpr additions (trivial default initialization,[58] unevaluated inline-assembly[59])
using scoped enums[60]
various changes to the spaceship-operator[61][62]
DR: minor changes to modules[63]
constinit keyword[64]
changes to concepts (removal of -> Type return-type-requirements[65])
(most of) volatile has been deprecated[66]
DR: [[nodiscard]] effects on constructors[67]
The new standard library concepts will not use PascalCase (rather standard_case, as rest of standard library)[68]
text formatting[69][70] (chrono integration,[71] corner case fixes[72])
bit operations[73]
constexpr INVOKE[74]
math constants[75]
consistency additions to atomics (std::atomic_ref<T>,[76] std::atomic<std::shared_ptr<T>>[77])
add the spaceship (<=>) operator to the standard library[78]
header units for the standard library[79]
synchronization facilities[80] (merged from: Efficient atomic waiting and semaphores,[81] latches and barriers,[82] Improving atomic_flag,[83] Don't Make C++ Unimplementable On Small CPUs[84])
std::source_location[85]
constexpr containers (std::string,[86] std::vector[87])
std::stop_token and joining thread (std::jthread)[88]
New (and changed) keywords
Many new keywords added (and the new "spaceship operator", operator <=>), such as concept, constinit,[64] consteval, co_await, co_return, co_yield, requires (plus changed meaning for export), and char8_t.[89] And explicit can take an expression since C++20.[90] (Most of) the use for the volatile keyword has been deprecated.[66]

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

C++ has added a number of attributes over the years, including new in C++20, [[likely]] and [[unlikely]]; and [[no_unique_address]].[91]

Features published as Technical Specifications
Parallelism TS v2[92] (including task blocks[93])
Networking TS v1[94]
Reflection TS v1[95]
Features deferred to a later standard
Contracts – a new study group (SG21) has been formed to work on a new proposal[96]
Reflection[97][98]
Metaclasses[99]
Executors[100]
Networking extensions,[101][102] including async, basic I/O services, timers, buffers and buffer-oriented streams, sockets, and Internet protocols (blocked by executors)
Properties[103]
Extended futures[104]
See also
C++
C++98
C++03
C++11
C++14
C++17
C11 (C standard revision)
C18 (C standard revision)