C++20
C++20 is the name for the revision of the ISO/IEC standard for the C++ programming language following C++17.[1] The standard was technically finalized[2] by WG21 at the meeting in Prague in February 2020,[3] approved on 4th September 2020,[4][5] and published by ISO in December 2020.[6]
C++ language revisions |
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History
Changes applied to the C++20 working draft in July 2017 (Toronto) include:[7]
- concepts (what made it into the standard is a cut-down version; also described as "Concepts Lite"[8])
- designated initializers
[=, this]
as a lambda capture- template parameter lists on lambdas
std::make_shared
andstd::allocate_shared
for arrays
Changes applied to the C++20 working draft in the fall meeting in November 2017 (Albuquerque) include:[9][10]
- 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>>
andstd::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:[11]
- 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 tostd::string_view
butspan
can mutate the referenced sequence)<version>
header
Changes applied to the C++20 working draft in the summer meeting in June 2018 (Rapperswil) include:[12]
contracts(later deferred to a later standard)[13]- 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)[14] (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:[15][16]
- 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:[17][18][19]
- contracts were removed (see list of features deferred to a later standard)[20]
- use of comma operator in subscript expressions has been deprecated[21]
- constexpr additions (trivial default initialization,[22] unevaluated inline-assembly[23])
- using scoped enums[24]
- various changes to the spaceship operator[25][26]
- DR: minor changes to modules[27]
constinit
keyword- changes to concepts (removal of
-> Type
return-type-requirements[28]) - (most of)
volatile
has been deprecated[29] - DR:
[[nodiscard]]
effects on constructors[30] - The new standard library concepts will not use PascalCase (rather standard_case, as the rest of the standard library)[31]
- text formatting[32][33] (chrono integration,[34] corner case fixes[35])
- bit operations[36]
constexpr INVOKE
[37]- math constants[38]
- consistency additions to atomics (
std::atomic_ref<T>
,[39]std::atomic<std::shared_ptr<T>>
[40]) - add the
<=>
operator to the standard library[41] - header units for the standard library[42]
- synchronization facilities[43] (merged from: Efficient atomic waiting and semaphores,[44] latches and barriers,[45] Improving atomic_flag,[46] Don't Make C++ Unimplementable On Small CPUs[47])
std::source_location
[48]- constexpr containers (
std::string
,[49]std::vector
[50]) std::stop_token
and joining thread (std::jthread
)[51]
Changes applied during the NB comment resolution in the fall meeting in November 2019 (Belfast) include:[52][53][54]
- 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 fromoperator==
.[55] This allows also for array members in class-type NTTP. - Floating-point types,[56] 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[57]std::span
s typedef was changed fromindex_type
tosize_type
to be consistent with the rest of the standard library[58]- 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,[59] P1391R4: Range constructor for
std::string_view
(constructor from iterator-pair of characters),[60] P1394R4: Range constructor forstd::span<ref>
,[61] P1870R1: forwarding-range<T> is too subtle[62]) - Initialization for
std::atomic<T>
has been changed to make it work with default and list initialization,[63]std::latch
andstd::barrier
can now report the maximum number of threads that the implementation supports through the new member functionmax()
std::weak_equality
andstd::strong_equality
have been removed as they are not used anymore- Algorithms in
<numeric>
have been madeconstexpr
- Missing feature-test macros for new or changed features of C++20 have been added[64]
Features
C++20 adds more new major features than C++14 or C++17.[65] Changes that have been accepted into or have been discussed for inclusion into C++20 include:[66]
Language
- concepts,[67] with terse syntax[68]
- modules[69]
- designated initializers[70] (based on the C99 feature, and common G++ extension)
[=, this]
as a lambda capture[71]- template parameter lists on lambdas[72]
- three-way comparison using the "spaceship operator",
operator <=>
- initialization of an additional variable within a range-based
for
statement[73] - lambdas in unevaluated contexts[74][75]
- default constructible and assignable stateless lambdas[74][76]
- allow pack expansions in lambda init-capture[74][77]
- string literals as template parameters[74][78]
- removing the need for
typename
in certain circumstances[79] - new standard attributes
[[no_unique_address]]
,[80][[likely]]
and[[unlikely]]
[81] - conditional
explicit
, allowing the explicit modifier to be contingent on a boolean expression[82] - expanded
constexpr
: virtual functions,[83] union,[84] try and catch,[85] dynamic_cast and typeid,[86] std::pointer_traits[87] - immediate functions using the new
consteval
keyword[88] - signed integers are now defined to be represented using two's complement (signed integer overflow remains undefined behavior)[89]
- a revised memory model[90]
- various improvements to structured bindings (interaction with lambda captures, static and thread_local storage duration)[91][92]
- coroutines[93]
using
on scoped enums[94]constinit
keyword[95]
Library
- ranges (The One Ranges Proposal)[96]
std::make_shared
andstd::allocate_shared
for arrays[97]- atomic smart pointers (such as
std::atomic<shared_ptr<T>>
andstd::atomic<weak_ptr<T>>
)[98] std::to_address
to convert a pointer to a raw pointer[99]- calendar and time-zone additions to
<chrono>
[100] std::span
, providing a view to a contiguous array (analogous tostd::string_view
butspan
can mutate the referenced sequence)[101]std::erase
andstd::erase_if
, simplifying element erasure for most standard containers[102]<version>
header[103]std::bit_cast<>
for type casting of object representations, with less verbosity thanmemcpy()
and more ability to exploit compiler internals[104]- feature test macros[105]
- various constexpr library bits[106]
- smart pointer creation with default initialization[107]
std::map::contains
method[108]- bit operations, such as leading/trailing zero/one count,[109] and log2 operations[110][111][112]
std::bind_front
[113]
New and changed keywords
Many new keywords added (and the new "spaceship operator", operator <=>
), such as concept
, constinit
,[95] consteval
, co_await
, co_return
, co_yield
, requires
(plus changed meaning for export
), and char8_t
.[114] And explicit
can take an expression since C++20.[115] Most of the uses of the volatile
keyword have been deprecated.[29]
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]]
[116]
Published as Technical Specifications
Deferred to a later standard
- Contracts – a new study group (SG21) has been formed to work on a new proposal[121]
- Reflection[122][123]
- Metaclasses[124]
- Executors[125]
- Networking extensions,[126][127] including async, basic I/O services, timers, buffers and buffer-oriented streams, sockets, and Internet protocols (blocked by executors)
- Properties[128]
- Extended futures[129]
Removed features and deprecation
C++20 also removed some features.[130]
- 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
andstd::result_of_t
.
Deprecated features include:
Compiler support
Developers of major compilers have prepared for the final C++20 standard by implementing experimental support for various features from the C++20 drafts.
- 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).[132] - EDG eccp started implementing C++20 features in version 5.0 and as of version 6.1 supports most C++20 core language features.[133]
- GCC added partial, experimental C++20 support in 2017[134] 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
.[135] - Microsoft Visual Studio supports some C++20 features through its
std:c++latest
option. An option/std:c++20
to enable C++20 mode is expected as soon as the compiler is C++20 feature complete.[136]
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External links
- 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