C++23

In the absence of face-to-face WG21 meetings during the COVID pandemic, few new features were added to the C++23 draft in early standards meetings. The following were added after the virtual WG21 meeting of 9 November 2020, where they were approved by straw polls:[8]

• Literal suffixes for std::size_t and the corresponding signed type[9]
• A member function contains for std::basic_string and std::basic_string_view, to check whether or not the string contains a given substring or character[10]
• A stacktrace library (<stacktrace>), based on Boost.Stacktrace[11]
• A type trait std::is_scoped_enum[12]
• The header <stdatomic.h>, for interoperability with C atomics[13]

After the virtual WG21 meeting of 22 February 2021, following features are added where they were approved by straw polls:[14]

• Removing unnecessary empty parameter list () from lambda expressions.[15]
• Repairing input range adaptors and counted_iterator.[16]
• Relax the requirements for time_point::clock.[17]
• std::visit for classes that are derived from std::variant.[18]
• Locks lock lockables.[19]
• Conditionally borrowed ranges.[20]
• std::to_underlying.[21]

After the summer 2021 ISO C++ standards plenary virtual meeting of June 2021, new features and defect reports were approved by straw polls:[22]

• Consteval if (if consteval).[23]
• Narrowing contextual conversions to bool.[24]
• Allowing duplicate attributes.[25]
• std::span-based string-stream (<spanstream>).[26]
• std::out_ptr() and std::inout_ptr().[27]
• constexpr for std::optional, std::variant,[28] and std::type_info::operator==.[29]
• Iterators pair constructors for std::stack (stack) and std::queue (queue).[30]
• Few changes of the ranges library:
  • Generalized starts_with and ends_with for arbitrary ranges.[31]
  • Renamed split_view to lazy_split_view and new split_view.[32]
  • Relaxing the constraint on join_view.[33]
  • Removing default_initializable constraint from concept view.[34]
  • Range constructor for std::basic_string_view.[35]
• Prohibiting std::basic_string and std::basic_string_view construction from nullptr.[36]
• std::invoke_r.[37]
• Improvements on std::format.[38]
• Adding default arguments for std::pair's forwarding constructor.[39]

After the autumn 2021 ISO C++ standards plenary virtual meeting of October 2021, new features and defect reports were approved by straw polls:[40]

• Non-literal variables, labels, and gotos in constexpr functions, but still ill-formed to evaluate them at compile-time.[41]
• Explicit this object parameter.[42]
• Changes on character sets and encodings.[43][44]
• New preprocessors: #elifdef and #elifndef.[45] Both directives were added to C23 (C language update) and GCC 12.[46]
• Allowing alias declarations in init-statement.[47]
• Overloading multidimensional subscript operator (e.g. arr[1, 2]).[48]
• Decay copy in language: auto(x) or auto{x}.[49]
• Changes in text formatting library:
  • Fixing locale handling in chrono formatters.[50]
  • Use of forwarding references in format arguments to allow std::generator-like types.[51]
• Addition of type alias std::pmr::stacktrace which is equivalent to std::basic_stacktrace<std::pmr::polymorphic_allocator>.[52]
• Changes in ranges library:
  • Refined definition of a view.[53]
  • Replacing function template std::ranges::istream_view with alias templates std::ranges::istream_view, std::ranges::wistream_view, and customization point object std::views::istream.[54]
  • zip range adaptor family:[55]
    • zip_view
    • zip_transform_view
    • adjacent_view (and std::views::pairwise being equivalent to std::views::adjacent<2>)
    • adjacent_transform_view (and std::views::pairwise_transform being equivalent to std::views::adjacent_transform<2>)
• std::move_only_function.[56]
• Monadic operations for std::optional.[57]
• Member function template std::basic_string::resize_and_overwrite.[58]
• Printing volatile pointers (volatile T*).[59]
• std::byteswap.[60]
• Heterogeneous erasure overloads for associative containers.[61]
• Every specialization of std::span and std::basic_string_view is trivially copyable.[62]
• Adding conditional noexcept specifications to std::exchange.[63]
• Revamped specification and use of integer-class types.[64]
• Clarify C headers. "The headers are not useful in code that is only required to be valid C++. Therefore, the C headers should be provided by the C++ standard library as a fully-supported, not deprecated part, but they should also be discouraged for use in code that is not polyglot interoperability code. [..] This proposal makes the C headers no longer deprecated, so there is no formal threat of future removal. The effective discouragement to use the C headers in pure C++ code is now spelled out explicitly as normative discouragement."[65]

After the virtual WG21 meeting of 7 February 2022, the following features are added where they were approved by straw polls:[66]

• Allowed attributes on the function call operator of a lambda[67]
• std::expected[68]
• constexpr for cmath and cstdlib[69]
• Function to mark unreachable code[70]
• ranges::to[71]
• A type trait to detect reference binding to temporary[72]
• Making std::unique_ptr constexpr[73]
• Pipe support for user-defined range adaptors[74]
• ranges::iota, ranges::shift_left and ranges::shift_right[75]
• views::join_with[76]
• Windowing range adaptors: views::chunk and views::slide[77]
• views::chunk_by[78]

After the virtual WG21 meeting of 25 July 2022, the following features and defect reports are added where they were approved by straw polls:[79]

• Made rewriting equality in expressions less of a breaking change.[80]
• Reverted the deprecation of bitwise assignment to volatile variables.[81]
• Added the #warning preprocessor directive.[82]
• Removed non-encodable wide character literals and multicharacter wide character literals.[83]
• Allowed labels to appear at the end of compound statements.[84]
• Added escape sequences delimited with curly braces for octal and hexadecimal numbers and universal character names.[85]
• Allowed constexpr functions to never be constant expressions.[86]
• Simplified some implicit move rules from C++20 and allowed implicit move when returning an rvalue reference.[87]
• Add a way to specify unicode characters by name. For example, U'\N{LATIN CAPITAL LETTER A WITH MACRON}' // Equivalent to U'\u0100'[88]
• Allowed operator() and lambdas to be static.[89]
• Allowed the this pointer and references of unknown origin to appear in constant expressions.[90]
• Allowed implementations to define extended floating-point types in addition to the three standard floating-point types. Added the type aliases std::float16_t, std::float32_t, std::float64_t, std::float128_t, std::bfloat16_t for these extended types accessible through the header <stdfloat>, their corresponding literal suffixes f16 f32 f64 f128 bf16or F16 F32 F64 F128 BF16 and added overloads to various standard library functions that take floats as arguments.[91]
• Added the [[assume(expression)]] attribute which allows the compiler to assume the provided expression is true to allow optimizations.[92]
• Made support for UTF-8 source files mandatory, providing a portable encoding for source files.[93]
• Allowed arrays of char and unsigned char to be initialized with UTF-8 string literals.[94]
• Removed the requirement that wchar_t can encode all characters of the extended character set, in effect allowing UTF-16 to be used for wide string literals.[95]
• Added std::mdspan, a multidimensional array view analogous to std::span.[96][97][98][99]
• flat_map and flat_set were added to the standard library.[100][101]
• Added the std::print and std::println functions for printing formatted text to stdout.[102]
• Provide the named modules std and std.compat for importing the standard library.[103]
• Added support for exclusive mode fstreams, analogous to the "x" flag in fopen.[104]
• Allowed std::format to handle Ranges.[105]
• Made std::string::substr use move semantics.[106]
• Added std::generator which implements a coroutine generator that models std::ranges::input_range[107]

After the hybrid WG21 meeting of 7 November 2022, the following features and defect reports are added where they were approved by straw polls:[108]

• Allowed operator[] to be static.[109]
• Allowed static and thread_local variables to appear in constexpr functions if they are usable in constant expressions.[110]
• consteval propagates upwards, that is, certain existing constexpr functions become consteval functions when those functions can already only be invoked during compile time.[111]
• Extended the lifetime of temporaries that appear in the for-range-initializer of a range-based for loop to cover the entire loop.[112]
• Reverted the deprecation of (all, not just bitwise) compound assignment to volatile variables.[113]
• Monadic functions for std::expected.[114]
• Synchronize the output of std::print with the underlying stream if the native Unicode API is used.[115]

• Removed the garbage collection support that was added in C++11, despite many GC implementations available for C++; "Garbage Collection in C++ is clearly useful for particular applications. However, Garbage Collection as specified by the Standard is not useful for those applications."[116]
• Deprecate std::aligned_storage and std::aligned_union[117]