ISO C++ 2020 In this implementation the -std=gnu++20 or -std=c++20 flag must be used to enable language and library features. See dialect options. The pre-defined symbol __cplusplus is used to check for the presence of the required flag. This section describes the C++20 and library TS support in mainline GCC, not in any particular release. The following table lists new library features that have been accepted into the C++20 working draft. The "Proposal" column provides a link to the ISO C++ committee proposal that describes the feature, while the "Status" column indicates the first version of GCC that contains an implementation of this feature (if it has been implemented). A dash (—) in the status column indicates that the changes in the proposal either do not affect the code in libstdc++, or the changes are not required for conformance. The "SD-6 Feature Test / Notes" column shows the corresponding macro or header from SD-6: Feature-testing recommendations for C++ (where applicable) or any notes about the implementation. Library Feature Proposal Status SD-6 Feature Test / Notes Compile-time programming Add constexpr modifiers to functions in <algorithm> and <utility> Headers P0202R3 10.1 __cpp_lib_constexpr_algorithms >= 201703L Constexpr for swap and swap related functions P0879R0 10.1 __cpp_lib_constexpr_algorithms >= 201806L Constexpr for std::complex P0415R1 9.1 __cpp_lib_constexpr_complex >= 201711L (since 9.4, see Note 1) P0595R2 std::is_constant_evaluated() P0595R2 9.1 __cpp_lib_is_constant_evaluated >= 201811L More constexpr containers P0784R7 10.1 __cpp_lib_constexpr_dynamic_alloc >= 201907L Making std::string constexpr P0980R1 12.1 __cpp_lib_constexpr_string >= 201907L Making std::vector constexpr P1004R2 12.1 __cpp_lib_constexpr_vector >= 201907L Constexpr in std::pointer_traits P1006R1 9.1 __cpp_lib_constexpr_memory >= 201811L (since 9.4, see Note 1) constexpr for <numeric> algorithms P1645R1 10.1 __cpp_lib_constexpr_numeric >= 201911L Constexpr iterator requirements P0858R0 9.1 __cpp_lib_array_constexpr >= 201803L __cpp_lib_string_view >= 201803L (both since 9.4, see Note 1) constexpr comparison operators for std::array P1023R0 10.1 __cpp_lib_array_constexpr >= 201806 Misc constexpr bits P1032R1 10.1 __cpp_lib_array_constexpr >= 201811L __cpp_lib_constexpr_functional >= 201811L __cpp_lib_constexpr_iterator >= 201811L __cpp_lib_constexpr_string_view >= 201811L __cpp_lib_constexpr_tuple >= 201811L __cpp_lib_constexpr_utility >= 201811L constexpr INVOKE P1065R2 10.1 __cpp_lib_constexpr_functional >= 201907L Transformation Trait remove_cvref P0550R2 9.1 __cpp_lib_remove_cvref >= 201711L (since 9.4, see Note 1) Implicit conversion traits and utility functions P0758R1 9.1 __cpp_lib_is_nothrow_convertible >= 201806L (since 9.4, see Note 1) The identity metafunction P0887R1 9.1 __cpp_lib_type_identity >= 201806L (since 9.4, see Note 1) unwrap_ref_decay and unwrap_reference P0318R1 9.1 __cpp_lib_unwrap_ref >= 201811L (since 9.4, see Note 1) Improving Completeness Requirements for Type Traits P1285R0 Partial Missing feature test macros P1353R0 9.1 Making std::underlying_type SFINAE-friendly P0340R3 9.1 Traits for [Un]bounded Arrays P1357R1 9.1 __cpp_lib_bounded_array_traits >= 201902L Layout-compatibility and pointer-interconvertibility traits P0466R5 12.1 __cpp_lib_is_layout_compatible >= 201907L __cpp_lib_is_pointer_interconvertible >= 201907L Integrating feature-test macros into the C++ WD P0941R2 5.1 <version> P0754R2 9.1 __has_include(<version>) Synchronization Atomic Ref P0019R8 10.1 __cpp_lib_atomic_ref >= 201806L Floating Point Atomic P0020R6 10.1 __cpp_lib_atomic_float >= 201711L C++ Synchronized Buffered Ostream P0053R7 11 __cpp_lib_syncbuf >= 201711L Manipulators for C++ Synchronized Buffered Ostream P0753R2 11 __cpp_lib_syncbuf >= 201803L Make std::memory_order a scoped enumeration P0439R0 9.1 The Curious Case of Padding Bits, Featuring Atomic Compare-and-Exchange P0528R3 Atomic shared_ptr P0718R2 12.1 __cpp_lib_atomic_shared_ptr >= 201711L std::stop_token and std::jthread P0660R10 10.1 __cpp_lib_jthread >= 201907L Rename condition_variable_any interruptible wait methods P1869R1 10.1 __cpp_lib_jthread >= 201911L Atomic waiting and notifying, std::semaphore, std::latch and std::barrier P1135R6 11.1 __cpp_lib_atomic_lock_free_type_aliases >= 201907L __cpp_lib_atomic_flag_test >= 201907L __cpp_lib_atomic_wait >= 201907L __cpp_lib_semaphore >= 201907L __cpp_lib_latch >= 201907L __cpp_lib_barrier >= 201907L Fixing Atomic Initialization P0883R2 10.1 __cpp_lib_atomic_value_initialization >= 201911L Ranges and Concepts Standard Library Concepts P0898R3 10.1 __cpp_lib_concepts >= 201806L Rename concepts to standard_case for C++20, while we still can P1754R1 10.1 __cpp_lib_concepts >= 201907L Wording for boolean-testable P1964R2 10.1 __cpp_lib_concepts >= 202002L The One Ranges Proposal P0896R4 10.1 __cpp_lib_ranges >= 201811L Input Range Adaptors P1035R7 10.1 __cpp_lib_ranges >= 201907L ranges compare algorithm are over-constrained P1716R3 10.1 __cpp_lib_ranges >= 201911L Remove CommonReference requirement from StrictWeakOrdering (a.k.a Fixing Relations) P1248R1 10.1 Ranges Design Cleanup P1252R2 10.1 Avoid template bloat for safe_ranges in combination with ‘subrange-y’ view adaptors. P1739R4 Time, dates, calendars, time zones Extending chrono to Calendars and Time Zones P0355R7 __cpp_lib_chrono >= 201803L Miscellaneous minor fixes for chrono P1466R3 __cpp_lib_chrono >= 201907L <chrono> zero(), min(), and max() should be noexcept P0972R0 9.1 Three-way comparison Library Support for the Spaceship (Comparison) Operator P0768R1 10.1 __cpp_lib_three_way_comparison >= 201711L Symmetry for spaceship P0905R1 10.1 Adding <=> to the standard library P1614R2 10.1 __cpp_lib_three_way_comparison >= 201907L Strings and text string::reserve Should Not Shrink P0966R1 11 char8_t: A type for UTF-8 characters and strings P0482R6 9.1 __cpp_lib_char8_t >= 201811L char8_t backward compatibility remediation P1423R3 10.1 __cpp_lib_char8_t >= 201907L Text formatting P0645R10 __cpp_lib_format >= 201907L Integration of chrono with text formatting P1361R2 __cpp_lib_format >= 201907L Printf corner cases in std::format P1652R1 __cpp_lib_format >= 201907L String Prefix and Suffix Checking P0457R2 9.1 __cpp_lib_starts_ends_with >= 201711L (since 9.4, see Note 1) Update The Reference To The Unicode Standard P1025R1 Containers span: bounds-safe views for sequences of objects P0122R7 10.1 __cpp_lib_span >= 201803L Usability Enhancements for std::span P1024R3 10.1 __cpp_lib_span >= 201902L Should Span be Regular? P1085R2 10.1 Fixed-size <span> construction from dynamic range P1976R2 10.1 __cpp_lib_span >= 202002L std::to_array P0325R4 10.1 __cpp_lib_to_array >= 201907L Checking for Existence of an Element in Associative Containers P0458R2 9.1 Comparing Unordered Containers P0809R0 Heterogeneous lookup for unordered containers P0919R3 11.1 __cpp_lib_generic_unordered_lookup >= 201811 Refinement Proposal for P0919 P1690R1 11.1 Adopt Consistent Container Erasure from Library Fundamentals 2 for C++20 P1209R0 9.1 __cpp_lib_erase_if >= 201811L Improving the Return Value of Erase-Like Algorithms P0646R1 9.1 __cpp_lib_list_remove_return_type >= 201806L Improving the Return Value of Erase-Like Algorithms II: Free erase/erase_if P1115R3 9.1 __cpp_lib_erase_if >= 202002L (defined to 201900L for GCC 9.1 and 9.2 so use > 201811L) Signed ssize() functions, unsigned size() functions P1227R2 10.1 __cpp_lib_ssize >= 201902L Memory management Utility to convert a pointer to a raw pointer P0653R2 8.1 __cpp_lib_to_address >= 201711L (since 9.4, see Note 1) Extending make_shared to Support Arrays P0674R1 12.1 __cpp_lib_shared_ptr_arrays >= 201707L Efficient sized delete for variable sized classes P0722R3 9.1 __cpp_lib_destroying_delete >= 201806L Utility functions to implement uses-allocator construction P0591R4 9.1 std::scoped_allocator_adaptor changes missing in 9.1.0 std::assume_aligned P1007R3 9.1 __cpp_lib_assume_aligned >= 201811L (since 9.4, see Note 1) Smart pointer creation with default initialization P1020R1 12.1 __cpp_lib_smart_ptr_for_overwrite >= 201811L Make stateful allocator propagation more consistent for operator+(basic_string) P1165R1 10.1 polymorphic_allocator<> as a vocabulary type P0339R6 9.1 __cpp_lib_polymorphic_allocator >= 201902L (since 9.4, see Note 1) LWG 2511: guaranteed copy elision for piecewise construction P0475R1 7.1 Miscellaneous nodiscard in the Library P0600R1 9.1 de-pessimize legacy algorithms with std::move P0616R0 9.1 Deprecate POD P0767R1 10.1 Treating Unnecessary decay P0777R1 9.1 Thou Shalt Not Specialize std Function Templates! P0551R3 Bit-casting object representations P0476R2 11 __cpp_lib_bit_cast >= 201806L Integral power-of-2 operations P0556R3 9.1 __cpp_lib_int_pow2 >= 201806L (since 9.4, see Note 1) On the names of low-level bit manipulation functions P1956R1 10.1 __cpp_lib_int_pow2 >= 202002L Safe integral comparisons P0586R2 10.1 __cpp_lib_integer_comparison_functions >= 202002L Reviewing Deprecated Facilities of C++17 for C++20 P0619R4 12.1 fpos Requirements P0759R1 — Add shift to <algorithm> P0769R2 10.1 __cpp_lib_shift >= 201806L Standard Library Specification in a Concepts and Contracts World P0788R3 explicit(bool) P0892R2 — Eradicating unnecessarily explicit default constructors from the standard library P0935R0 9.1 std::function move constructor should be noexcept P0771R1 7.2 Simplified partial function application P0356R5 9.1 __cpp_lib_bind_front >= 201811L bind_front should not unwrap reference_wrapper P1651R0 9.3 __cpp_lib_bind_front >= 201907L reference_wrapper for incomplete types P0357R3 9.1 Fixing operator>>(basic_istream&, CharT*) (LWG 2499) P0487R1 11.1 variant and optional should propagate copy/move triviality P0602R4 8.3 A sane variant converting constructor P0608R3 10.1 visit<R>: Explicit Return Type for visit P0655R1 9.1 P0899R1 - LWG 3016 is not a defect P0899R1 7.1 Editorial Guidance for merging P0019r8 and P0528r3 P1123R0 — Cleaning up Clause 20 P1148R0 Completing the Rebase of Library Fundamentals, Version 3, Working Draft P1210R0 Alternative Wording for P0907R4 Signed Integers are Two's Complement P1236R1 — I Stream, You Stream, We All Stream for istream_iterator P0738R2 10.1 Mandating the Standard Library: Clause 16 - Language support library P1458R1 — Mandating the Standard Library: Clause 18 - Diagnostics library P1459R1 — Mandating the Standard Library: Clause 20 - Strings library P1462R1 — Mandating the Standard Library: Clause 21 - Containers library P1463R1 10.1 Mandating the Standard Library: Clause 22 - Iterators library P1464R1 Make create_directory() Intuitive P1164R1 8.3 Treated as a DR for C++17 Target Vectorization Policies from Parallelism V2 TS to C++20 P1001R2 9.1 __cpp_lib_execution >= 201902L (since 9.4, see Note 1) Merge Coroutines TS into C++20 working draft P0912R5 10.1 __cpp_lib_coroutines >= 201902L Endian just Endian P0463R1 8.1 __cpp_lib_endian >= 201907L Bit operations P0553R4 10.1 __cpp_lib_bitops >= 201907L (since 9.4, see Note 1) Well-behaved interpolation for numbers and pointers P0811R3 9.1 __cpp_lib_interpolate >= 201902L Mathematical constants P0631R8 10.1 __cpp_lib_math_constants >= 201907L std::source_location P1208R6 11 __cpp_lib_source_location >= 201907L Efficient access to std::basic_stringbuf's Buffer P0408R7 11 Note 1: This feature is supported in older releases but the __cpp_lib macro is not defined to the right value (or not defined at all) until the version shown in parentheses.