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(experimental features)=
Libc++ implements technical specifications (TSes) and ships them as experimental features that users are free to try out. The goal is to allow getting feedback on those experimental features.
However, libc++ does not provide the same guarantees about those features as it does for the rest of the library. In particular, no ABI or API stability is guaranteed, and experimental features are deprecated once the non-experimental equivalent has shipped in the library. This document outlines the details of that process.
The “end game” of a Technical Specification (TS) is to have the features in there added to a future version of the C++ Standard. When this happens, the TS can be retired. Sometimes, only part of at TS is added to the standard, and the rest of the features may be incorporated into the next version of the TS.
Adoption leaves library implementors with two implementations of a feature, one in namespace std, and the other in namespace std::experimental. The first one will continue to evolve (via issues and papers), while the other will not. Gradually they will diverge. It's not good for users to have two (subtly) different implementations of the same functionality in the same library.
When a feature is adopted into the main standard, we implement it in namespace std. Once that implementation is complete, we then create a deprecation warning for the corresponding experimental feature warning users to move off of it and to the now-standardized feature.
These deprecation warnings are guarded by a macro of the form _LIBCPP_NO_EXPERIMENTAL_DEPRECATION_WARNING_<FEATURE>, which can be defined by users to disable the deprecation warning. Whenever possible, deprecation warnings are put on a per-declaration basis using the [[deprecated]] attribute, which also allows disabling the warnings using -Wno-deprecated-declarations.
After 2 releases of LLVM, the experimental feature is removed completely (and the deprecation notice too). Using the experimental feature simply becomes an error. Furthermore, when an experimental header becomes empty due to the removal of the corresponding experimental feature, the header is removed. Keeping the header around creates incorrect assumptions from users and breaks __has_include.
Most (but not all) of the features of the LFTS were accepted into C++17.
| Section | Feature | Shipped in std | To be removed from std::experimental | Notes |
|---|---|---|---|---|
| 2.1 | uses_allocator construction | 5.0 | 7.0 | |
| 3.1.2 | erased_type | n/a | Not part of C++17 | |
| 3.2.1 | tuple_size_v | 5.0 | 7.0 | Removed |
| 3.2.2 | apply | 5.0 | 7.0 | Removed |
| 3.3.1 | All of the _v traits in <type_traits> | 5.0 | 7.0 | Removed |
| 3.3.2 | invocation_type and raw_invocation_type | n/a | Not part of C++17 | |
| 3.3.3 | Logical operator traits | 5.0 | 7.0 | Removed |
| 3.3.3 | Detection Idiom | 5.0 | Only partially in C++17 | |
| 3.4.1 | All of the _v traits in <ratio> | 5.0 | 7.0 | Removed |
| 3.5.1 | All of the _v traits in <chrono> | 5.0 | 7.0 | Removed |
| 3.6.1 | All of the _v traits in <system_error> | 5.0 | 7.0 | Removed |
| 3.7 | propagate_const | n/a | Not part of C++17 | |
| 4.2 | Enhancements to function | Not yet | ||
| 4.3 | searchers | 7.0 | 9.0 | |
| 5 | optional | 5.0 | 7.0 | Removed |
| 6 | any | 5.0 | 7.0 | Removed |
| 7 | string_view | 5.0 | 7.0 | Removed |
| 8.2.1 | shared_ptr enhancements | Not yet | Never added | |
| 8.2.2 | weak_ptr enhancements | Not yet | Never added | |
| 8.5 | memory_resource | 16.0 | 18.0 | Removed |
| 8.6 | polymorphic_allocator | 16.0 | 18.0 | Removed |
| 8.7 | resource_adaptor | n/a | Not part of C++17 | |
| 8.8 | Access to program-wide memory_resource objects | 16.0 | 18.0 | Removed |
| 8.9 | Pool resource classes | 16.0 | 18.0 | Removed |
| 8.10 | monotonic_buffer_resource | 16.0 | 18.0 | Removed |
| 8.11 | Alias templates using polymorphic memory resources | 16.0 | 18.0 | Removed |
| 8.12 | Non-owning pointers | n/a | Not part of C++17 | |
| 11.2 | promise | n/a | Not part of C++17 | |
| 11.3 | packaged_task | n/a | Not part of C++17 | |
| 12.2 | search | 7.0 | 9.0 | |
| 12.3 | sample | 5.0 | 7.0 | Removed |
| 12.4 | shuffle | Not part of C++17 | ||
| 13.1 | gcd and lcm | 5.0 | 7.0 | Removed |
| 13.2 | Random number generation | Not part of C++17 | ||
| 14 | Reflection Library | Not part of C++17 |
The FileSystem TS was accepted (in totality) for C++17. The FileSystem TS implementation was shipped in namespace std in LLVM 7.0, and was removed in LLVM 11.0 (due to the lack of deprecation warnings before LLVM 9.0).
Some (most) of the Parallelism TS was accepted for C++17. We have not yet shipped an implementation of the Parallelism TS.
The Coroutines TS was accepted for C++20. An implementation of the Coroutines TS was shipped in LLVM 5.0 in namespace std::experimental, and C++20 Coroutines shipped in LLVM 14.0. The implementation of the Coroutines TS in std::experimental has been removed in LLVM 17.0.
The Networking TS is not yet part of a shipping standard, and there is discussion around removing it. Libc++ never shipped an implementation of the Networking TS and does not plan to do so in the future, unless the C++ Standards Committee expresses a desire to merge the Networking TS into the IS (which is unlikely at this point).
The Ranges TS was accepted for C++20. We will not ship an implementation of the Ranges TS, however we are actively working on the implementation of C++20 Ranges.
The Concepts TS was accepted for C++20. We will not ship an implementation of the Concepts TS, however we are shipping an implementation of C++20 Concepts.
The Concurrency TS was adopted in Kona (2015). None of the Concurrency TS was accepted for C++17. We have not yet shipped an implementation of the Concurrency TS.
% +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
% | Section | Feature | Shipped in std | To be removed from std::experimental | Notes |
% +=========+=======================================================+====================+==========================================+=========================+
% | 2.3 | class template future | | | |
% +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
% | 2.4 | class template shared_future | | | |
% +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
% | 2.5 | class template promise | | | Only using future |
% +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
% | 2.6 | class template packaged_task | | | Only using future |
% +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
% | 2.7 | function template when_all | | | Not part of C++17 |
% +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
% | 2.8 | class template when_any_result | | | Not part of C++17 |
% +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
% | 2.9 | function template when_any | | | Not part of C++17 |
% +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
% | 2.10 | function template make_ready_future | | | Not part of C++17 |
% +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
% | 2.11 | function template make_exeptional_future | | | Not part of C++17 |
% +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
% | 3 | latches and barriers | | | Not part of C++17 |
% +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+
% | 4 | Atomic Smart Pointers | | | Adopted for C++20 |
% +---------+-------------------------------------------------------+--------------------+------------------------------------------+-------------------------+