Experimental Features¶

Overview
Background
Design
Status of TSes

Overview ¶

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.

Background ¶

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.

Design ¶

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.

Status of TSes ¶

Library Fundamentals TS V1 and V2 ¶

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`	Not yet
8.6	`polymorphic_allocator`	Not yet
8.7	`resource_adaptor`		n/a	Not part of C++17
8.8	Access to program-wide `memory_resource` objects	Not yet
8.9	Pool resource classes	Not yet
8.10	`monotonic_buffer_resource`	Not yet
8.11	Alias templates using polymorphic memory resources	Not yet
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

FileSystem TS ¶

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).

Parallelism TS V1 and V2 ¶

Some (most) of the Parallelism TS was accepted for C++17. We have not yet shipped an implementation of the Parallelism TS.

Coroutines 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 will be removed in LLVM 16.0.

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