Experimental Features

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 will be 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 is not yet part of a shipping standard. We are shipping (as of v5.0) an implementation of the Coroutines TS in namespace std::experimental.

Networking TS

The Networking TS is not yet part of a shipping standard. We have not yet shipped an implementation of the Networking TS.

Ranges TS

The Ranges TS is not yet part of a shipping standard. We have not yet shipped an implementation of the Ranges TS.

Concepts TS

The Concepts TS is not yet part of a shipping standard, but it has been adopted into the C++20 working draft. We have not yet shipped an implementation of the Concepts TS.

Concurrency TS

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.