| RFC-0193: Supported C++ Versions | |
|---|---|
| Status | Accepted |
| Areas |
|
| Description | Drop C++14 support from the SDK and clarify how support for C++ versions will be decided and supported in the future. |
| Gerrit change | |
| Authors | |
| Reviewers | |
| Date submitted (year-month-day) | 2022-07-29 |
| Date reviewed (year-month-day) | 2022-10-03 |
Summary
Drop C++14 support from the SDK and clarify how support for C++ versions will be decided and supported in the future.
Motivation
The C++ SDK currently claims to support C++14 but there aren't effective tests in the Fuchsia tree to prevent C++14 incompatible code from making its way into the SDK. This breaks users and results in roll-backs.
The Fuchsia tree is built with C++17. Many libraries and frameworks in the Fuchsia tree that would be useful for out of tree developers depend on C++17 features. In particular the new FIDL C++ bindings require C++17.
There are no longer any users of the SDK that compile as C++14 so changing the supported version is just a matter of changing the documentation.
Stakeholders
Facilitator: cpu@google.com
Reviewers: johngro@google.com, dschuyler@google.com, abarth@google.com
Consulted: yifeit@google.com, sebmarchand@google.com, phosek@google.com, eieio@google.com, schottm@google.com, jamesr@google.com, gevalentino@google.com, jyknight@google.com
Design
Terminology
A C++ version refers to a revision of the ISO C++ standard. We refer to it by short-hand like C++14, C++17, C++20, etc.
The SDK C++ versions is a list of C++ versions that are documented to be supported by the Fuchsia SDK and are used by the build systems shipped as part of the SDK. That list currently includes C++14 and C++17.
Dropping C++14
The SDK documentation and C++ in Zircon SHALL be updated to reflect that C++14 is no longer supported.
Fuchsia Tree C++ Version
The C++ version used to compile most code that runs on Fuchsia in the Fuchsia tree MUST be an SDK C++ version. Other C++ versions MAY be used in the Fuchsia tree for small, isolated codebases or for host-only code, but this is discouraged.
C++ Version Testing
A core set of tests MUST be built and run for every SDK C++ version as part of the Fuchsia commit queue. This set of tests includes all C++ tests in the CTS and API tests for C++ FIDL bindings, but may include more.
Adding C++ Versions
A new SDK C++ version may be added by writing an RFC to propse that addition and having it approved.
Only C++ versions that are published by the ISO C++ committee may be added to the C++ versions.
As part of adding an SDK C++ version the Fuchsia build MUST be updated to build and run the C++ tests identified above with the added C++ version.
Removing C++ Versions
_SDK C++ version_s may be removed by writing an RFC to propose that removal and having it approved.
When removing an SDK C++ version it MUST be removed from the SDK documentation and SHOULD be removed from the Fuchsia build system.
Implementation
Dropping C++14
This is just a documentation change.
C++ Version Testing
After C++14 is dropped C++17 will be the only SDK C++ version. Tests are already built and run with C++17.
Before another version can be added the Fuchsia build system will need to be updated to allow tests to be built and run with multiple C++ versions.
Performance
N/A
Ergonomics
It will be much easier to update C++ code that is shipped in the SDK because every change will be tested against every supported C++ version.
Backwards Compatibility
All current (in October 2022) Fuchsia users build with C++17 so there are no backwards compatibility issues.
Security considerations
N/A
Privacy considerations
N/A
Testing
See above.
Documentation
The SDK documentation and C++ in Zircon will be updated to reflect that C++14 is no longer supported.
This RFC serves as the documentation for the policy for adding and removing C++ versions in the future.