JEP 347: Enable C++14 Language Features

OwnerKim Barrett
TypeInfrastructure
ScopeImplementation
StatusIntegrated
Release16
Componenthotspot / other
Discussionhotspot dash dev at openjdk dot java dot net
Reviewed byDavid Holmes, Mikael Vidstedt
Endorsed byMikael Vidstedt
Created2018/07/23 14:30
Updated2020/09/01 23:35
Issue8208089

Summary

Allow the use of C++14 language features in JDK C++ source code, and give specific guidance about which of those features may be used in HotSpot code.

Goals

Through JDK 15, the language features used by C++ code in the JDK have been limited to the C++98/03 language standards. With JDK 11, the code was updated to support building with newer versions of the C++ standard, although it does not yet use any new features. This includes being able to build with recent versions of various compilers that support C++11/14 language features.

The purpose of this JEP is to formally allow C++ source code changes within the JDK to take advantage of C++14 language features, and to give specific guidance about which of those features may be used in HotSpot code.

Non-Goals

This JEP does not propose any usage or style changes for C++ code in the JDK that is outside of HotSpot. The rules are already different for HotSpot and non-HotSpot code. For example, C++ exceptions are used in some non-HotSpot code, but are disallowed in HotSpot by build-time options. However, build consistency requirements will make the newer language features available for use in all C++ code in the JDK.

Description

Changes to the Build System

To take advantage of C++14 language features some build-time changes are required, with specifics depending on the platform compiler. Minimum acceptable versions of the various platform compilers also need to be specified. The desired language standard should be specified explicitly; later compiler versions might default to a later, and possibly incompatible, language standard.

Changes to C++ Usage in HotSpot code

The existing restrictions and best-practice recommendations for C++ usage in HotSpot code are based on the C++98/03 language standard, and described in the HotSpot Style Guide.

We will add similar restrictions and guidelines for features of more recent language standards to that document. They will be described by a table of permitted features and another of excluded features. Use of permitted features may be unconditional or may have some restrictions or additional guidance. Use of excluded features is forbidden in HotSpot code.

There is a third category, undecided features, about which HotSpot developers have not reached a consensus, or possibly discussed at all. Use of these features is also forbidden.

Note, however, that the use of some language features may not be immediately obvious and may accidentally slip in anyway, since the compiler will accept them. As always, the code review process is the main defense against this.

Proposed changes to a feature's categorization are approved by rough consensus of the HotSpot Group members, as determined by the Group Lead. Such changes must be documented in updates to the Style Guide.

Lists of new features for C++11 and C++14, along with links to their descriptions, can be found in the online documentation for some of the compilers and libraries:

As a rule of thumb, permitting features which simplify writing code and, especially, reading code, are encouraged.

HotSpot has largely avoided using the C++ Standard Library. Some of the reasons for that may be obsolete (in particular, bugs encountered in early versions), while others may still be applicable (minimizing dependencies). Categorizing pieces of the Standard Library should go through the same process as language features.

An initial set of feature categorizations for HotSpot follows.

Permitted

Excluded

Similar lists for some other projects

Risk and Assumptions

There may be other platforms with toolchains that do not yet support the C++14 language standard.

There may be bugs in the support for some new features by some compilers.