Fix Version/s: None
Allow the use of selected C++14 language features in the HotSpot C++ source code.
Through JDK 12, the language features utilized by C++ code in the JDK have been limited to the C++98/03 language standards. With JDK 11, the code has been updated to support building with newer versions of the C++ standard, although it is not yet using 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 HotSpot to take advantage of some C++14 standard language features.
Changes to the Build System
To take advantage of C++14 language features some build-time changes may be required, 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.
For Windows: Visual Studio 2017 is required for JDK 11. (Earlier
versions will generate a configure-time warning and may or may not
work.) For Visual Studio 2017 the default C++ standard is C++14. The
/std:c++14 option should be added. Support for older versions will be
For Linux: Replace the
-std=gnu++98 compiler option with
The minimum supported version of gcc is TBD.
For macOS: Replace the
-std=gnu++98 compiler option with
The minimum supported version of clang is TBD.
For Solaris: Add the
-std=c++14 compiler option. Remove the existing
standard library selection option (
-library=stlport4). Remove the
linking options for using stlport4 or no library (
-nolib). The minimum supported version of the compiler is
Oracle Developer Studio 12.6.
Changes to C++ Usage
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.
Similar restrictions and guidelines for features of more recent language standards will be added 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.
(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.)
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:
- C++ Standards Support in GCC
- C++ Support in Clang
- Visual C++ Language Conformance
- libstdc++ Status
- libc++ Status
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.
constexpr(n3652) is perhaps the key feature distinguishing C++14 from C++11. The
constexprfeature will permit the elimination of some clumsy macro code in favor of
constexprfunctions. This feature is also the foundation for simplified metaprogramming idioms. See mpl11 and mpl11_2.
Sized deallocation (n3778) — Syntax is already in use, because of Visual Studio.
Static assertions (n1720) — Replaces HotSpot STATIC_ASSERT macro, providing better error messages.
Important metaprogramming tool. Needed to implement a simplified function template SFINAE utility.
Right angle brackets (n1757) — Eliminates an annoying syntactic wart.
Default template arguments for function templates (CWG D226) — In addition to being useful in its own right, this is needed to implement a simplified function template SFINAE utility.
Template aliases (n2258) — Typedefs with template parameters. Provides syntax for partial specializations of a class template, rather than using inheritance (in sometimes inappropriate ways). Also used as metafunctions in simplified approach to metaprogramming mpl11 and mpl11_2.
Strongly-typed enums (n2347) — Allows explicit control of the underlying type for an enum, rather than leaving it potentially implementation defined (and varying between implementations). Also allows strong typing for enum classes, eliminating implicit conversions. It is recommended that scoped-enums be used when the enumerators are indeed a logical set of values. Use of unscoped-enums is permitted, though ordinary constants should be preferred when the automatic initializer feature isn't used. The enum-base should always be specified explicitly, rather than leaving it dependent on the range of the enumerator values and the platform.
Delegating constructors (n1986) — Eliminate some code duplication and simplification by allowing specialized constructors to chain to more general constructors.
Explicit conversion operators (n2437) — Use to make some existing conversion operators safe.
Standard Layout Types (n2342)
Defaulted and deleted functions (n2346)
Dynamic initialization and destruction with concurrency (n2660) — Thread-safe function-local statics.
<type_traits>is a core metaprogramming library. It eliminates the need for many of the HotSpot metaprogramming utilities, which were modeled on corresponding parts of this library.
finalvirtual specifiers for classes and virtual functions (n2928), (n3206), (n3272) — The
finalspecifier permits devirtualization of virtual function calls. This can provide better performance than relying on the compiler's use of techniques such as points-to analysis or speculative devirtualization. The
overridesspecifier for virtual functions, which is also described in the referenced papers, may be considered at a later time.
Local and unnamed types as template parameters (n2657) — Allows local definition of used-once helper classes to be placed near the point of use when the use is a template, rather than requiring such helpers be defined at namespace scope.
New string and character literals
- New character types (n2249)
- Unicode string literals (n2442)
- Raw string literals (n2442)
- Universal character name literals (n2170)
HotSpot doesn't need any of the new character and string literal types.
User-defined literals (n2765) — User-defined literals should not be added casually, but only through a proposal to add a specific UDL.
Inline namespaces (n2535) — HotSpot makes very limited use of namespaces.
using namespacedirectives. In particular, don't use
using namespace std;to avoid needing to qualify Standard Library names.
Propagating exceptions (n2179) — HotSpot does not permit the use of exceptions, so this feature isn't useful.
Similar lists for some other projects:
Google C++ Style Guide — Currently targeting C++11 and should not use C++14
C+ 11 and C +14 use in Chromium — Categorizes features as allowed, banned, or to be discussed.
llvm Coding Standards — Currently targeting C++11 and should not use C++14.
Using C++ in Mozilla code — C++14 support is required.
Risk and Assumptions
There may be other platforms with toolchains that do not yet support the C++14 language standard. In particular, the IBM XLC++ compiler used by the aix_ppc port provides very little C++11/14 support; version 13.1.3 seems to be the most recent version, and provides a small subset of the new language and library features, and will be rendered unusable once HotSpot code starts using almost any of the above proposed permitted features.
There may be bugs in the support for some new features by some compilers.