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RFC 68: C++11 Compilation Mode
Author: | Kurt Schwehr |
Contact: | schwehr@… / schwehr@… |
Started: | 2017-Apr-11 |
Status: | Work-In-Progress |
Proposed Switch: | 2017-Oct-1 |
This RFC is based on trac.osgeo.org/geos/wiki/RFC5 GEOS RFC 5 by Mateusz Łoskot.
Summary
The document proposes to switch to C++11 compilation mode as default throughout the whole C++ source code of GDAL.
The goal of the document is to request and achieve agreement on using C++11 as the minimum required version of the C++ programming language standard.
Motivation
The C++11 is the first major update of the C++ standard since 1998. (C++03 was a bug fix release.)
Having fewer versions of C++ to support will reduce the load on developers and testing systems.
The C++11 features aim to promote writing clean, compact, type-safe and fast code. It also delivers better feature-wise compatibility with the C language (C99).
The Wikipedia article at http://en.wikipedia.org/wiki/C++11 does a great job describing all changes in C++11 extensively.
The std::auto_ptr
smart pointer, together with a bunch of other features, has been deprecated and will be removed from C++17. Features like std::unique_ptr
provide much stronger replacements.
The new compilers provide better diagnostics.
Enabling C++11 compilation mode will improve the programming environment making it much friendlier than C++98.
A social factor: since (many) C++ programmers no longer enjoy C++98, allowing C++11 mode may increase potential for new contributions.
Compilers Landscape
Summary of compilers supported by GDAL with their minimal versions required to compile source code based on
http://en.cppreference.com/w/cpp/compiler_support C++11 features.
C++11
GCC | 4.8.1+ | C++11 status | Debian 8 (stable), Ubuntu 15.04+, Ubuntu 14.04 ppa:ubuntu-toolchain-r/test , Fedora 19+, RHEL7
|
Clang | 3.3+ | C++11 status | Debian 8 (stable), Ubuntu 14.04+, Fedora 19+, CentOS 6(?) |
MSVC | 12.0+ (2013) | C++11 status | n/a |
C++14
The C++14 compilers are listed for comparison only:
GCC | 4.9+ |
Clang | 3.4+ |
MSVC | 14.0+ (2015) |
Plan
This proposal only requests agreement for the C++11 compilation mode
switch in the current trunk
branch only.
If accepted, currently available build configurations (Autotools, CMake, NMake) will be updated to switch the compilation mode to C++11.
This proposal does not suggest any detailed roadmap of big refactoring of the GDAL C++ codebase.
The GDAL codebase is around 150 KLOC and given the available man-power to LOCs ratio, such one-step refactoring would not be feasible.
Instead, the task will be tackled with the baby step approach gradually transforming the codebase according to priorities set along the way. Any disruptive refactoring, changes in interfaces of C++ classes, breaking changes in C++ API must be announced and discussed on the mailing list or the bug tracker.
IMPORTANT: C++11 refactoring must not change the C API or break C API compatibility, unless agreed upon based on prior RFC proposed.
However, if the proposal is accepted, any new C++ code written for GDAL can be C++11-compliant.
Prior acceptance of this proposal is necessary in order to start any source code refactoring using C++11 features.
Once accepted, first step will be to update the build configurations to require C++11-compliant compiler.
Issues
This section outlines issues potentially caused by upgrade to C++11 language.
- C++11 destructors, by default, have now the new exception
specification of
nothrow(true)
. Destructors of GDAL classes should be reviewed and any that are allowed/expected to throw exceptions must be marked withnothrow(false)
. Otherwise, any user of the existing GDAL codebase would find the program terminating whenever GDAL destructor throws an exception. Such review would be beneficial anyway.
Release
First release of GDAL with C++11 compiler requirement could be 2.3.0.
C++14
This section clarifies status of C++14 support in GDAL.
- Once C++11 is adopted as default compilation mode, GDAL developers and maintainers must ensure it also successfully compiles in C++14 and C++17 modes.
- Are contributors allowed to add
ifdef
's for C++14 and C++17? No.
- Is there a plan to upgrade to C++14 or C++17 to allow use of the C++ latest features? No, there is no plan. It is, however, recognized, such motion may be put to the vote around 2020.
References
- http://en.cppreference.com/w/cpp/compiler_support C++ compiler support
Self-assigned development constraints
The changes should have moderate impact on the existing GDAL/OGR code base, and particularly on most of its code, that lies in drivers. Existing users of the GDAL/OGR API should also be moderately impacted by the changes, if they do not need to use the new offered capabilities.
GDAL has been working well with C++11 builds for a number of years now, so there should be no externally visible changes for the initial removal of C++03 support.
Core changes: summary
- Change configure.ac to remove
with_cpp11
flag, always use C++11, and fail if AX_CXX_COMPILE_STDCXX_11 does not find C++11 - Remove continuous build targets that do not support C++11 from Travis-CI and AppVeyor
- Remove #if HAVE_CXX11 and leave only the C++11 code
find . -name \*.h -o -name \*.cpp | xargs egrep 'HAVE_CXX11'
- CPL_STATIC_ASSERT -> static_asset
- NULL -> nullptr
- CPL_OVERRIDE -> override and remove redundant virtual
- - MAX or -max() -> std::numeric_limits<T>::lowest()
Core changes: details
TODO
Changes in SWIG bindings
TODO
The switch does not impact the C
API used by the SWIG bindings.
However, there may be some code that can be removed.
Potential changes that are *NOT* included in this RFC
There are many C++11 features not address in this RFC. See https://en.wikipedia.org/wiki/C%2B%2B11
- attributes
auto
consexpr
cstdint
delete
anddefault
for member functions- enum classes
- initializer lists and
std::initializer_list
- lambda
- range based for loops
- std::regex
- rvalue references
- smart pointers
std::unique_ptr
andstd::shared_ptr
- New string literals: u8"An UTF-8 string", u"An UTF-16 str", U"An UTF-32 str", R"xml(raw content)xml"
std::thread
,thread_local
and related- tuples
- And more...
Backward compatibility
The C API should not be impacted.
GDAL 2.2.x and older will continue to have C++03 support.
Documentation
TODO
This RFC and ?
Testing
Very few changes have been made so that the existing autotest suite still passes.
Version numbering
Although the above describes changes should have very few impact on existing applications of the C API, some behavior changes, C++ level changes and the conceptual changes are thought to deserve a 2.3 version number.
Implementation
Implementation will be done by Kurt Schwehr.
Related RFCs
There are a few related past RFCs that have never been adopted but strongly relate to RFC 68:
- GEOS RFC 5: C++11 Compilation Mode: GEOS is switching to requiring C++11 to build.
Voting history
No vote taken yet.