RFC 68: C++11 Compilation Mode

Author:	Kurt Schwehr
Contact:	schwehr@… / schwehr@…
Started:	2017-Apr-11
Status:	Work-In-Progress

This RFC is based on trac.osgeo.org/geos/wiki/RFC5 GEOS RFC 5 by Mateusz Łoskot.

Summary

The document proposes to switch to en.wikipedia.org/wiki/C%2B%2B11 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.)

The C++11 features aim to promote writing clean, compact, type-safe and fast code. It also delivers better feature-wise compatibility with C language (C99).

The Wikipedia article at 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.

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 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 with nothrow(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

• en.cppreference.com/w/cpp/compiler_support C++ compiler support

Self-assigned development constraints

*TODO*

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.

Core changes: summary

TODO

Core changes: details

TODO

• > > to >>

Other

TODO

Changes to Travis-CI

Changes to AppVeyor

Changes in drivers

TODO

Changes in utilities

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.

• Python and Java bindings:
• Perl and CSharp: make sure that it still compiles but some work would have

Potential changes that are *NOT* included in this RFC

TODO

"Natural" evolutions of current RFC :

Further steps :

Backward compatibility

TODO

Documentation

TODO

Testing

TODO

Very few changes have been made so that the existing autotest suite still passes. Additions have been made to test the GDALOpenEx() API and the methods "imported" from OGRDataSource into GDALDataset.

Version numbering

TODO

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

TODO

Implementation will be done by Kurt Schwehr.

Related RFCs

TODO

There are a few related past RFCs that have never been adopted but strongly relate to RFC 68:

• trac.osgeo.org/geos/wiki/RFC5 GEOS RFC 5: C++11 Compilation Mode. GEOS is switching to requiring C++11 to build.

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