C++ style guide

A lot of GNU Octave is written in C++. This document details the C++ style used by the GNU Octave project.

Being part of the GNU project, Octave inherits the GNU coding standards.

See also the GNU Octave Octave style guide for how to write m-files.

Formatting

Indentation

Use only spaces, and indent 2 spaces at a time.

We use spaces for indentation. Absolutely do not use tabs in your code. You should probably set your editor to emit spaces when you hit the tab key.

Line Length

Keep the length of source lines to 79 characters or less, for maximum readability in the widest range of environments. This is inherited from the GNU Coding Standards.

Namespace

All code should be in the octave namespace. This is an ongoing project. We are still moving existing classes into namespaces but all new classes should go somewhere into the "octave" namespace. There is 1 extra level for namespaces inside octave to be used with care, we don't want too many namespaces. Ask before creating a new namespace.

Indent namespaces as any other block. Emacs and other editors can do this automatically.
Define namespace on the .cc files;
Do not use "using X" directives;
Do not declare anything on the std namespace;

Code: namespace style on a .h file

// Note indentation
namespace octave
{
  namespace math
  {
    class foo
    {
    public:
      foo (...);
    };
  }
}

Code: namespace style on a .cc file

// Note indentation and that functions are not defined
// as "octave::math::foo:foo"
namespace octave
{
  namespace math
  {
    foo::foo (...)
    {
      ...;
    }
  }
}

Header Files

Order of Includes

Use the following order with an empty line between each section:

config.h
The C++ wrappers for C headers (cstdlib, cassert, etc.)
C++ standard library headers (iostream, list, map, etc.)
Other POSIX headers (sys/types.h, unistd.h, etc., no need to use #if defined (HAVE_FOO_H) if existence is guaranteed because of gnulib)
Other library header files (glpk.h, curl.h, etc., should be protected by #if defined (HAVE_FOO_H) since they may be missing on the build system)
Octave's liboctave headers
Octave's libinterp headers

Each grouping of header files should be alphabetized unless there is some specific reason to not do that. The only case where that is true is in oct-parse.in.yy and there is a comment in the file for that one.

Other C++ features

C++11

C++11 features are generally allowed. Check if the feature you want to use has been already used. If not, ask on the mailing list.

auto

Use of auto is allowed only where it helps readability and local variables.

Never use auto for class members.
Do not use auto unless the type really is obscure.
Beware of copy when using auto in for loops. Pass by reference and use const unless you're dealing with simple types such as int. See 'auto' uses and for-range loops on the maintainers mailing list for more details.

C++14

Do not use C++14 features. Octave is widely used in very old systems and we want them to be able to use up to date versions of Octave. Building a recent compiler in such systems is not a trivial task so the limitation must happen in Octave.

An exception: code that requires C++14 feature must also implement an alternative code in the absence of said feature. In such case, use a configure check. This increases maintenance a lot, must be used sparsely, and requires approval from other maintainers.

#if defined (HAVE_THIS_C14_FEATURE)
  // code that really needs it
#else
  // alternative code in its absence
#endif