C++ style guide

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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.



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.

When indenting, indent the statement after control structures (like if, while, etc.). If there is a compound statement, indent both the curly braces and the body of the statement (so that the body gets indented by two indents). This format is known as "GNU style" and is an option for some code formatting tools. Example indenting:

if (have_args)
    idx.push_back (first_args);
    have_args = false;
  idx.push_back (make_value_list (args, arg_nm, tmp));

If you have nested if statements, use extra braces for extra clarification.

Split long expressions in such a way that a continuation line starts with an operator rather than identifier. If the split occurs inside braces, continuation should be aligned with the first char after the innermost braces enclosing the split. Example:

SVD::type type = ((nargout == 0 || nargout == 1)
                  ? SVD::sigma_only
                  : (nargin == 2) ? SVD::economy : SVD::std);

Non indenting whitespace

Consider putting extra braces around a multi-line expression to make it more readable, even if they are not necessary. Also, do not hesitate to put extra braces anywhere if it improves clarity.

The negation operator is written with a space between the operator and its target, e.g., ! A.

Declarations of pointers have the '*' character cuddled with the name of the variable.

unsigned int *pointer_variable;

However, references have the '&' character cuddled with the type of the variable.

unsigned int& reference_variable;

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.

Function headers

Format function headers like this:

static bool
matches_patterns (const string_vector& patterns, int pat_idx,
                  int num_pat, const std::string& name)

The return type of the function and any modifiers are specified on the first line. The function name on the second line should start in column 1, and multi-line argument lists should be aligned on the first char after the open parenthesis. You should put a space before the left open parenthesis and after commas, for both function definitions and function calls.


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
      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 (...)


Use lowercase names if possible. Uppercase is acceptable for variable names consisting of 1-2 letters. Do not use mixed case names.

Header Files

Order of Includes

In source files (not headers files), use the following order with an empty line between each section:

  1. config.h
  2. The C++ wrappers for C headers (cstdlib, cassert, etc.)
  3. C++ standard library headers (iostream, list, map, etc.)
  4. 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)
  5. Octave's liboctave headers
  6. Octave's libinterp headers
  7. Octave's libgui headers

Other POSIX headers (sys/types.h, unistd.h, etc., should not be included directly into Octave sources. For portability, use a wrapper function. Anything you need is probably already available as a wrapper around a function or header provided by gnulib. See the files in liboctave/wrappers. This is necessary because although gnulib is great for portability, it does not generally work well with C++.

Similarly, Windows headers should not be included directly into Octave sources. Wrappers that provide the needed functionality in an OS-independent way should be used instead. This is almost always possible and there are many examples in the Octave sources.

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.

There is a strict ordering of header files/libraries that must be followed. There are no exceptions to these rules:

  • The functions in liboctave/wrappers may only use headers and symbols from gnulib, standard libraries, or OS-dependent libraries. They must not use any headers or symbols from other parts of liboctave, libinterp, or libgui.
  • liboctave must not use any headers or symbols from libinterp or libgui. It must be fully functional without the interpreter or GUI.
  • libinterp must not use any headers or symbols from libgui. It must be fully functional without the GUI.

As much as possible, header files should be independent of other header files.

Header files must not include config.h. Instead, they should begin by including octave-config.h.

Header files should not use any "#if defined (HAVE_FEATURE)" conditionals. This is not quite true yet, but we are almost there. No new conditionals may be added.

C++ features


Use references when passing variables that will be changed to subroutines rather than the C-style method of passing pointers.

When passing variables that are large, but will not be changed in a subroutine (read-only) , consider using 'const' references. This helps avoid overflowing the finite stack capacity of a program while still ensuring that read-only access is enforced.


When an empty string is required, use "", rather than creating an empty string object with std::string ().


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++ style casting

Always use one of the four C++ long style casting forms (static_cast, dynamic_cast, reinterpret_cast, const_cast) rather than C-style forms (type cast (new_type) variable or the function form new_type (variable)).

C++11 features

A C++11 compatible compiler is required for building Octave. Please make use of all C++11 features.

C++14, C++17, C++20 features

Try to avoid C++14, C++17, or C++20 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.

If the implementation using a C++14, C++17, or C++20 feature is very beneficial, make it optional via configure feature detection or also implement an alternative code in the absence of said feature. In any case, please get in contact with the Octave maintainers on Discourse.

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


Doxygen documentation can be a great help when developing octave however the current state has a lot of room for improvement. For more information about Doxygen in Octave look at Doxygen.

Doxygen Style Guide

Doxygen allows for a variety of commenting styles. In order to maintain uniformity across the entire project the following rules should be applied:

  • For Doxygen comments use only //! and NOT /*! ... */, regardless of the size of the block comment
  • Use @ for any Doxygen Special Commands
  • Use as little formatting as possible. Restrict to Markdown and avoid HTML-markup.
  • Do NOT use the @brief command, the first sentence will automatically be used as the summary description.
  • The first sentence should describe briefly what the function does and end with a period.
  • Leave a blank line between the Doxygen comment and function definition.

An example of properly used Doxygen would look like:

//! Does something interesting with its arguments.
//! Long comment with **bold** special commands.
//! @param some_param Really should figure out what to do.
//! @param another_param Does something cool with @p some_param.
//! And some example using syntax highlighting:
//! @code{.cc}
//! double v = 1.0;
//! int n = 2;
//! some_function (v, n);
//! @endcode

some_function (double some_param, int another_param)
  // ...


FIXME notes

The preferred comment mark for places that may need further attention is with FIXME: comments.