Doxygen Style Guide

We write all the API documentation with Doxygen. The add.h and add.cpp are reference implementations of the style guide.

Language and Locale

  • Documentation should be in English.
  • All dates are YYYY-MM-DD.
  • We strongly discourage the use of Unicode symbols.
    • Symbols inside TeX math modes are fine.
  • Numbers which are return values should not be in math mode.
  • We use snake_case for the variables and functions.
  • Use . at the end of each list item
  • When adding in-text references, any style (APA, MLA, Chicago, IEEE, etc.) is allowed.
    • It is a good practice to add to the relevant .bib file in the repository.


  • Doxygen markup is preferred for single words.
    • @a for italics (parameters).
    • @b for bold (single emphasis).
    • @c for monospace (for file names / code symbols).
  • For multiple words, we use the markdown syntax.
  • Never use HTML.


// Do
//!< @note
//!< \note
  • We use //!< for inline documentation (see above) so keep the caveats in mind from the Doxygen manual.
  • Non documenting // and /**/ are used to describe steps of an algorithm in-place.

Documentation Blocks

  • We use the /** style.
* @note This is acceptable


  • The author of the documentation is supposed to ensure that the documentation is legible in the code.
  • We use one space before, and two spaces after *.
* @note This is acceptable since it has two spaces between the * and the word
  • We use one space to indicate a paragraph.
* @note This is acceptable since it has two spaces between the * and the word.
* It is also an acceptable paragraph because of the leading indentation.
* @warning This is a pointlessly long paragraph to demonstrate the @b wrong
* way to indent a paragraph
  • Lists are to be indented for legibility as well.
* @note I am a good list:
* - Item 1.
* - Item 2.
* - SubItem 1.
* - SubItem 2.
* - Item 3.
* - SubItem 1.

Source Files

Each file shall begin with a @file block as shown:

* @file blah.extension
* @author SymEngine Developers
* @date 2021-02-25
* @brief A very brief but compelling explanation of this file
* Created on: 2012-07-11
* This is a single paragraph which really makes you feel like reading the rest
* of this file because it is really very interesting and has a bunch of @ref
* tags to make things easier

The salient points are:

  • @file must be present and is simply the filename.
  • @author is always SymEngine Developers.
    • We let git handle more granular attribution.
  • @date is the date the file was last modified.
    • This is inclusive of documentation changes.
  • @brief should be a single line about the contents of the file.
  • Created on: is not a Doxygen directive, but should be present.
  • The paragraph is meant to describe the logical layout of the file.
    • @ref tags are meant to allow the user to jump to relevant sections more easily.

Note that this block is to appear before any #ifndef and #include preprocessor directives.


It is in the .h file that @ingroup and @defgroup directives are used for grouping. The logical grouping follows the layout of the tests directory.


To maintain the logical division of .cpp and .h files, we disallow long comments in header files. This is also to reduce the compilation time when changing minor documentation[^1]. Anything longer than one line should be replaced with a short description which is expanded on in a @note in the corresponding .cpp. Acceptable special directives are (in order):

  • @brief for a pithy description of the entity.
  • @pre for describing preconditions (optional).
  • @see for related functions.
  • @param one for each input parameter.
    • @param[out] is not used, we have the .cpp to describe the effects of void functions.
  • @return one for each possible return value with a line on the condition.
    • Keep this short, the .cpp has the implementation details.
    • void functions should @return Void. (including the .).
  • @relatesalso takes a single class and groups the function with the class in the output (optional).
    • This is preceded by a line.

Note that since C++ is strongly typed, there is no need to describe the type of inputs (@param) or the outputs.

For example:

* @brief The best number generator in the universe
* @pre Does assume a known universe
* @see `other_best` for an example of another number generator
* @param seed is the reproducibility helper
* @param am_awesome is an indicator of awesomeness
* @return the best number if the caller is awesome
* @return a random number if the caller is not awesome
* int best_number(const int &seed, std::string am_awesome);

Inline Functions

These are typically simply a @return directive in a single line //!<; but should be used sparingly. If the function in question requires more documentation it probably should not be inline.

Code Files

  • The order of function definition must match the declaration in the header file

Here we expect:

  • @details for explanations of the overall algorithm, speed concerns, etc.
  • @note for longer details of the parameters or other important issues.
  • @warning for pitfalls.
  • Other special directives as required.
  • @see is allowed in both headers and code files, but only sparingly.
* @details The best number is determined by a complicated algorithm described
* over many paragraphs with equations here
* @f$ \mathbb{N}^3\to\mathbb{R}^N @f$
int best_number(const int &seed, std::string am_awesome){
return 0; // Clearly the documentation is lying to you

This formulation; with comments in the code, to augment the documentation blocks is the most legible method.

Source Code


  • We do expect private variables to be documented.

Classes descriptions have the @class directive declared before @brief in .h files and before @details in the .cpp as well.


  • All documentation blocks must precede the function being documented.

External Tools

We expect the build system to be able to find graphviz for dot.

[^1]: Changing a header triggers the recompilation of the entire project.