This post outlines a basic workflow for C++ projects using Doxygen, Sphinx, and Exhale.

## Background

My project proposal for documenting Symengine was recently accepted for the Google Summer of Docs initiative. In the past I have been more than happy to document C++ code using only Doxygen (with pretty fantastic results), while keeping example usage separate (d-SEAMS wiki). Though this is still a feasible method, a monolithic multi-project setup might benefit from Sphinx, which is what will be covered.

### Series

This post is the first in a series based on best C++ documentation practices for Sphinx+Doxygen workflows.

1. Documenting C++ with Doxygen and Sphinx - Exhale <– You are here!
2. Publishing Doxygen and Sphinx with Nix and Rake
3. Documenting C++ with Doxygen and Sphinx - doxyrest (TBD)
4. Adding Tutorials to Sphinx Projects (TBD)

## Goals

A couple of goals informed this approach:

• We expect our documentation to link to the source files
• We expect a lot of Python developers to contribute
• Hence Sphinx
• We would like to write .ipynb files into the docs
• Another reason to use Sphinx (via MyST{NB})

## Folder Structure

tree -d \$prj/ -L 2

.
├──docs
│  ├──Doxygen
│  └──Sphinx
├──nix
│  └──pkgs
├──projects
│  └──symengine
└──scripts
8directories

Essentially we have a scripts directory to store basic build scripts, and two kinds of documentation folders.

## Basic Doxygen

The doxygen setup is beautifully simple:

cd docs/Doxygen
doxygen -g
# Easier to edit
mv Doxyfile Doxyfile.cfg


Since Doxyfile should be updated by subsequent versions of doxygen, it is best to separate the project settings. We will therefore modify some basic settings in a separate file

touch Doxyfile-prj.cfg
vim Doxyfile-prj.cfg # or whatever


Edit the file to be (minimally):

@INCLUDE                = "./Doxyfile.cfg"
GENERATE_HTML           = NO
GENERATE_XML            = YES
XML_PROGRAMLISTING = NO

# Project Stuff
PROJECT_NAME           = "myProject"
PROJECT_BRIEF          = "Dev docs"
OUTPUT_DIRECTORY       = "./gen_docs"

# Inputs
INPUT                  = "./../../projects/symengine/symengine"
RECURSIVE              = NO


With this we will now be able to obtain the xml files for the rest of this setup.

## Exhale

For our first attempt, we will focus on the automation of Sphinx using the exhale tool.

# Basic setup
poetry init


Now we can generate the basic Sphinx structure.

# Separate source and build
sphinx-quickstart --sep --makefile docs/Sphinx \
--project "My Proj" \
--author "Juurj" \
--release "latest" \
--language "en"


This allows us to generate the Sphinx documentation we require, with some changes to the docs/Sphinx/source/config.py file (lifted from the exhale documentation):

extensions = [
'breathe',
'exhale',
]

# -- Exhale configuration ---------------------------------------------------
# Setup the breathe extension
breathe_projects = {
"My Proj": "./../../Doxygen/gen_docs/xml"
}
breathe_default_project = "My Proj"

# Setup the exhale extension
exhale_args = {
# These arguments are required
"containmentFolder":     "./api",
"rootFileName":          "library_root.rst",
"rootFileTitle":         "Library API",
"doxygenStripFromPath":  "..",
# Suggested optional arguments
"createTreeView":        True,
# TIP: if using the sphinx-bootstrap-theme, you need
# "treeViewIsBootstrap": True,
}

# Tell sphinx what the primary language being documented is.
primary_domain = 'cpp'

# Tell sphinx what the pygments highlight language should be.
highlight_language = 'cpp'


We also need to add the output to the index.rst use the following:

.. toctree::
:maxdepth: 2
:caption: Contents:

api/library_root


At this point we are ready to manually build our documentation.

cd docs/Doxygen
doxygen Doxyfile-prj.cfg
cd ../Sphinx
make html


This is still pretty cumbersome though. We can view our documentation in a more pleasant manner with darkhttpd.

darkhttpd docs/Sphinx/build/html


With this we now beautify the documentation (with the sphinx_book_theme):

poetry add sphinx-book-theme


We need to set the theme as well (in the Sphinx config.py file):

html_theme = 'sphinx_book_theme'


This leads to some pretty documentation.

## Conclusions

At this point, we have a basic setup, which we can tweak with a bunch of themes, and/or different parsers, but this is still pretty rough around the edges. However, a caveat of this setup is that the actual contents of the source are not visible in the generated documentation. In the next post, we will look at automating this setup for deploying with Travis.