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Doxygen and BOOST_PP

There are many topics I’d like to broach here… In order to start the writing process anew, a first, small, easy post.


You might have noticed: I really enjoy messing with the infamous C preprocessor. Besides, I also took the habit of using Doxygen to automatically generate some documentation for my C++ project, as it offers a few interesting features in my opinion.

  • Unlike a separate hand-written API documentation, a documentation generated from the source code itself does not suffer the risk of being out of sync.
  • When there’s no other documentation available, a Doxygen generated documentation provides a good-enough source indexation and layout, which makes browsing for info a bit easier than just waltzing through source files.
  • Having a usable documentation can be just a matter of adding simple comments to the source code.

Overall, it is a very good solution for the lazy sloth I am, considering the limited scale of my projects. And furthermore, it can generate inclusion or inheritance graphs, and THAT is cool.


But in order to generate the documentation, Doxygen has to parse the source code. And due to BOOST_PP and to some similar macros usage, my code is sometimes somewhat… obfuscated. Usually, this means that the generated documentation is incomplete, or even sometimes plainly wrong. Here are two examples of the absurd or unreadable output it can yield…


The solution came from Doxygen’s filtering options. They allow one to specify how to preprocess a file before it is parsed. And with this, rather than letting Doxygen try to expand macros, we can use the tool that’s made for it: the compiler. The only tricky thing was to preserve comments and include directives, in order for Doxygen to do its job properly; but it was nothing a few sed calls couldn’t solve.

Ultimately, the resulting script is surprisingly small. Here it is in its entirety (you can also download it from here).

#! /usr/bin/env bash

CCOPTS="-C -x c++ -std=c++11 -I include -I src"
G1="__________B $(date +%s) B__________"
G2="__________E $(date +%s) E__________"

function surround()
    egrep    "^# *include" "$FILE" | grep -v '\.hxx.$'
    echo "$G1"
    egrep -v "^# *include" "$FILE"
    echo "$G2"

egrep "^# *include" "$FILE" | grep -v '\.hxx.$'
surround                       \
    | cpp $CCOPTS -            \
    | sed -n -e "/$G1/,/$G2/p" \
    | sed "/$G1\|$G2\|^#/d"

So, well, there it is. It isn’t much. But if, one day, you have to use Doxygen on some C++ source code which is full of macros… well, then, that day, you’ll be ready. :)

Thank you for reading!