Utilities for converting from C/C++ include guards to #pragma once and back again.
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Utilities for converting from C/C++ include guards to #pragma once and back again.

Why Convert?

Include guards suck. They're tiring to type and tedious to update. Worse, the task of updating boilerplate leaves room for copy/paste errors, or other mistakes. #pragma once is simpler and less error prone. That's why you should convert to #pragma once.

Alas, though #pragma once is available on all the most commonly used compilers, it's not available on every compiler. Perhaps one day you'll add support for a platform with a barebones compiler with no support for #pragma once and you'll have to convert back. That's ok. It's easy!

What exactly is guardonce?

There are three main tools provided by guardonce:

  1. checkguard helps find any broken include guards you may already have in your project. These should be addressed before converting.

  2. guard2once converts files with include guards into files with #pragma once directives. This ensures your entire project is consistently using #pragma once.

  3. once2guard converts files with #pragma once directives back into files with include guards. This ensures your entire project is consistently using include guards.

How to use:

First, check your project for broken headers. To recursively search your project directories for the names of all files that lack proper include guards, use the following command, substituting your project's directory for the quoted string:

checkguard -r "source_directory"

By default, checkguard is very forgiving. It accepts either #pragma once or anything that looks like an include guard. If you know that all your guards should match some format, you can be more strict by using -p to specify a pattern to check against.

If certain files are not supposed to have include guards, feel free to leave them be. Files without include guards are ignored by this next step.

Now, all that remains is converting the headers to use #pragma once:

guard2once -r "source_directory"

You're done! Double check that the result matches your expectations and start using #pragma once in your new code. Know that if you ever need to switch back, it's as simple as:

once2guard -r "source_directory"

If the default guard style doesn't appeal to you, there are a few options to customize it. Maybe take a look through once2guard --help or check out a walkthrough for some examples.

How to Install:

Whether you use Python 2 or Python 3, these tools can be installed with pip. Run python -m pip install guardonce and you're off to the races.

If you'd rather not use pip, it is possible to instead just run from the repository. However, you'll need to use slightly different commands. Add the repository to your PYTHONPATH and invoke the tools as python modules, as illustrated below.

Linux / OSX

git clone https://github.com/cgmb/guardonce.git
export PYTHONPATH="$(pwd)/guardonce"
python -m guardonce.checkguard -r ~/myproject


git clone https://github.com/cgmb/guardonce.git
set "PYTHONPATH=%CD%\guardonce"
python -m guardonce.checkguard -r ~/myproject

Note that on Windows you might need to invoke guardonce via python -m even if you install with pip.