[Discuss-gnuradio] Converting from #ifndef/#define include guards to #pragma once globally

[Marcus Müller]

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Hello community,

after finding a minor mistake in a header include file this week, I've
wanted to make sure that the GR header files have proper,
non-conflicting include guards.

So I want to open the discussion whether GR would want to change from
#ifndef-based include guards to #pragma once.

What I noticed is:
* there were 881 files that contained the classical
        #ifdef MACRO_WHATEVER
        #define MACRO_WHATEVER
        ...
        #endif /*maybe some comment*/
  scheme in current master
* replacing these with #pragma once has not damaged anything (as far
as one simple make test can tell)
* one minor mis-#define was found only which I believe is very
reassuring for our overall code quality (1) (2).

I've created a script (3) that checks (4) whether c++ files follow the
classical include guard methodology, and replaces that with a #pragma
once. cmake && make && make test worked fine on the first run :) which
made me confident enough to push everything (changed headers and the
script) to a branch:

https://github.com/marcusmueller/gnuradio/tree/convert_to_pragma_once

It would be great if people running other toolchains than I do
(gcc4.8.2/x86_64) would check if it breaks stuff.

If there is enough interest and consensus that we might want to switch
over to #pragma once (instead of just doing this "for testing purposes
only"), I think it'd be worthwhile to tackle things that stayed
untouched during automatic conversion:
* .t templates
* the newmod architecture

Greetings,
Marcus

(1) see my pull request against master
(2) however, vocoder/gsm/config.h uses #ifndef CONFIG_H, which I think
is a little bit asking for trouble
(3)
https://github.com/marcusmueller/gnuradio/blob/convert_to_pragma_once/dtools/bin/convert_ifndef_to_pragma_once
(4) checking is done by running it through the C preprocessor once to
get rid of comments and check the first line against an #ifndef macro,
and a second time to check whether defining that macro causes the file
to semantically disappear.
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