[Development] Using '#pragma once' instead of include guards?

[Volker Hilsheimer]

> On 11 Oct 2022, at 22:11, Thiago Macieira <thiago.macieira at intel.com> wrote:
> 
> On Tuesday, 11 October 2022 12:25:13 PDT Kyle Edwards via Development wrote:
>> Speaking as co-maintainer of CMake, we have effectively required #pragma
>> once to build CMake itself since August 2017, we officially codified
>> this as policy in September 2020, and we will soon be writing a
>> clang-tidy plugin to enforce this in our CI. We have not received any
>> complaints about it. Just my $0.02.
> 
> Thanks for the information. This confirms what we already knew that all systems 
> and compilers where Qt would be compiled do support it.
> 
> However, neither Qt Creator nor CMake are libraries. They are not comparable.


Thanks all for sharing your insights and digging up the previous discussions as well.

The summary of all this then seems to be:

- ok to use '#pragma once’ in headers that are not designed to be included by Qt users, i.e. in tools, applications, examples and demos, tests
- for everything else, in particular for public and, for consistency’s sake - private headers in Qt, we continue to use conventional include guards

Rationale: #pragma once is not well enough defined and not part of the standard, and we cannot make any assumptions about how Qt is installed, used as part of a larger SDK etc. So best to stay conservative.

If that’s not entirely off, then I’d like to put this into https://wiki.qt.io/Coding_Conventions [1], preempting perhaps a new thread on this topic in a few years.

Volker

[1]: And since that page seems rather outdated - e.g. we do use dynamic_cast in Qt today, and the suggestion to normalize signals and slots should rather suggest to make connections via PMF syntax - perhaps it’s time to move this to a QUIP where we can discuss and review such changes in gerrit. I won’t have time to do that for a while (perhaps ditto for https://wiki.qt.io/Qt_Coding_Style), but perhaps someone else wants to give this a shot.