i am by no means a C++ expert, but isn't "pragma once" frowned upon?
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No, this is completely wrong. Pragma once is non-standard compiler directive. It might be supported by some compilers such as msvc but technically it is not even C++.
There are only two options: include guards, and modules.
I think you're suggesting that you don't need to make up the names for include guards because all tools / IDEs for C++ write them for you automatically anyway. But that isn't my experience. Many IDEs don't write include guards for you automatically ... because everybody uses #pragma once already.
> #pragma once is broken by design
I think you're referring to the historical problem with #pragma once, which is that it can be hard for the compiler to identify what is really the same file (and therefore shouldn't be included a second time). If you hard link to the same file, or soft link to it, is it the same? What if the same file is mapped to two different mount points? What if genuinely different files have the same contents (e.g., because the same library is included from two different installation paths)? In practice, soft/hard links to the same file are easily detectable, and anything more obscure indicates such a weird problem with your setup that you surely have bigger issues. #pragma once is fine.
(Historically, it also had the benefit that compilers would know not to even re-read the header, whereas with traditional include guards they would need to re-include the file (e.g. in case the whole file is not wrapped in the #ifdef, or in case something else has undefined it since) only to then discard the contents. I've even seen coding guidelines requiring external include guards wrapped around every use of headers with #include <...>. Yuck! But modern compilers can work out when include guards are meant to mean that so today that difference probably no longer exists.)
They don't. They are not C++ and at most they are compiler-specific.
It's fine if you opt to not write C++ and instead target specific compilers instead. Just don't pretend it's not frowned upon or kosher.
The whole point is that it's not supported and it's not standard, thus using #pragma once needlessly introduced the risk of having the code break.
You should ask yourself what are you doing and why are you using non-standard constructs that may or may not work, specially when it's rather obvious and trivial to just use include guards. Using #pragma once isn't even qualify as being clever to gain anything.
Since you seem to be more knowledgeble about this, I'm curious to know which C++ compilers lack support? I know that at least the 3 big ones do (GCC, Clang, and MSVC) and they have for a very long time.
If the standards still don't have a proper replacement for include guards, then too bad for the standards. The C++ standard wasn't aware of multithreading before C++11, this didn't stop people from writing multithreaded programs.
As to why - #pragma once is just cleaner to look at.
In the C++ community (as lots of other things are), rejecting `#pragma once` is a long-standing tradition of worshipping the decaying body of prehistoric compilers for
It's unclear what benefits this approach has achieved, but don't disturb it, or else.
But given that I haven't seen any mention of that issue in other comments, I wonder if it really is an issue.
Plus, it's nicer to read than #ifndef FOO_BAR_BAZ_PROJ_DIR1_DIR2_DIR3_FILE_H
#endif /* FOO_BAR_BAZ_PROJ_DIR1_DIR2_DIR3_FILE_H */
On every file.
`#pragma once` seems to be far preferred for internal code, there's an argument for being strictly conforming if you're putting out a library. I've converted stuff to `#ifndef` before sharing it, but I think heavy C++ people usually type `#pragma once` in the privacy of their own little repository.
- `spdlog`: `#pragma once` https://github.com/gabime/spdlog/blob/v1.x/include/spdlog/as...
- `absl`: `#ifndef` https://github.com/abseil/abseil-cpp/blob/master/absl/base/a...
- `zpp_bits`: `#ifndef` https://github.com/eyalz800/zpp_bits/blob/main/zpp_bits.h
- `stringzilla` `#ifndef` https://github.com/ashvardanian/StringZilla/blob/main/includ...