gcc complains "pragma once" in *.cpp file

[bigcat26]

gcc version

c++ (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

compiler output:

[  1%] Building CXX object CMakeFiles/casbin.dir/casbin/enforcer.cpp.o
/usr/bin/c++    -g   -std=gnu++11 -o CMakeFiles/casbin.dir/casbin/enforcer.cpp.o -c /home/bigcat26/work/lab/third_party/casbin/casbin/enforcer.cpp
/home/bigcat26/work/lab/third_party/casbin/casbin/enforcer.cpp:17:9: warning: #pragma once in main file
   17 | #pragma once
      |         ^~~~
[  3%] Building CXX object CMakeFiles/casbin.dir/casbin/enforcer_cached.cpp.o
/usr/bin/c++    -g   -std=gnu++11 -o CMakeFiles/casbin.dir/casbin/enforcer_cached.cpp.o -c /home/bigcat26/work/lab/third_party/casbin/casbin/enforcer_cached.cpp
/home/bigcat26/work/lab/third_party/casbin/casbin/enforcer_cached.cpp:17:9: warning: #pragma once in main file
   17 | #pragma once
      |         ^~~~
[  5%] Building CXX object CMakeFiles/casbin.dir/casbin/internal_api.cpp.o
/usr/bin/c++    -g   -std=gnu++11 -o CMakeFiles/casbin.dir/casbin/internal_api.cpp.o -c /home/bigcat26/work/lab/third_party/casbin/casbin/internal_api.cpp
/home/bigcat26/work/lab/third_party/casbin/casbin/internal_api.cpp:17:9: warning: #pragma once in main file
   17 | #pragma once
      |         ^~~~
[  7%] Building CXX object CMakeFiles/casbin.dir/casbin/management_api.cpp.o
/usr/bin/c++    -g   -std=gnu++11 -o CMakeFiles/casbin.dir/casbin/management_api.cpp.o -c /home/bigcat26/work/lab/third_party/casbin/casbin/management_api.cpp
/home/bigcat26/work/lab/third_party/casbin/casbin/management_api.cpp:17:9: warning: #pragma once in main file
   17 | #pragma once
      |         ^~~~
[  8%] Building CXX object CMakeFiles/casbin.dir/casbin/model/function.cpp.o
/usr/bin/c++    -g   -std=gnu++11 -o CMakeFiles/casbin.dir/casbin/model/function.cpp.o -c /home/bigcat26/work/lab/third_party/casbin/casbin/model/function.cpp
/home/bigcat26/work/lab/third_party/casbin/casbin/model/function.cpp:17:9: warning: #pragma once in main file
   17 | #pragma once
      |         ^~~~
[ 10%] Building CXX object CMakeFiles/casbin.dir/casbin/rbac_api.cpp.o
/usr/bin/c++    -g   -std=gnu++11 -o CMakeFiles/casbin.dir/casbin/rbac_api.cpp.o -c /home/bigcat26/work/lab/third_party/casbin/casbin/rbac_api.cpp
/home/bigcat26/work/lab/third_party/casbin/casbin/rbac_api.cpp:17:9: warning: #pragma once in main file
   17 | #pragma once
      |         ^~~~
[ 12%] Building CXX object CMakeFiles/casbin.dir/casbin/rbac_api_with_domains.cpp.o
/usr/bin/c++    -g   -std=gnu++11 -o CMakeFiles/casbin.dir/casbin/rbac_api_with_domains.cpp.o -c /home/bigcat26/work/lab/third_party/casbin/casbin/rbac_api_with_domains.cpp
/home/bigcat26/work/lab/third_party/casbin/casbin/rbac_api_with_domains.cpp:17:9: warning: #pragma once in main file
   17 | #pragma once
      |         ^~~~
[ 14%] Building CXX object CMakeFiles/casbin.dir/casbin/util/built_in_functions.cpp.o
/usr/bin/c++    -g   -std=gnu++11 -o CMakeFiles/casbin.dir/casbin/util/built_in_functions.cpp.o -c /home/bigcat26/work/lab/third_party/casbin/casbin/util/built_in_functions.cpp
/home/bigcat26/work/lab/third_party/casbin/casbin/util/built_in_functions.cpp:17:9: warning: #pragma once in main file
   17 | #pragma once
      |         ^~~~

[hsluoyz]

@bigcat26 can you make a PR to fix it?

[hsluoyz]

@Zxilly

[Zxilly]

got it, working on this

[bigcat26]

find . -name '*.cpp' |xargs sed -i -z -e  "s/#pragma once\n\n//"
# commit & push

[Zxilly]

@bigcat26 seems #pragma once was added deliberately
ref to: b69e3a3, 18b41bc and 52929cb
Warnings were explicitly ignored.
Maybe prepared for some magic usage?

[bigcat26]

@bigcat26 seems #pragma once was added deliberately
ref to: b69e3a3, 18b41bc and 52929cb
Warnings were explicitly ignored.
Maybe prepared for some magic usage?

I love magic.
I just think replace '#ifdef/#define/#endif' with '#pragma once' in header files is reasonable..
Pragma_once

[Zxilly]

@bigcat26 so just add #pragma once in *.h?
I think they can work together.

[divy9881]

MS VC++ requires #pragma once for its efficient execution on windows platform. We cannot do much about it. That's a trade-off we need to make while building the same code on linux. I have prepared 2 separate routines in Makefile script to build the library.

make

The above sets off the warnings. But, under development environment we do need warning logs. Under such cases, we build using

make debug

[hsluoyz]

@divy9881 @bigcat26 can we wrap the #pragma once into a ifdef? Like:

#ifdef WINDOWS_MSVC
#pragma once
#endif

[bigcat26]

1. In my knowledge, pragma once is a compiler directive, it just work at compile time, it is replacement of header guard macro.
2. MSVC compiler support "precompile header" help speed up compile progress.
3. Is there any document describes how "pragma once" optimize msvc program runtime speed?

[Zxilly]

@bigcat26 https://docs.microsoft.com/en-us/cpp/preprocessor/once?view=msvc-160

We recommend the #pragma once directive for new code because it doesn't pollute the global namespace with a preprocessor symbol. It requires less typing, it's less distracting, and it can't cause symbol collisions. Symbol collisions are errors caused when different header files use the same preprocessor symbol as the guard value. It isn't part of the C++ Standard, but it's implemented portably by several common compilers.

[bigcat26]

@bigcat26 https://docs.microsoft.com/en-us/cpp/preprocessor/once?view=msvc-160

We recommend the #pragma once directive for new code because it doesn't pollute the global namespace with a preprocessor symbol. It requires less typing, it's less distracting, and it can't cause symbol collisions. Symbol collisions are errors caused when different header files use the same preprocessor symbol as the guard value. It isn't part of the C++ Standard, but it's implemented portably by several common compilers.

It has an effect similar to the include guard idiom, which uses preprocessor macro definitions to prevent multiple inclusions of the contents of the file.

[EmperorYP7]

I don't think there is any noticeable compile time performance difference between #pagma once and include-guards on MSVC++ which is evident in this SO discussion. Include guards are more generic and won't throw warnings on compilation with Clang and GCC on MacOS and Linux (tested it myself). May I go ahead and create a PR for this?

[github-actions]

🎉 This issue has been resolved in version 1.11.6 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[hsluoyz]

Good work!