[CMake] target_compile_features() uses incorrect C++ version

[Kim Walisch]

Hi,

I have realized that my C++ primesieve project (
https://github.com/kimwalisch/primesieve) is
compiled using -std=gnu++11 with GCC 7.3 and CMake 3.10 (on Ubuntu 18.10
x64) even
though the default C++ version used by GCC 7.3 is C++14 as checked below:

$ g++ -dM -E -x c++ /dev/null | grep -F __cplusplus
#define __cplusplus 201402L

Note that I have not set CXX_STANDARD manually in my CMakeLists.txt so I
was expecting
that my project would be compiled either with -std=gnu++14 or without any
C++ version flag.
I spent some time investigating where the -std=gnu++11 flag comes from and
I found that it is
related to the use of target_compile_features():

target_compile_features(primesieve PRIVATE cxx_auto_type)

This code tells the compiler that my program uses features from C++11 and
that the compiler
should enable C++11 if the default C++ version of the compiler is e.g.
C++98. I would
however expect that if the default C++ version of the compiler is > C++11
CMake would not
add -std=gnu++11 to the compiler flags.

For me this is a CMake bug.

Thanks,
Kim Walisch

[Deniz Bahadir]

Post by Kim Walisch
Hi,
I have realized that my C++ primesieve project
(https://github.com/kimwalisch/primesieve) is
compiled using -std=gnu++11 with GCC 7.3 and CMake 3.10 (on Ubuntu 18.10
x64) even
$ g++ -dM -E -x c++  /dev/null | grep -F __cplusplus
#define __cplusplus 201402L
Note that I have not set CXX_STANDARD manually in my CMakeLists.txt so I
was expecting
that my project would be compiled either with -std=gnu++14 or without
any C++ version flag.
I spent some time investigating where the -std=gnu++11 flag comes from
and I found that it is
target_compile_features(primesieve PRIVATE cxx_auto_type)
This code tells the compiler that my program uses features from C++11
and that the compiler
should enable C++11 if the default C++ version of the compiler is e.g.
C++98. I would
however expect that if the default C++ version of the compiler is >
C++11 CMake would not
add -std=gnu++11 to the compiler flags.
For me this is a CMake bug.
Thanks,
Kim Walisch

Hi Kim,

this does indeed sound like some unwanted behavior.

I recommend you open an issue in CMake's issue-tracker
(https://gitlab.kitware.com/cmake/cmake/issues) and copy your
email-content there.

Best regards,
Deniz Bahadir
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[Deniz Bahadir]

https://gitlab.kitware.com/cmake/cmake/issues/18686
Greetings,
Kim

Thanks, great.

Best regards,
Deniz

PS: Next time better answer to the mailing-list. ;-) This is probably
interesting for others that read your first email on the list, too.

Post by Kim Walisch
Hi,
I have realized that my C++ primesieve project
(https://github.com/kimwalisch/primesieve) is
compiled using -std=gnu++11 with GCC 7.3 and CMake 3.10 (on

Ubuntu 18.10

Post by Kim Walisch
x64) even
though the default C++ version used by GCC 7.3 is C++14 as
$ g++ -dM -E -x c++  /dev/null | grep -F __cplusplus
#define __cplusplus 201402L
Note that I have not set CXX_STANDARD manually in my

CMakeLists.txt so I

Post by Kim Walisch
was expecting
that my project would be compiled either with -std=gnu++14 or

without

Post by Kim Walisch
any C++ version flag.
I spent some time investigating where the -std=gnu++11 flag comes

from

Post by Kim Walisch
and I found that it is
target_compile_features(primesieve PRIVATE cxx_auto_type)
This code tells the compiler that my program uses features from

C++11

Post by Kim Walisch
and that the compiler
should enable C++11 if the default C++ version of the compiler is

e.g.

Post by Kim Walisch
C++98. I would
however expect that if the default C++ version of the compiler is >
C++11 CMake would not
add -std=gnu++11 to the compiler flags.
For me this is a CMake bug.
Thanks,
Kim Walisch

Hi Kim,
this does indeed sound like some unwanted behavior.
I recommend you open an issue in CMake's issue-tracker
(https://gitlab.kitware.com/cmake/cmake/issues) and copy your
email-content there.
Best regards,
Deniz Bahadir
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