CMake does not set the compiler option -std to gnu17 or c++17 although I set the target_compile_features to cxx_std_17

Usage
1

I updated the versions of GCC and CMake. After the update CMake no longer sets the right command line option for the c++ standard.

Here is how I set it:

add_library(project_options INTERFACE)
target_compile_features(project_options INTERFACE cxx_std_17)

and then I link to the project_options

target_link_libraries(app PRIVATE project_options)

With GCC 8.3 (on rhel) this sets the std compile flag to -std=gnu17

Now it is set to -std=c++11

If I set the compiler globally it works as expected:

set(CMAKE_CXX_STANDARD 17)

What am I missing? Or is this a bug in the new Tools?

My System:

  • GCC: 11.1.0
  • CMake: 3.20.2

This is a cross post from:
c++ - CMake does not set the compiler option -std to gnu17 or c++17 although I set the target_compile_features to cxx_std_17 - Stack Overflow

2

This has probably something to do with the fact that GCC11 defaults to -std=gnu++17
https://gcc.gnu.org/gcc-11/changes.html

1 Like
3

Interesting.

But why is then the standard set to c++11, if I use GCC11

4

This sounds like a regression to me. What version of CMake and GCC worked before?

5

It also did not work with the CMake Version 19.x and GCC11
But it worked with CMake Version 19.x and GCC10.2

6

OK, sounds like an environmental regression then. We are in the process of updating our CI to Fedora 34 which includes GCC 11, so this might show up there.

FYI @brad.king

1 Like
7

Thank you!
Could you write here once this is fixed?

8

This is working as designed. The cxx_std_17 feature is documented here. It does not mean “pass -std=c++17”. It means “run the compiler in a mode that supports C++17”. If the default compiler mode supports C++17, no flag is added.

gcc 11 defaults to C++17, and so does not need any -std= flag to satisfy cxx_std_17. On Fedora 34:

$ g++ --version
g++ (GCC) 11.1.1 20210428 (Red Hat 11.1.1-1)
$ g++ -x c++ /dev/null -E -dM | grep __cplusplus
#define __cplusplus 201703L
1 Like
9

Ok thank you, this makes perfect sense So why is there the c++11 flag after setting it? Maybe inherited from somewhere else? Can I override it?

10

Please post an example CMakeLists.txt file demonstrating how a -std=c++11 flag is showing up.

11

Thank you very much for your help!
The problem was that a fetched and build dependency sets the global standard to set(CMAKE_CXX_STANDARD 11).
But I think it is quite cumbersome that even though I require a target to be built with c++17 it is not. Is there another option then setting it globally?

12

I cannot reproduce that either:

$ cat ../CMakeLists.txt
cmake_minimum_required(VERSION 3.19)
project(Forum3299 CXX)
set(CMAKE_CXX_STANDARD 11)
add_library(example STATIC example.cxx)
target_compile_features(example PRIVATE cxx_std_17)

$ cmake ..
-- The CXX compiler identification is GNU 11.0.1
...

$  grep CMAKE_CXX_STANDARD_COMPUTED_DEFAULT CMakeFiles/*/CMakeCXXCompiler.cmake
set(CMAKE_CXX_STANDARD_COMPUTED_DEFAULT "17")

$ make VERBOSE=1 
...
/usr/bin/c++   -std=gnu++17 -o CMakeFiles/example.dir/example.cxx.o -c /.../example.cxx
...
13

I also wasn’t able to reproduce a minimal not working example.

I will try to figure out was causes the problem. Thank you for helping!

14

Your gcc was 11.0

It is v11.1 that come with c++17 as default

15

I came accross this today as well. It’s quite annoying because some of the lint (e.g. clangd) rely on compiler_commands.json for the settings. They don’t know gcc version and hence cannot infer --std

16

See my above reply. The behavior of cxx_std_## not always adding -std= is now also documented here. If you want explicit -std= options to appear in compile_commands.json, you need to set CMAKE_CXX_STANDARD to the desired standard level. That setting uses the given standard unless a cxx_std_## feature needs a higher one, and therefore should always pass a -std= flag.

1 Like
17

Hello,

I am working with CMake 3.22.1, emacs IDE and clangd 12 as static analyzer. I have an interface library with a set of tests, and even if I set CXX_STANDARD 17 either for my library or for my tests, I don’t get the -std=… option in the compile_commands.json. I also tried setting CMAKE_CXX_STANDARD, but that doesn’t work either. Am I missing something?

18

Brad linked to the 3.23 docs. I suspect that 3.23 might be where that behavior was introduced.

19

Oops thanks for the remark!
I managed to force the flag though, by adding the CXX_STANDARD_REQUIRED ON property along with the CXX_STANDARD one.
Thanks for you help.