Skip to content
Snippets Groups Projects

[CMake] Rewrite C++ standard detection

Merged [CMake] Rewrite C++ standard detection
feature/cmake-rewrite-c++-standard-detection into master
Merged Steffen Müthing requested to merge feature/cmake-rewrite-c++-standard-detection into master
  1. Feb 17, 2016
  3. Feb 15, 2016
    • Steffen Müthing's avatar
      [CMake] It's -std=c++0x, of course · 0b825996
      Steffen Müthing authored 
      Oops.
      0b825996
    • Steffen Müthing's avatar
      [CMake] Remove unused variable · 840eb992
      Steffen Müthing authored
      840eb992
    • Steffen Müthing's avatar
      [CMake] Add function to allow modules to require a minimum C++ standard · c5e82ed2
      Steffen Müthing authored 
      This patch adds a new function dune_require_cxx_standard() that any Dune
module can call to require support for at least a given C++
standard. If the compiler doesn't meet the requirements, the function
will report the required and the actually available standard versions
and abort with a fatal error.

Moreover, it knows about CXX_MAX_STANDARD and will tell the user if the
value of that variable is below the requirements. This avoids desperate
users that have a shiny beta of GCC 6 with C++17 support wondering why
their own module using shiny C++17 concepts stubbornly fails to
build... ;-)
      c5e82ed2
    • Steffen Müthing's avatar
      [CMake] Rewrite C++ standard detection test · daa2e9f4
      Steffen Müthing authored 
      This patch is a complete rewrite of the existing C++ standard version
test that tries the various -std=c++xy flags and tries to figure out
which version of the standard to enable.

While my cleaned-up version of the old implementation generally works,
it has a few problems:

- it is not really extensible to newer standards (that will lead to the
  same kind of mess as before).

- More importantly, there is no way to say: I want C++11, but not
  C++14 (or, in the future, I want C++14, but not C++17). Being able to
  do so is important for testing and compatibility reason.

So this patch mostly starts from scratch: There is a list of supported
C++ standards (currently 17,14,11) and corresponding lists of compiler
tests and compiler flags to switch the compiler to the respective
standards (there might be different values for the switch, e.g. some
compilers accept -std=c++1y, but not -std=c++14, and recent versions of
GCC reject -std=c++1x).

The code then just iterates through these versions and flags until it
finds one that works. If that flag / version combo also passes the
compile test, the maximum supported standard version is recorded in the
variable CXX_MAX_SUPPORTED_STANDARD and the test stops. If the test
fails to find a working combination, it issues a warning to the user and
only records support for C++03.

The test can be influenced by two CMake variables:

- DISABLE_CXX_VERSION_CHECK already existed in the old version of the
  test. It completely disables all testing and requires the user to
  manually set CXX_MAX_SUPPORTED_STANDARD to the correct
  value. Moreover, any required compiler command line switches have to
  be manually added to CMAKE_CXX_FLAGS.

- CXX_MAX_STANDARD defines the maximum version of the standard that the
  build system will try to enable. With this new switch, it becomes
  possible to limit the compiler to an older version of the standard
  than what it could theoretically support. For now, this defaults to
  C++14.
      daa2e9f4
    • Steffen Müthing's avatar
      [CMake] Clean up the C++ version flag detection and add final test that is always run · 79bd9732
      Steffen Müthing authored 
      This patch cleans up the logic of the C++ version flag detection code,
which had accumulated quite a bit of cruft over time.

Moreover, it also adds a new test for basic C++11 compliance which is
always run, even if DISABLE_CXX_VERSION_CHECK has been set. I think the
original code was supposed to also error out in this case, but I didn't
really do so.

With this patch, DISABLE_CXX_VERSION_CHECK really only disables testing
for the various -std=c++xy flags, but the build system will now always
make sure that your compiler runs (at least) in C++11 mode.
      79bd9732
    • Steffen Müthing's avatar
      [CMake] Don't change CMAKE_REQUIRED_FLAGS when permanently setting -std=... · 1856b715
      Steffen Müthing authored 
      CMAKE_REQUIRED_FLAGS is explicitly designed for temporary flags used in
CMake's various check_<something> macros, so don't store any test
outcome in there.
      1856b715
    • Steffen Müthing's avatar
      [CMake] Don't pollute build-specific CXX_FLAGS with global compiler flags · 4f024207
      Steffen Müthing authored 
      The C++ version flag check used to set the -std=... flag for both the
basic CMAKE_CXX_FLAGS and the build-specific variants of it. That is
unnecessary because CMAKE concatenates the base variable and the
build-specific variant (if a build type has been set by the
user). Having both is actually rather unhelpful, as any downstream
changes will also have to be done to the complete set of variables.
      4f024207
    • Steffen Müthing's avatar
      [CMake] Move C++14 check into separate function · acdec4ee
      Steffen Müthing authored
      acdec4ee
    • Steffen Müthing's avatar
      [CMake][Cleanup] Consolidate include statements at beginning of module · 45cdc4b5
      Steffen Müthing authored
      45cdc4b5
    • Steffen Müthing's avatar
      [CMake] Remove unnecessary cmake_{push,pop}_check_state() pair · 820990a3
      Steffen Müthing authored
      820990a3
    • Steffen Müthing's avatar
      [CMake][Bugfix] Remove spurious cmake_pop_check_state() · 45f1352f
      Steffen Müthing authored
      45f1352f