[CMake] -std=c++17 vs -std=c++1y

[Robert Maynard]

If you have standard required enabled the compiler version shouldn't decay
down to 14 when asking for 17.

I recommend having a small test repo and using cmake --trace/--trace-expand
to determine where the gnu++1y is coming from. It is not clear when looking
at Modules/Compiler/Clang-CXX.cmake how gnu++1y would be used unless CMake
is thinking you are asking for C++14 on clang 3.5.
On Sun, May 6, 2018 at 1:35 PM Andrew Brownsword <
andrew.e.brownsword at gmail.com> wrote:

> Well, I am specifying:

> set(CMAKE_CXX_STANDARD_REQUIRED ON)
> set(CMAKE_CXX_STANDARD 17)
> set(CMAKE_CXX_EXTENSIONS ON)

> and the compiler line contains “-std=gnu++1y”.  I’m using a very recent
source build of CLang and libc++.  Any idea why cmake would fall back to 1y
in this case?





> On May 6, 2018, at 9:05 AM, Jean-Michaël Celerier <
jeanmichael.celerier at gmail.com> wrote:

> Note that c++17 is c++1z. c++1y was C++14.



> -------
> Jean-Michaël Celerier
> http://www.jcelerier.name

> On Sun, May 6, 2018 at 5:00 PM, Andrew Brownsword <
andrew.e.brownsword at gmail.com> wrote:

>> Hi,

>> Is there a way to get cmake 3.11 to emit 17 vs 1y when setting
CMAKE_CXX_STANDARD?  I’m finding that it seems to be a common practice to
use the value of the preprocessor define __cplusplus (which is a date), and
the value differs depending on whether 17 or 1y is used (at least with
CLang).  I haven’t figured out how to get cmake to emit -std=c++17, it
always seems to use 1y, which is causing problems with code that assumes
the __cplusplus date reflects the c++17 standardization date.

>> Thanks,
>>     Andrew

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