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

[Rolf Eike Beer]

Am Sonntag, 6. Mai 2018, 08:00:14 schrieb Andrew Brownsword:
> 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.

My bet is: that code is wrong. For every compiler I have seen the rules have 
been:

-std=c++AA is used as long as the standard is not fixed

-afterwards std=c++YY is introduced, and -std=c++AA is an alias for that

So it does not matter which flag you use when the compiler supports both, but 
you can't of course properly test on the value of __cplusplus in older 
compilers because you don't know in which year/month the standard will be 
released.

Eike
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