Re: [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.brownsw...@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.celer...@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.brownsw...@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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Re: [CMake] -std=c++17 vs -std=c++1y

[Andrew Brownsword]

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 
>  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 
> mailto:andrew.e.brownsw...@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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Re: [CMake] -std=c++17 vs -std=c++1y

[Jean-Michaël Celerier]

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.brownsw...@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
>
> --
>
> Powered by www.kitware.com
>
> Please keep messages on-topic and check the CMake FAQ at:
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> information on each offering, please visit:
>
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> opensource/opensource.html
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Re: [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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[CMake] -std=c++17 vs -std=c++1y

[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.

Thanks,
   Andrew

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