Re: [cmake-developers] Wrong cxx-extension flags

[Kornel Benko]

Am Donnerstag, 14. Juli 2016 um 16:06:17, schrieb Brad King 

> On 07/14/2016 03:59 PM, Kornel Benko wrote:
> >> What you can do is not add your own flag and instead do
> >>
> >>  set(CMAKE_CXX_STANDARD 14)
> >>  set(CMAKE_CXX_EXTENSIONS OFF)
> > 
> > I did this, as it seemed least invasive.
> > But now, instead of '-std=gnu++1' I got '-std=c++11' which is not better.
> > Of course, since QT5.7 explicitly requires   C++11 features, we don't need 
> > C++14.
> 
> I don't quite follow here.  What flag to you want to get?

I meant we don't need to test for c++14.

> Don't add your own flag explicitly.  Some combination of
> the above settings can get the flag you need.

I have to stick also with older cmake versions, so bear with me.

Nonetheless, I tested some combinations already, and it all looks good.

> -Brad

Kornel


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Re: [cmake-developers] Wrong cxx-extension flags

[Kornel Benko]

Am Donnerstag, 14. Juli 2016 um 15:27:00, schrieb Brad King 

> On 07/14/2016 02:48 PM, Kornel Benko wrote:
> > I am asking about '-std=gnu++11'.
> On 07/14/2016 02:45 PM, Kornel Benko wrote:
> > The new cmake-files from QT5.7 are somehow responsible that this happens.
> 
> The file `lib/cmake/Qt5Core/Qt5CoreConfigExtras.cmake` in Qt 5.7 contains:
> 
>   set_property(TARGET Qt5::Core PROPERTY INTERFACE_COMPILE_FEATURES 
> cxx_decltype)
> 
> This tells CMake that dependents of this library need to be compiled as
> at least C++11.  This causes `-std=gnu++11` to be added automatically.

Thanks Brad, that was what I was searching for.

> What you can do is not add your own flag and instead do
> 
>  set(CMAKE_CXX_STANDARD 14)
> 
> or set the CXX_STANDARD property of specific targets.  That tells CMake
> to build as C++14 so it will add -std=gnu++14.  If you don't want the
> extensions then also set
> 
>  set(CMAKE_CXX_EXTENSIONS OFF)l

I did this, as it seemed least invasive.
But now, instead of '-std=gnu++1' I got '-std=c++11' which is not better.
Of course, since QT5.7 explicitly requires   C++11 features, we don't need 
C++14.

> Relevant docs:
> 
>  https://cmake.org/cmake/help/v3.6/manual/cmake-compile-features.7.html
>  https://cmake.org/cmake/help/v3.6/variable/CMAKE_CXX_STANDARD.html
>  https://cmake.org/cmake/help/v3.6/variable/CMAKE_CXX_EXTENSIONS.html
> 
> -Brad

Kornel

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Re: [cmake-developers] Wrong cxx-extension flags

[Brad King]

On 07/14/2016 02:48 PM, Kornel Benko wrote:
> I am asking about '-std=gnu++11'.
On 07/14/2016 02:45 PM, Kornel Benko wrote:
> The new cmake-files from QT5.7 are somehow responsible that this happens.

The file `lib/cmake/Qt5Core/Qt5CoreConfigExtras.cmake` in Qt 5.7 contains:

  set_property(TARGET Qt5::Core PROPERTY INTERFACE_COMPILE_FEATURES 
cxx_decltype)

This tells CMake that dependents of this library need to be compiled as
at least C++11.  This causes `-std=gnu++11` to be added automatically.

What you can do is not add your own flag and instead do

 set(CMAKE_CXX_STANDARD 14)

or set the CXX_STANDARD property of specific targets.  That tells CMake
to build as C++14 so it will add -std=gnu++14.  If you don't want the
extensions then also set

 set(CMAKE_CXX_EXTENSIONS OFF)

Relevant docs:

 https://cmake.org/cmake/help/v3.6/manual/cmake-compile-features.7.html
 https://cmake.org/cmake/help/v3.6/variable/CMAKE_CXX_STANDARD.html
 https://cmake.org/cmake/help/v3.6/variable/CMAKE_CXX_EXTENSIONS.html

-Brad
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Re: [cmake-developers] Wrong cxx-extension flags

[Kornel Benko]

Am Donnerstag, 14. Juli 2016 um 14:36:43, schrieb Brad King 

> In lyx I see the
> 
>  development/cmake/modules/FindCXX11Compiler.cmake

I had to reread.
Yes, I know this file too. There is set the '-std=c++14'. But I am asking about 
'-std=gnu++11'.

Kornel

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Re: [cmake-developers] Wrong cxx-extension flags

[Kornel Benko]

Am Donnerstag, 14. Juli 2016 um 14:36:43, schrieb Brad King 

> On 07/14/2016 01:59 PM, Kornel Benko wrote:
> > I was trying to compile lyx with QT5.7.
> > ...
> > found that the flags in the flag.cmake files were mismatched.
> > 
> > Therefore I started searching, where the extra ' --std=gnu++11' came
> > from, and found it in Modules/Compiler/GNU-CXX.cmake.
> 
> It appears there but only in a table.  Other settings are required
> to get the table entry to be used.  See the CXX_STANDARD target
> property:
> 
>  https://cmake.org/cmake/help/v3.6/prop_tgt/CXX_STANDARD.html
> 
> Unless you or the project is setting that (perhaps via the
> CMAKE_CXX_STANDARD variable) then CMake won't add any flags.
> 
> In lyx I see the
> 
>  development/cmake/modules/FindCXX11Compiler.cmake

No, this file is only used for QT4.
The new cmake-files from QT5.7 are somehow responsible that this happens.
If I compile with QT5.6 (which also has files like Qt5CoreConfig.cmake) no such 
problem occurs.

> source file that adds such flags.  Please ask them about it.

I know, I am one of the creators. But, as already said, this is not the case 
here.

> -Brad

Kornel

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Re: [cmake-developers] Wrong cxx-extension flags

[Brad King]

On 07/14/2016 01:59 PM, Kornel Benko wrote:
> I was trying to compile lyx with QT5.7.
> ...
> found that the flags in the flag.cmake files were mismatched.
> 
> Therefore I started searching, where the extra ' --std=gnu++11' came
> from, and found it in Modules/Compiler/GNU-CXX.cmake.

It appears there but only in a table.  Other settings are required
to get the table entry to be used.  See the CXX_STANDARD target
property:

 https://cmake.org/cmake/help/v3.6/prop_tgt/CXX_STANDARD.html

Unless you or the project is setting that (perhaps via the
CMAKE_CXX_STANDARD variable) then CMake won't add any flags.

In lyx I see the

 development/cmake/modules/FindCXX11Compiler.cmake

source file that adds such flags.  Please ask them about it.

-Brad

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Re: [cmake-developers] Wrong cxx-extension flags

[Kornel Benko]

Am Donnerstag, 14. Juli 2016 um 13:35:06, schrieb Brad King 

> On 07/14/2016 01:13 PM, Kornel Benko wrote:
> > -- The CXX compiler identification is GNU 6.1.0
> > -- CMAKE_CXX11_EXTENSION_COMPILE_OPTION = -std=gnu++11
> > I expected here "-- CMAKE_CXX11_EXTENSION_COMPILE_OPTION = -std=c++14".
> 
> That is the option for C++11 with extensions.  See
> CMAKE_CXX14_EXTENSION_COMPILE_OPTION for C++14 with extensions.
> Both of these are internal table entries for CMake's generators
> and not meant for direct use by projects.
> 
> What are you actually trying to do?
> 
> -Brad

I was trying to compile lyx with QT5.7.
The effect was, that the compilation failed. Searching for a reason I found 
that the flags in the flag.cmake files were mismatched.

Here one example:

# CMAKE generated file: DO NOT EDIT!
# Generated by "Unix Makefiles" Generator, CMake Version 3.6

# compile CXX with /usr/local/gcc6.1/bin/g++
CXX_FLAGS = -Wall -Wunused-parameter --std=c++14 -fno-strict-aliasing  
-Wall -Wunused-parameter --std=c++14 -fno-strict-aliasing -O0 -g3 -D_DEBUG   
-DBOOST_USER_CONFIG="" -fPIC --std=gnu++11

...

Therefore I started searching, where the extra ' --std=gnu++11' came from, and 
found it in Modules/Compiler/GNU-CXX.cmake.

To verify, I made the output of this variable.
To be clear, I did not want to do anything with the variable 
CMAKE_CXX14_EXTENSION_COMPILE_OPTION.

Kornel


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Re: [cmake-developers] Wrong cxx-extension flags

[Brad King]

On 07/14/2016 01:13 PM, Kornel Benko wrote:
> -- The CXX compiler identification is GNU 6.1.0
> -- CMAKE_CXX11_EXTENSION_COMPILE_OPTION = -std=gnu++11
> I expected here "-- CMAKE_CXX11_EXTENSION_COMPILE_OPTION = -std=c++14".

That is the option for C++11 with extensions.  See
CMAKE_CXX14_EXTENSION_COMPILE_OPTION for C++14 with extensions.
Both of these are internal table entries for CMake's generators
and not meant for direct use by projects.

What are you actually trying to do?

-Brad

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