Build error in include/nana/deploy.hpp redefinition of clamp

[Red-Portal]

while building with gcc 7 I got this error

[ 98%] Building CXX object CMakeFiles/BoardPlatformerClient.dir/main.cpp.o
In file included from /usr/include/c++/7.1.1/functional:64:0,
                 from /home/msca8h/projects/board_platformer/src/client/nana/include/nana/gui/detail/general_events.hpp:21,
                 from /home/msca8h/projects/board_platformer/src/client/nana/include/nana/gui/programming_interface.hpp:17,
                 from /home/msca8h/projects/board_platformer/src/client/nana/include/nana/gui/wvl.hpp:18,
                 from /home/msca8h/projects/board_platformer/src/client/nana/include/nana/gui.hpp:1,
                 from /home/msca8h/projects/board_platformer/src/client/main.cpp:1:
/usr/include/c++/7.1.1/bits/stl_algo.h:3719:5: error: redefinition of ‘template<class _Tp> constexpr const _Tp& std::clamp(const _Tp&, const _Tp&, const _Tp&)’
     clamp(const _Tp& __val, const _Tp& __lo, const _Tp& __hi)
     ^~~~~
In file included from /home/msca8h/projects/board_platformer/src/client/nana/include/nana/deploy.hpp:20:0,
                 from /home/msca8h/projects/board_platformer/src/client/nana/include/nana/paint/../basic_types.hpp:16,
                 from /home/msca8h/projects/board_platformer/src/client/nana/include/nana/paint/graphics.hpp:18,
                 from /home/msca8h/projects/board_platformer/src/client/nana/include/nana/gui/effects.hpp:15,
                 from /home/msca8h/projects/board_platformer/src/client/nana/include/nana/gui/programming_interface.hpp:16,
                 from /home/msca8h/projects/board_platformer/src/client/nana/include/nana/gui/wvl.hpp:18,
                 from /home/msca8h/projects/board_platformer/src/client/nana/include/nana/gui.hpp:1,
                 from /home/msca8h/projects/board_platformer/src/client/main.cpp:1:
/home/msca8h/projects/board_platformer/src/client/nana/include/nana/stdc++.hpp:151:21: note: ‘template<class T> constexpr const T& std::clamp(const T&, const T&, const T&)’ previously declared here
  constexpr const T& clamp(const T& v, const T& lo, const T& hi)
                     ^~~~~
make[2]: *** [CMakeFiles/BoardPlatformerClient.dir/build.make:63: CMakeFiles/BoardPlatformerClient.dir/main.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:68: CMakeFiles/BoardPlatformerClient.dir/all] Error 2
make: *** [Makefile:130: all] Error 2

is this because I'm using a new compiler?
or compiler flags perhaps?

[cnjinhao]

Yes, because of new compiler.

The detection of std::clamp in c++defines.hpp

#if (!defined(__cpp_lib_clamp)) || (__cpp_lib_clamp < 201603)
#	ifndef _enable_std_clamp
#		define _enable_std_clamp
#	endif
#endif

[Red-Portal]

Should I open a PR fix for that?

[cnjinhao]

That's would be great. Thanks in advance.

[Red-Portal]

Alright I'll work on that

[Red-Portal]

#222 fixed

[codicodi]

Similar error occurs with VS2017 when compiling with /std:c++latest:

c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.10.25017\include\algorithm(4033,2): error C2995: 'const T &std::clamp(const T &,const T &,const T &)': function template has already been defined

Wouldn't it be better to define custom 'clamp' in namespace nana? Defining stuff is in namespace std is undefined behaviour according to the C++ standard and breaks compilation as soon as compiler starts to conform to c++17 :(

[cnjinhao]

Hi, @codicodi
Thank you for reporting this issue, the bug is fixed now.

I know the restriction that C++ standard describes. Essentially, I am not trying to introduce new customized names into the namespace std, but the unimplemented standard compenent. The purpose of defining clamp in namespace std is to avoid creating a duplicate with standard. But to reduce the complexity, there are some other unimplemented standard compenents are defined in namespace nana, such as filesystem, any and optional.

[Red-Portal]

Maybe making a macro switch such as

#if C++17
     #define nana::clamp std::clamp
#else
     #define enable_nana_clamp
#endif

Would do the job?
Though we'll have to change all references to std::clamp to nana::clamp.
I guess this would solve your concern

[cnjinhao]

Thank you for the suggestion, this way is used for adopting nana::file system to std::experimental::filesystem.
The conflict is generated because of failure of feature test. Now it is fixed.