如何在Oct文件中使用C ++ 17功能

Question

I'm trying to use std::optional (a C++17 feature) in a Oct file for GNU Octave. I have upgraded to g++ version 9.1.0 and set linux environment vars 'CC' and 'CXX' to /usr/bin/gcc and /usr/bin/g++ respectively. Running getenv on these variables from within Octave shows that the correct compiler is set. However, when running `mkoctfile', I get the following error:

oct_interface.cpp: In function ‘octave_value_list Fmibc(const octave_value_list&, int)’:
oct_interface.cpp:863:10: error: ‘optional’ is not a member of ‘std’
  863 |     std::optional<double> dub_opt = {};
      |          ^~~~~~~~
oct_interface.cpp:863:10: note: ‘std::optional’ is only available from C++17 onwards
oct_interface.cpp:863:19: error: expected primary-expression before ‘double’
  863 |     std::optional<double> dub_opt = {};
      |                   ^~~~~~
warning: mkoctfile: building exited with failure status

As pointed out in the comments, the appropriate flag must be passed to the compiler in order to use C++17 features. So, a more precise question might be how to configure mkoctfile to pass the -std=c++17 flag to the compiler.

Answer 1

The problem is that the C++17 flag is not being passed to the compiler, as is pointed out in a comment by Human-Compiler. Compiler flags can be passed in two ways: as part of the call to the Oct-file utility, ie mkoctfile -std=c++17 , or by setting an Octave environment variable, ie setenv("CXXFLAGS", "-std=c++17")