why the first example in xtensor can't run? - c++

Question

I install the xtensor by conda
conda install -c conda-forge xtensor
and I find the position where the xtensor is installed,
path_xtensor="/amax/home/user/miniconda3/pkgs/xtensor-0.23.10-h4bd325d_0/include/"
path_xtl="/amax/home/user/miniconda3/pkgs/xtl-0.7.2-h4bd325d_1/include/"
the first example is given by the xtensor docs,
i.e.
#include <iostream>
#include <xtensor/xarray.hpp>
#include <xtensor/xio.hpp>
#include <xtensor/xview.hpp>
int main(int argc, char* argv[])
{
xt::xarray<double> arr1
{{1.0, 2.0, 3.0},
{2.0, 5.0, 7.0},
{2.0, 5.0, 7.0}};
xt::xarray<double> arr2
{5.0, 6.0, 7.0};
xt::xarray<double> res = xt::view(arr1, 1) + arr2;
std::cout << res << std::endl;
return 0;
}
I followed that and run the command in linux servers
g++ -I path_xtensor -I path_xtl x.cpp -o x
but the command cause error, the log is too long, the following is partly:
/amax/home/user/miniconda3/pkgs/xtensor-0.23.10-h4bd325d_0/include/xtensor/xfunction.hpp:931:63: error: ‘int xt::xfunction_iterator<F, CT>::deref_impl’ is not a static data member of ‘class xt::xfunction_iterator<F, CT>’
/amax/home/user/miniconda3/pkgs/xtensor-0.23.10-h4bd325d_0/include/xtensor/xfunction.hpp:931:63: error: template definition of non-template ‘int xt::xfunction_iterator<F, CT>::deref_impl’
/amax/home/user/miniconda3/pkgs/xtensor-0.23.10-h4bd325d_0/include/xtensor/xfunction.hpp:931:58: error: ‘index_sequence’ is not a member of ‘std’
inline auto xfunction_iterator<F, CT...>::deref_impl(std::index_sequence<I...>) const -> reference
^
/amax/home/user/miniconda3/pkgs/xtensor-0.23.10-h4bd325d_0/include/xtensor/xfunction.hpp:936:29: warning: variadic templates only available with -std=c++11 or -std=gnu++11
template <class F, class... CT>
^
/amax/home/user/miniconda3/pkgs/xtensor-0.23.10-h4bd325d_0/include/xtensor/xfunction.hpp:937:30: warning: variadic templates only available with -std=c++11 or -std=gnu++11
template <std::size_t... I>
^
...
/amax/home/user/miniconda3/pkgs/xtensor-0.23.10-h4bd325d_0/include/xtensor/xfunction.hpp: In member function ‘void xt::xfunction_stepper<F, CT>::step_leading()’:
/amax/home/user/miniconda3/pkgs/xtensor-0.23.10-h4bd325d_0/include/xtensor/xfunction.hpp:1058:14: error: ‘ame a type
virtual const char what() const noexcept override { return "bad_variant_access"; }
^
/amax/home/user/miniconda3/pkgs/xtl-0.7.2-h4bd325d_1/include/xtl/xvariant_impl.hpp:914:38: note: C++11 ‘noexcept’ only available with -std=c++11 or -std=gnu++11
/amax/home/user/miniconda3/pkgs/xtl-0.7.2-h4bd325d_1/include/xtl/xvariant_impl.hpp:914:25: error: looser throw specifier for ‘virtual const char mpark::bad_variant_access::what() const’
virtual const char what() const noexcept override { return "bad_variant_access"; }
^
In file included from /usr/include/c++/5/ios:39:0,
from /usr/include/c++/5/ostream:38,
from /usr/include/c++/5/iostream:39,
from x.cpp:24:
/usr/include/c++/5/exception:68:25: error: overriding ‘virtual const char std::exception::what() const throw ()’
virtual const char* what() const _GLIBCXX_USE_NOEXCEPT;
^
In file included from /amax/home/user/miniconda3/pkgs/xtl-0.7.2-h4bd325d_1/include/xtl/xvariant.hpp:13:0,
from /amax/home/user/miniconda3/pkgs/xtensor-0.23.10-h4bd325d_0/include/xtensor/xstrided_view.hpp:20,
from /amax/home/user/miniconda3/pkgs/xtensor-0.23.10-h4bd325d_0/include/xtensor/xgenerator.hpp:27,
from /amax/home/user/miniconda3/pkgs/xtensor-0.23.10-h4bd325d_0/include/xtensor/xbuilder.hpp:31,
from /amax/home/user/miniconda3/pkgs/xtensor-0.23.10-h4bd325d_0/include/xtensor/xmanipulation.hpp:13,
from /amax/home/user/miniconda3/pkgs/xtensor-0.23.10-h4bd325d_0/include/xtensor/xmath.hpp:28,
from /amax/home/user/miniconda3/pkgs/xtensor-0.23.10-h4bd325d_0/include/xtensor/xcontainer.hpp:25,
from /amax/home/user/miniconda3/pkgs/xtensor-0.23.10-h4bd325d_0/include/xtensor/xarray.hpp:20,
from x.cpp:25:
/amax/home/user/miniconda3/pkgs/xtl-0.7.2-h4bd325d_1/include/xtl/xvariant_impl.hpp:917:3: error: expected unqualified-id before ‘[’ token
[[noreturn]] inline void throw_bad_variant_access() {
^
x.cpp:38:1: error: expected ‘}’ at end of input
}
Thanks in advance for any help/suggestion.

Answer 1

Is the following command the full command line you execute?
g++ -I path_xtensor -I path_xtl x.cpp -o x
Then it is missing two helpful options:
To ensure the compiler is using a modern c++ standard, add an option like -std=c++11 or -std=c++14 or even newer standards, depending what your compiler supports and what you want to use in the remainder of your project. xtensor requires at least c++11.
To ensure that the compiler will use good CPU optimizations for the binary, add flags that enable general performance optimizations like -O2 and specific hardware optimizations for your current CPU like -march=native, unless you want your resulting binary to be portable to other computers (with a less modern CPU). This can give an huge performance boost with xtensor.
Does that help?