为什么 xtensor 中的第一个示例无法运行？

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 , ie

#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?