shared generator expressions

I would like to wrap generator expressions in a function like in the following example.

inline auto build(int n) {
    auto x = xt::make_xshared(xt::arange(n));
    return x*x;
}

xt::xtensor<double, 1> array;
array = build(10);

Currently this gives an error

/usr/include/xtensor/xexpression.hpp:575:9: error:
    ‘class xt::xgenerator<xt::detail::arange_generator<int, int, int>, int,
    std::array<long unsigned int, 1> >’ has no member named ‘linear_begin’
  575 |         XTENSOR_FORWARD_METHOD(linear_begin)

As far as I understand, this is currently only possible for xexpressions of evaluated expressions.
My current workarounds are:

Evaluating the generator. This sacrifices laziness.
Passing array by reference. This is not idiomatic and only works if the assignment results in a evaluation.

I think that what happens is that the return forces the evaluation of the expression. (It is a bit tricky to see from you error snippet)

For a minimum example I took the following code

#include <xtensor/xtensor.hpp>

inline auto build(int n) {
    auto x = xt::make_xshared(xt::arange(n));
    return x*x;
}

int main() {
    xt::xtensor<double, 1> array;
    array = build(10);
}

xtensor 0.24.6, xtl 0.7.5, GCC 12.2.1.
This gave the following errors:

In file included from /usr/include/xtensor/xaccumulator.hpp:18,
                 from /usr/include/xtensor/xmath.hpp:27,
                 from /usr/include/xtensor/xcontainer.hpp:25,
                 from /usr/include/xtensor/xtensor.hpp:20,
                 from test.cpp:1:
/usr/include/xtensor/xexpression.hpp: In instantiation of ‘class xt::xshared_expression<xt::xgenerator<xt::detail::arange_generator<int, int, int>, int, std::array<long unsigned int, 1> > >’:
test.cpp:4:30:   required from here
/usr/include/xtensor/xexpression.hpp:575:9: error: ‘class xt::xgenerator<xt::detail::arange_generator<int, int, int>, int, std::array<long unsigned int, 1> >’ has no member named ‘linear_begin’
  575 |         XTENSOR_FORWARD_METHOD(linear_begin)
      |         ^
/usr/include/xtensor/xexpression.hpp:576:9: error: ‘class xt::xgenerator<xt::detail::arange_generator<int, int, int>, int, std::array<long unsigned int, 1> >’ has no member named ‘linear_end’
  576 |         XTENSOR_FORWARD_METHOD(linear_end)
      |         ^
/usr/include/xtensor/xexpression.hpp:577:9: error: ‘const class xt::xgenerator<xt::detail::arange_generator<int, int, int>, int, std::array<long unsigned int, 1> >’ has no member named ‘linear_begin’
  577 |         XTENSOR_FORWARD_CONST_METHOD(linear_begin)
      |         ^
/usr/include/xtensor/xexpression.hpp:578:9: error: ‘const class xt::xgenerator<xt::detail::arange_generator<int, int, int>, int, std::array<long unsigned int, 1> >’ has no member named ‘linear_end’
  578 |         XTENSOR_FORWARD_CONST_METHOD(linear_end)
      |         ^
/usr/include/xtensor/xexpression.hpp:579:9: error: ‘const class xt::xgenerator<xt::detail::arange_generator<int, int, int>, int, std::array<long unsigned int, 1> >’ has no member named ‘linear_cbegin’; did you mean ‘get_cbegin’?
  579 |         XTENSOR_FORWARD_CONST_METHOD(linear_cbegin)
      |         ^
/usr/include/xtensor/xexpression.hpp:580:9: error: ‘const class xt::xgenerator<xt::detail::arange_generator<int, int, int>, int, std::array<long unsigned int, 1> >’ has no member named ‘linear_cend’
  580 |         XTENSOR_FORWARD_CONST_METHOD(linear_cend)
      |         ^
/usr/include/xtensor/xexpression.hpp:582:9: error: ‘class xt::xgenerator<xt::detail::arange_generator<int, int, int>, int, std::array<long unsigned int, 1> >’ has no member named ‘linear_rbegin’
  582 |         XTENSOR_FORWARD_METHOD(linear_rbegin)
      |         ^
/usr/include/xtensor/xexpression.hpp:583:9: error: ‘class xt::xgenerator<xt::detail::arange_generator<int, int, int>, int, std::array<long unsigned int, 1> >’ has no member named ‘linear_rend’
  583 |         XTENSOR_FORWARD_METHOD(linear_rend)
      |         ^
/usr/include/xtensor/xexpression.hpp:584:9: error: ‘const class xt::xgenerator<xt::detail::arange_generator<int, int, int>, int, std::array<long unsigned int, 1> >’ has no member named ‘linear_rbegin’
  584 |         XTENSOR_FORWARD_CONST_METHOD(linear_rbegin)
      |         ^
/usr/include/xtensor/xexpression.hpp:585:9: error: ‘const class xt::xgenerator<xt::detail::arange_generator<int, int, int>, int, std::array<long unsigned int, 1> >’ has no member named ‘linear_rend’
  585 |         XTENSOR_FORWARD_CONST_METHOD(linear_rend)
      |         ^
/usr/include/xtensor/xexpression.hpp:586:9: error: ‘const class xt::xgenerator<xt::detail::arange_generator<int, int, int>, int, std::array<long unsigned int, 1> >’ has no member named ‘linear_crbegin’
  586 |         XTENSOR_FORWARD_CONST_METHOD(linear_crbegin)
      |         ^
/usr/include/xtensor/xexpression.hpp:587:9: error: ‘const class xt::xgenerator<xt::detail::arange_generator<int, int, int>, int, std::array<long unsigned int, 1> >’ has no member named ‘linear_crend’
  587 |         XTENSOR_FORWARD_CONST_METHOD(linear_crend)
      |         ^

It appears this and the issue I ran into share the same underlying issue: #2916. From the bit of detective work I've done its an interface issue, the methods generated by the macros that are yelling in the above code need to be gracefully disabled for certain types.

I found a solution but have yet to get to a point I can confirm it doesn't break outside my specific use case.

#define XTENSOR_FORWARD_CONST_METHOD(name)                     \
    decltype(auto) name() const                                \
        requires( requires(const E& e){ e.name(); } )          \
    {                                                          \
        return m_ptr->name();                                  \
    }

#define XTENSOR_FORWARD_METHOD(name)                  \
    decltype(auto) name()                             \
        requires( requires(E& e){ e.name(); } )       \
    {                                                 \
        return m_ptr->name();                         \
    }

Does the pull request #2918 also fix this issue?

Yes it should.

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shared generator expressions #2673

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tdegeus commented on Mar 31, 2023

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