What parameter type should I use to accept xexpressions?

Suppose I have the xtensor xexpression waffle.

xt::xtensor_fixed<double, xt::xshape<1, 4>, xt::layout_type::column_major> open = {{1., 3., 2., 5.}};
xt::xtensor_fixed<double, xt::xshape<1, 4>, xt::layout_type::column_major> close = { {5., 1., 6., 2.} };
auto waffle = xt::equal(open, close);

Considering the type of waffle is: xt::detail::xfunction_type_t<xt::detail::not_equal_to, xt::xtensor_fixed<double, xt::xshape<1U, 366U>, xt::layout_type::column_major> &, xt::xtensor_fixed<double, xt::xshape<1U, 366U>, xt::layout_type::column_major> &>

I want to pass waffle into a class constructor:

class WaffleWrapper {
public:
    ??? waffle;
    WaffleWrapper(??? wafflein) {
        ??? waffle = wafflein;
    }
};

What would I use instead of ??? so that I can pass waffle into it?

e.g.

auto waffle = xt::equal(open, close);
WaffleWrapper example(waffle);

Like I commented, as a rule-of-thumb your class should own its data. You should thus create a data-container, which will force the evaluation of the xfunction. Thereafter you run no risk on pointing to data that may have gone out-of-scope.

If you are worried about loosing the ability to use the fixedness of your array you could consider templating the class:

#include <xtensor/xarray.hpp>
#include <xtensor/xfixed.hpp>
#include <xtensor/xview.hpp>
#include <xtensor/xio.hpp>

template<class T>
class WaffleWrapper {
public:
    T waffle;

    WaffleWrapper(const T& wafflein) {
        waffle = wafflein;
    }
};

int main()
{
    using fixed_double = xt::xtensor_fixed<double, xt::xshape<1, 4>, xt::layout_type::column_major>;
    using fixed_int = xt::xtensor_fixed<int, xt::xshape<1, 4>, xt::layout_type::column_major>;

    fixed_double open = {{1., 3., 2., 5.}};
    fixed_double close = {{5., 1., 6., 2.}};
    auto waffle = xt::equal(open, close);

    WaffleWrapper<fixed_int> example(waffle);

    return 0;
}

Note that I abbreviated your typenames for readability.