如何创建 boost::histogram 作为类成员并在类构造函数中初始化它？

Question

鉴于

boost::histogram

的返回类型是高度模板化的，目前尚不清楚如何创建

boost::histogram::make_histogram()

对象作为类成员并使用

make_histogram()

在类构造函数中初始化它。

入门指南给出了使用包装器结构的示例，如下所示：

struct HolderOfStaticHistogram {
    using axes_t = std::tuple<
        boost::histogram::axis::regular<>,
        boost::histogram::axis::integer<>
    >;
    using hist_t = boost::histogram::histogram<axes_t>;
    hist_t hist_;
};

int main() {
    using namespace boost::histogram;

    HolderOfStaticHistogram hs;
    hs.hist_ = make_histogram(axis::regular<>(5, 0, 1), axis::integer<>(0, 3));
}

但是，将

make_histogram

放入构造函数中不会编译，例如：

struct HolderOfStaticHistogram {
    using axes_t = boost::histogram::axis::regular<>;
    using hist_t = boost::histogram::histogram<axes_t>;
    hist_t hist_;

    HolderOfStaticHistogram() {
        hist_ = boost::histogram::make_histogram(
            boost::histogram::axis::regular<>(5, 0, 1));
    }
};

int main() {
    auto hist_container = HolderOfStaticHistogram();
}

/opt/compiler-explorer/libs/boost_1_80_0/boost/histogram/detail/axes.hpp:310:5: error: no member named 'resize' in 'boost::histogram::axis::regular<double, boost::use_default, boost::use_default, boost::use_default>'
  t.resize(sizeof...(Us));
  ~ ^
/opt/compiler-explorer/libs/boost_1_80_0/boost/histogram/histogram.hpp:98:13: note: in instantiation of function template specialization 'boost::histogram::detail::axes_assign<boost::histogram::axis::regular<double, boost::use_default, boost::use_default, boost::use_default>, boost::histogram::axis::regular<double, boost::use_default, boost::use_default, boost::use_default> >' requested here
    detail::axes_assign(axes_, std::move(unsafe_access::axes(rhs)));
            ^
<source>:53:15: note: in instantiation of function template specialization 'boost::histogram::histogram<boost::histogram::axis::regular<double, boost::use_default, boost::use_default, boost::use_default>, boost::histogram::unlimited_storage<std::allocator<char> > >::operator=<std::tuple<boost::histogram::axis::regular<double, boost::use_default, boost::use_default, boost::use_default> >, boost::histogram::unlimited_storage<std::allocator<char> > >' requested here
        hist_ = boost::histogram::make_histogram(
     

一种解决方法是在调用

decltype

的表达式上使用

make_histogram

，如下所示：

#include <boost/histogram.hpp>

class HistContainer {
public:
    using axis_t = boost::histogram::axis::regular<>;
    using hist_t = decltype(boost::histogram::make_histogram(axis_t(10, 0, 10)));
    hist_t hist;

    HistContainer() {
        hist = boost::histogram::make_histogram(axis_t(10, 0, 10));
    }
};

int main() {
    auto hist_container = HistContainer();
}

但是，这并不理想，因为我们必须构造

histogram

对象两次。拥有

boost::histogram

作为班级成员的正确方法是什么？

Answer 1

缺少一个元组：

using axes_t = std::tuple<boost::histogram::axis::regular<> >;

此外，使用基类/成员初始值设定项列表：

住在Coliru

#include <boost/histogram.hpp>
namespace bh = boost::histogram;

struct HolderOfStaticHistogram {
    using axes_t = std::tuple<bh::axis::regular<>>;
    using hist_t = bh::histogram<axes_t>;
    hist_t hist_;

    HolderOfStaticHistogram() : hist_{bh::make_histogram(bh::axis::regular<>(5, 0, 1))} {}
};

int main() {
    auto hist_container = HolderOfStaticHistogram();
}