使用 std:array 通過模板非類型參數初始化 Boost 直方圖軸？

Question

我在初始化提升直方圖軸時遇到了一些麻煩。 以下似乎導致數組類型和迭代器與指向第一個元素的指針之間不匹配。 見解/建議將不勝感激。 謝謝。

#include <iostream>
#include <array>
#include <boost/histogram/axis.hpp>

template<class C, const C& array_>      
class Foo {
protected:
        typedef boost::histogram::axis::variable<>      VaxisType;
        inline static VaxisType                 axis_{array_};
        std::array<unsigned char, array_.size()>        bins_;
public:
        Foo() { }
};

template<class C, const C& array_>     
class Bar : public Foo<C, array_> {
public:
        using Base_Type = Foo<C, array_>;
        Bar() : Base_Type{} { }
        auto idx(double x) { return Base_Type::axis_.index(x); }
};

template<class Base = float>
class FS {
private:
        static const unsigned nbins = 3;
        typedef std::array<double, nbins>       AType;
        static constexpr AType bdrs{2.0, 3.0, 4.0};
        Bar<AType, bdrs>        blah;
public:
        FS() : blah{} { }
        auto idx(double x) { return blah.idx(x); }
};

int
main()
{
        FS      x1;
        std::cout << x1.idx(3.5) << "\n";
        return 0;
}

生成的錯誤信息比較長（因為嘗試了模板匹配），但其實質如下：

%  g++ -std=c++17 -o boost_no_workie-2{,.cc}

In file included from /usr/include/boost/histogram/axis.hpp:13,
                 from boost_no_workie-2.cc:4:
/usr/include/boost/histogram/axis/variable.hpp: In instantiation of ‘boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::variable(It, It, boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::metadata_type, boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::allocator_type) [with It = const std::array<double, 3>*; <template-parameter-2-2> = boost::histogram::detail::requires_iterator<const std::array<double, 3>*, void>; Value = double; MetaData = boost::use_default; Options = boost::use_default; Allocator = std::allocator<double>; boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::metadata_type = std::__cxx11::basic_string<char>; boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::allocator_type = std::allocator<double>]’:
/usr/include/boost/histogram/axis/variable.hpp:122:77:   required from ‘boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::variable(std::initializer_list<_Up>, boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::metadata_type, boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::allocator_type) [with U = std::array<double, 3>; Value = double; MetaData = boost::use_default; Options = boost::use_default; Allocator = std::allocator<double>; boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::metadata_type = std::__cxx11::basic_string<char>; boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::allocator_type = std::allocator<double>]’
boost_no_workie-2.cc:10:28:   required from ‘Foo<std::array<double, 3>, FS<float>::bdrs>::VaxisType Foo<std::array<double, 3>, FS<float>::bdrs>::axis_’
boost_no_workie-2.cc:22:47:   required from ‘auto Bar<C, array_>::idx(double) [with C = std::array<double, 3>; const C& array_ = FS<float>::bdrs]’
boost_no_workie-2.cc:34:40:   required from ‘auto FS<Base>::idx(double) [with Base = float]’
boost_no_workie-2.cc:41:25:   required from here
/usr/include/boost/histogram/axis/variable.hpp:95:18: error: no match for ‘operator<=’ (operand types are ‘const std::array<double, 3>’ and ‘__gnu_cxx::__alloc_traits<std::allocator<double>, double>::value_type’ {aka ‘double’})
   95 |       if (*begin <= v.back())

... skip ...
/usr/include/boost/histogram/axis/variable.hpp:93:5:   required from ‘boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::variable(It, It, boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::metadata_type, boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::allocator_type) [with It = const std::array<double, 3>*; <template-parameter-2-2> = boost::histogram::detail::requires_iterator<const std::array<double, 3>*, void>; Value = double; MetaData = boost::use_default; Options = boost::use_default; Allocator = std::allocator<double>; boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::metadata_type = std::__cxx11::basic_string<char>; boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::allocator_type = std::allocator<double>]’
/usr/include/boost/histogram/axis/variable.hpp:122:77:   required from ‘boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::variable(std::initializer_list<_Up>, boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::metadata_type, boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::allocator_type) [with U = std::array<double, 3>; Value = double; MetaData = boost::use_default; Options = boost::use_default; Allocator = std::allocator<double>; boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::metadata_type = std::__cxx11::basic_string<char>; boost::histogram::axis::variable<Value, MetaData, Options, Allocator>::allocator_type = std::allocator<double>]’
boost_no_workie-2.cc:10:28:   required from ‘Foo<std::array<double, 3>, FS<float>::bdrs>::VaxisType Foo<std::array<double, 3>, FS<float>::bdrs>::axis_’
boost_no_workie-2.cc:22:47:   required from ‘auto Bar<C, array_>::idx(double) [with C = std::array<double, 3>; const C& array_ = FS<float>::bdrs]’
boost_no_workie-2.cc:34:40:   required from ‘auto FS<Base>::idx(double) [with Base = float]’
boost_no_workie-2.cc:41:25:   required from here
/usr/include/c++/9/ext/new_allocator.h:145:20: error: cannot convert ‘const std::array<double, 3>’ to ‘double’ in initialization
  145 |  noexcept(noexcept(::new((void *)__p)
      |                    ^~~~~~~~~~~~~~~~~~
  146 |        _Up(std::forward<_Args>(__args)...)))

Answer 1

有趣的。 編譯您的示例在 GCC 10 上給了我 ICE（這始終是編譯器錯誤）。

我開始減少直到它編譯。 為了簡單起見，我使用了 c++17 自動非類型模板 arguments ：

實時編譯器資源管理器

#include <array>
#include <fmt/ranges.h>

template <auto const& array_> struct Foo {
    static constexpr int N = array_.size();
};

template <auto const& array_> struct Bar : Foo<array_> {
    using Base_Type = Foo<array_>;
};

struct FS {
    static const unsigned nbins = 3;
    static constexpr std::array<double, nbins> bdrs{2.0, 3.0, 4.0};
    Bar<bdrs> blah;
};

int main() {
    FS x1;
    fmt::print(FMT_STRING("{}: {}\n"), x1.blah.N, x1.bdrs);
}

印刷

3: {2.0, 3.0, 4.0}

（如預期的那樣具有出色的代碼生成）。

也許我們可以從這里開始，找到問題重新出現的地方。 （會在家庭事務后回來）。

更新：

• 這是 FS<> 上的模板跳閘：

  ICES： https://godbolt.org/z/onaEor

• 我認為這可能是模板參數推導，但事實並非如此。

  還有 ICES： https://godbolt.org/z/hxMWPj

總而言之，我認為這應該報告。 如果您選擇報告，CC 主干會提供更多信息： https://godbolt.org/z/489a5z - 還可以考慮使用 Creduce 進一步最小化

解決方法

有趣的是，當您在 FS 上使用模板參數時，事情就會變得混亂。 （這給了我一種預感，它仍然涉及論證推理機制）。

固定： https://godbolt.org/z/TWsas6

template <typename F>
struct FS {
    static const unsigned nbins = 3;
    static constexpr std::array<F, nbins> bdrs{2.0, 3.0, 4.0};
    Bar<bdrs> blah;
};

int main() {
    FS<float> x1;
    fmt::print(FMT_STRING("{}: {}\n"), x1.blah.N, x1.bdrs);
}

替換回您的原始示例會導致不同的編譯錯誤，這似乎是“正常”的 Boost Histogram 類型錯誤，所以我將其留給您：

https://godbolt.org/z/9Tx3n3 （來自您的確切代碼的相同錯誤，只有關鍵更改： https://godbolt.org/z/sMMcWT