使用 std:array 通过模板非类型参数初始化 Boost 直方图轴？

Question

我在初始化提升直方图轴时遇到了一些麻烦。以下似乎导致数组类型和迭代器与指向第一个元素的指针之间不匹配。见解/建议将不胜感激。谢谢。

#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;
}

生成的错误信息比较长（因为尝试了模板匹配），但其实质如下：

%  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。编译您的示例给了我 GCC 10 上的 ICE（这始终是一个编译器错误）。

我开始减少直到它编译。我用的是c++17 auto non-type template arguments for simplicity:

Live on Compiler Explorer

#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);
}

打印

3: {2.0, 3.0, 4.0}

（如预期的那样具有出色的代码生成）。

也许我们可以从这里开始，找出问题再次出现的地方。 （会在家庭事情结束后回来）。

更新：

• 是 FS 上的模板跳闸：

  ICES：https://godbolt.org/z/onaEor

• 我以为可能是模板参数推导，但不是。

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

总而言之，我认为这应该被举报。如果您选择报告，CC 中继会提供更多信息：https://godbolt.org/z/489a5z - 还可以考虑使用 Creduce 进一步最小化

解决方法

有趣的是，当您使用 FS 上的模板参数时，事情会变得混乱。 （这让我有一种预感，它仍然涉及论证推理机制）。

已修复：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);
}

替换回您的原始示例会导致不同的编译错误，这似乎是“正常”的 Boost Histogram 类型错误，所以我将其留给您：

https://godbolt.org/z/9Tx3n3（来自您的确切代码的相同错误，只有关键更改：https://godbolt.org/z/sMMcWT