initializing Boost histogram axes using std:array via template non-type argument?

Question

I'm having some trouble in initializing a boost histogram axis. The following appears to cause a mismatch between the type of the array and iterator vs pointer to first element. And insights/suggestions would be appreciated. thanks.

#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;
}

The error message generated is quite long (because of attempted template matching), but the essence of it is as follows:

%  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

Interesting. Compiling your sample gave me ICE on GCC 10 (which is always a compiler bug).

I started reducing till it did compile. I used 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);
}

Prints

3: {2.0, 3.0, 4.0}

(with excellent codegen as expected).

Perhaps we can take it from here, to find where the problem is reintroduced. (Will be back after family things).

UPDATE:

• It's the template on FS<> that trips:

  ICES: https://godbolt.org/z/onaEor

• I thought it might have been the template argument deduction, but it isn't.

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

All in all I think this should be reported. CC trunk gives a more info should you chose to report: https://godbolt.org/z/489a5z - also consider using Creduce to minimize further

Workaround

Interestingly things get unstuck when you use the template argument on FS. (This gives me a hunch it still involves the argument deduction machinery).

FIXED: 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);
}

Substituting back into your original sample leads to different compilation errors, which seem to be "normal" Boost Histogram type errors, so I'll leave that for you:

https://godbolt.org/z/9Tx3n3 (same error coming from your exact code with only the crucial changes: https://godbolt.org/z/sMMcWT