add is_quotrem_order

[gpeterhoff]

Problem: std::Xdiv_t can be implemented as {quot, rem} or {rem, quot} (what a mess). This means that structured binding are not unique: auto [a, b] = std::div(x, y); is a quot or rem or is b quot or rem ? is_quotrem_order.hpp

//  Distributed under the Boost Software License Version 1.0 https://www.boost.org/LICENSE_1_0.txt
//  Copyright Gero Peterhoff

#ifndef BOOST_TYPE_TRAITS_IS_QUOTREM_ORDER_HPP
#define BOOST_TYPE_TRAITS_IS_QUOTREM_ORDER_HPP

#include <boost/type_traits/is_bool.hpp>
#include <type_traits>
#include <cmath>

namespace boost
{
namespace detail
{
template <typename Type>
inline constexpr bool   is_quotrem_order()  noexcept
{
    static_assert(std::is_arithmetic<Type>::value, "invalid type");

    //  std::div need signed integer
    using value_type = typename std::conditional_t
        <
            std::is_floating_point<Type>::value || boost::is_bool<Type>::value,
            int,
            typename std::make_signed<Type>::type
        >;
    using div_type = decltype(std::div(value_type{}, value_type{}));

    constexpr div_type
        div{value_type{0}, value_type{1}};

    return div.quot == value_type{0};
}
}   //  detail

template <typename Type> struct is_quotrem_order : public std::integral_constant
<
    bool,
    detail::is_quotrem_order<Type>()
>   {};

#if !defined(BOOST_NO_CXX17_INLINE_VARIABLES)
template <typename Type> inline constexpr bool is_quotrem_order_v = is_quotrem_order<Type>::value;
#endif
}   //  boost

#endif  //  BOOST_TYPE_TRAITS_IS_QUOTREM_ORDER_HPP

[jzmaddock]

I think this may be overkill: std::div is constexpr from C++23 only making this solution require C++23 as well, but off the top of my head a neater solution might be:

template <class D>
auto div_to_tuple(const D& d)
{
    return std::make_tuple(d.quot, d.rem);
}

So now you can write:

auto [q,r] = div_to_tuple(std::div(a,b));

and this works from C++11 onwards I think.

[gpeterhoff]

No. There is std::div. Nothing more. Unfortunately, I can't tell you why the C++ committee is not able to do this in a standardized way, let alone provide std::div for unsigned.