mp-units/src/include/units/unit.h

// The MIT License (MIT)
//
// Copyright (c) 2018 Mateusz Pusz
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

#pragma once

#include "bits/tools.h"
#include "dimension.h"

namespace units {

  template<Dimension D, Ratio R>
    requires (R::num > 0)
  struct unit : upcast_base<unit<D, R>> {
    using dimension = D;
    using ratio = R;
  };

  // is_unit

  namespace detail {

    template<typename T>
    inline constexpr bool is_unit = false;

    template<Dimension D, Ratio R>
    inline constexpr bool is_unit<unit<D, R>> = true;

  }

  template<typename T>
  concept bool Unit =
      std::is_empty_v<T> &&
      detail::is_unit<typename T::base_type> &&
      std::experimental::ranges::DerivedFrom<T, typename T::base_type>;

}  // namespace units