| 1 | /*! |
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| 2 | @file |
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| 3 | Forward declares `boost::hana::IntegralConstant`. |
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| 4 | |
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| 5 | Copyright Louis Dionne 2013-2022 |
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| 6 | Distributed under the Boost Software License, Version 1.0. |
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| 7 | (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) |
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| 8 | */ |
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| 9 | |
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| 10 | #ifndef BOOST_HANA_FWD_CONCEPT_INTEGRAL_CONSTANT_HPP |
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| 11 | #define BOOST_HANA_FWD_CONCEPT_INTEGRAL_CONSTANT_HPP |
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| 12 | |
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| 13 | #include <boost/hana/config.hpp> |
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| 14 | |
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| 15 | |
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| 16 | namespace boost { namespace hana { |
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| 17 | //! @ingroup group-concepts |
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| 18 | //! The `IntegralConstant` concept represents compile-time integral values. |
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| 19 | //! |
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| 20 | //! The `IntegralConstant` concept represents objects that hold a |
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| 21 | //! `constexpr` value of an integral type. In other words, it describes |
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| 22 | //! the essential functionality provided by `std::integral_constant`. |
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| 23 | //! An `IntegralConstant` is also just a special kind of `Constant` |
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| 24 | //! whose inner value is of an integral type. |
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| 25 | //! |
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| 26 | //! |
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| 27 | //! Minimal complete definition |
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| 28 | //! --------------------------- |
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| 29 | //! The requirements for being an `IntegralConstant` are quite simple. |
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| 30 | //! First, an `IntegralConstant` `C` must be a `Constant` such that |
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| 31 | //! `Tag::value_type` is an integral type, where `Tag` is the tag of `C`. |
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| 32 | //! |
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| 33 | //! Secondly, `C` must have a nested `static constexpr` member named |
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| 34 | //! `value`, such that the following code is valid: |
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| 35 | //! @code |
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| 36 | //! constexpr auto v = C::value; |
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| 37 | //! @endcode |
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| 38 | //! Because of the requirement that `Tag::value_type` be an integral type, |
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| 39 | //! it follows that `C::value` must be an integral value. |
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| 40 | //! |
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| 41 | //! Finally, it is necessary to specialize the `IntegralConstant` template |
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| 42 | //! in the `boost::hana` namespace to tell Hana that a type is a model |
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| 43 | //! of `IntegralConstant`: |
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| 44 | //! @code |
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| 45 | //! namespace boost { namespace hana { |
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| 46 | //! template <> |
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| 47 | //! struct IntegralConstant<your_custom_tag> { |
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| 48 | //! static constexpr bool value = true; |
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| 49 | //! }; |
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| 50 | //! }} |
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| 51 | //! @endcode |
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| 52 | //! |
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| 53 | //! |
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| 54 | //! Refined concept |
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| 55 | //! --------------- |
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| 56 | //! 1. `Constant` (free implementation of `value`)\n |
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| 57 | //! The `value` function required to be a `Constant` can be implemented |
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| 58 | //! as follows for `IntegralConstant`s: |
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| 59 | //! @code |
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| 60 | //! value<C>() == C::value |
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| 61 | //! @endcode |
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| 62 | //! The `to` function must still be provided explicitly for the model |
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| 63 | //! of `Constant` to be complete. |
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| 64 | //! |
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| 65 | //! |
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| 66 | //! Concrete models |
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| 67 | //! --------------- |
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| 68 | //! `hana::integral_constant` |
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| 69 | template <typename C> |
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| 70 | struct IntegralConstant; |
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| 71 | }} // end namespace boost::hana |
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| 72 | |
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| 73 | #endif // !BOOST_HANA_FWD_CONCEPT_INTEGRAL_CONSTANT_HPP |
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| 74 | |
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