74 lines
2.6 KiB
C++
74 lines
2.6 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::IntegralConstant`.
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@copyright Louis Dionne 2013-2017
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FWD_CONCEPT_INTEGRAL_CONSTANT_HPP
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#define BOOST_HANA_FWD_CONCEPT_INTEGRAL_CONSTANT_HPP
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#include <boost/hana/config.hpp>
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namespace boost { namespace hana {
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//! @ingroup group-concepts
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//! The `IntegralConstant` concept represents compile-time integral values.
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//!
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//! The `IntegralConstant` concept represents objects that hold a
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//! `constexpr` value of an integral type. In other words, it describes
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//! the essential functionality provided by `std::integral_constant`.
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//! An `IntegralConstant` is also just a special kind of `Constant`
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//! whose inner value is of an integral type.
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//!
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//!
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//! Minimal complete definition
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//! ---------------------------
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//! The requirements for being an `IntegralConstant` are quite simple.
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//! First, an `IntegralConstant` `C` must be a `Constant` such that
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//! `Tag::value_type` is an integral type, where `Tag` is the tag of `C`.
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//!
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//! Secondly, `C` must have a nested `static constexpr` member named
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//! `value`, such that the following code is valid:
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//! @code
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//! constexpr auto v = C::value;
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//! @endcode
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//! Because of the requirement that `Tag::value_type` be an integral type,
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//! it follows that `C::value` must be an integral value.
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//!
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//! Finally, it is necessary to specialize the `IntegralConstant` template
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//! in the `boost::hana` namespace to tell Hana that a type is a model
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//! of `IntegralConstant`:
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//! @code
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//! namespace boost { namespace hana {
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//! template <>
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//! struct IntegralConstant<your_custom_tag> {
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//! static constexpr bool value = true;
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//! };
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//! }}
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//! @endcode
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//!
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//!
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//! Refined concept
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//! ---------------
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//! 1. `Constant` (free implementation of `value`)\n
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//! The `value` function required to be a `Constant` can be implemented
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//! as follows for `IntegralConstant`s:
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//! @code
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//! value<C>() == C::value
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//! @endcode
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//! The `to` function must still be provided explicitly for the model
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//! of `Constant` to be complete.
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//!
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//!
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//! Concrete models
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//! ---------------
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//! `hana::integral_constant`
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template <typename C>
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struct IntegralConstant;
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}} // end namespace boost::hana
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#endif // !BOOST_HANA_FWD_CONCEPT_INTEGRAL_CONSTANT_HPP
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