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Diffstat (limited to 'boost/hana/fwd/concept/integral_constant.hpp')
| -rw-r--r-- | boost/hana/fwd/concept/integral_constant.hpp | 73 |
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diff --git a/boost/hana/fwd/concept/integral_constant.hpp b/boost/hana/fwd/concept/integral_constant.hpp new file mode 100644 index 0000000000..ba82c52931 --- /dev/null +++ b/boost/hana/fwd/concept/integral_constant.hpp @@ -0,0 +1,73 @@ +/*! +@file +Forward declares `boost::hana::IntegralConstant`. + +@copyright Louis Dionne 2013-2016 +Distributed under the Boost Software License, Version 1.0. +(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) + */ + +#ifndef BOOST_HANA_FWD_CONCEPT_INTEGRAL_CONSTANT_HPP +#define BOOST_HANA_FWD_CONCEPT_INTEGRAL_CONSTANT_HPP + +#include <boost/hana/config.hpp> + + +BOOST_HANA_NAMESPACE_BEGIN + //! @ingroup group-concepts + //! The `IntegralConstant` concept represents compile-time integral values. + //! + //! The `IntegralConstant` concept represents objects that hold a + //! `constexpr` value of an integral type. In other words, it describes + //! the essential functionality provided by `std::integral_constant`. + //! An `IntegralConstant` is also just a special kind of `Constant` + //! whose inner value is of an integral type. + //! + //! + //! Minimal complete definition + //! --------------------------- + //! The requirements for being an `IntegralConstant` are quite simple. + //! First, an `IntegralConstant` `C` must be a `Constant` such that + //! `Tag::value_type` is an integral type, where `Tag` is the tag of `C`. + //! + //! Secondly, `C` must have a nested `static constexpr` member named + //! `value`, such that the following code is valid: + //! @code + //! constexpr auto v = C::value; + //! @endcode + //! Because of the requirement that `Tag::value_type` be an integral type, + //! it follows that `C::value` must be an integral value. + //! + //! Finally, it is necessary to specialize the `IntegralConstant` template + //! in the `boost::hana` namespace to tell Hana that a type is a model + //! of `IntegralConstant`: + //! @code + //! namespace boost { namespace hana { + //! template <> + //! struct IntegralConstant<your_custom_tag> { + //! static constexpr bool value = true; + //! }; + //! }} + //! @endcode + //! + //! + //! Refined concept + //! --------------- + //! 1. `Constant` (free implementation of `value`)\n + //! The `value` function required to be a `Constant` can be implemented + //! as follows for `IntegralConstant`s: + //! @code + //! value<C>() == C::value + //! @endcode + //! The `to` function must still be provided explicitly for the model + //! of `Constant` to be complete. + //! + //! + //! Concrete models + //! --------------- + //! `hana::integral_constant` + template <typename C> + struct IntegralConstant; +BOOST_HANA_NAMESPACE_END + +#endif // !BOOST_HANA_FWD_CONCEPT_INTEGRAL_CONSTANT_HPP |