fwd/concept/integral_constant.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::IntegralConstant`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_FWD_CONCEPT_INTEGRAL_CONSTANT_HPP
0011 #define BOOST_HANA_FWD_CONCEPT_INTEGRAL_CONSTANT_HPP
0012
0013 #include <boost/hana/config.hpp>
0014
0015
0016 namespace boost { namespace hana {
0017     //! @ingroup group-concepts
0018     //! The `IntegralConstant` concept represents compile-time integral values.
0019     //!
0020     //! The `IntegralConstant` concept represents objects that hold a
0021     //! `constexpr` value of an integral type. In other words, it describes
0022     //! the essential functionality provided by `std::integral_constant`.
0023     //! An `IntegralConstant` is also just a special kind of `Constant`
0024     //! whose inner value is of an integral type.
0025     //!
0026     //!
0027     //! Minimal complete definition
0028     //! ---------------------------
0029     //! The requirements for being an `IntegralConstant` are quite simple.
0030     //! First, an `IntegralConstant` `C` must be a `Constant` such that
0031     //! `Tag::value_type` is an integral type, where `Tag` is the tag of `C`.
0032     //!
0033     //! Secondly, `C` must have a nested `static constexpr` member named
0034     //! `value`, such that the following code is valid:
0035     //! @code
0036     //!     constexpr auto v = C::value;
0037     //! @endcode
0038     //! Because of the requirement that `Tag::value_type` be an integral type,
0039     //! it follows that `C::value` must be an integral value.
0040     //!
0041     //! Finally, it is necessary to specialize the `IntegralConstant` template
0042     //! in the `boost::hana` namespace to tell Hana that a type is a model
0043     //! of `IntegralConstant`:
0044     //! @code
0045     //!     namespace boost { namespace hana {
0046     //!         template <>
0047     //!         struct IntegralConstant<your_custom_tag> {
0048     //!             static constexpr bool value = true;
0049     //!         };
0050     //!     }}
0051     //! @endcode
0052     //!
0053     //!
0054     //! Refined concept
0055     //! ---------------
0056     //! 1. `Constant` (free implementation of `value`)\n
0057     //! The `value` function required to be a `Constant` can be implemented
0058     //! as follows for `IntegralConstant`s:
0059     //! @code
0060     //!     value<C>() == C::value
0061     //! @endcode
0062     //! The `to` function must still be provided explicitly for the model
0063     //! of `Constant` to be complete.
0064     //!
0065     //!
0066     //! Concrete models
0067     //! ---------------
0068     //! `hana::integral_constant`
0069     template <typename C>
0070     struct IntegralConstant;
0071 }} // end namespace boost::hana
0072
0073 #endif // !BOOST_HANA_FWD_CONCEPT_INTEGRAL_CONSTANT_HPP