ceph/src/boost/libs/hana/include/boost/hana/fwd/concept/integral_constant.hpp

   1 /*!
   2 @file
   3 Forward declares `boost::hana::IntegralConstant`.
   4
   5 @copyright Louis Dionne 2013-2016
   6 Distributed under the Boost Software License, Version 1.0.
   7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   8  */
   9
  10 #ifndef BOOST_HANA_FWD_CONCEPT_INTEGRAL_CONSTANT_HPP
  11 #define BOOST_HANA_FWD_CONCEPT_INTEGRAL_CONSTANT_HPP
  12
  13 #include <boost/hana/config.hpp>
  14
  15
  16 BOOST_HANA_NAMESPACE_BEGIN
  17     //! @ingroup group-concepts
  18     //! The `IntegralConstant` concept represents compile-time integral values.
  19     //!
  20     //! The `IntegralConstant` concept represents objects that hold a
  21     //! `constexpr` value of an integral type. In other words, it describes
  22     //! the essential functionality provided by `std::integral_constant`.
  23     //! An `IntegralConstant` is also just a special kind of `Constant`
  24     //! whose inner value is of an integral type.
  25     //!
  26     //!
  27     //! Minimal complete definition
  28     //! ---------------------------
  29     //! The requirements for being an `IntegralConstant` are quite simple.
  30     //! First, an `IntegralConstant` `C` must be a `Constant` such that
  31     //! `Tag::value_type` is an integral type, where `Tag` is the tag of `C`.
  32     //!
  33     //! Secondly, `C` must have a nested `static constexpr` member named
  34     //! `value`, such that the following code is valid:
  35     //! @code
  36     //!     constexpr auto v = C::value;
  37     //! @endcode
  38     //! Because of the requirement that `Tag::value_type` be an integral type,
  39     //! it follows that `C::value` must be an integral value.
  40     //!
  41     //! Finally, it is necessary to specialize the `IntegralConstant` template
  42     //! in the `boost::hana` namespace to tell Hana that a type is a model
  43     //! of `IntegralConstant`:
  44     //! @code
  45     //!     namespace boost { namespace hana {
  46     //!         template <>
  47     //!         struct IntegralConstant<your_custom_tag> {
  48     //!             static constexpr bool value = true;
  49     //!         };
  50     //!     }}
  51     //! @endcode
  52     //!
  53     //!
  54     //! Refined concept
  55     //! ---------------
  56     //! 1. `Constant` (free implementation of `value`)\n
  57     //! The `value` function required to be a `Constant` can be implemented
  58     //! as follows for `IntegralConstant`s:
  59     //! @code
  60     //!     value<C>() == C::value
  61     //! @endcode
  62     //! The `to` function must still be provided explicitly for the model
  63     //! of `Constant` to be complete.
  64     //!
  65     //!
  66     //! Concrete models
  67     //! ---------------
  68     //! `hana::integral_constant`
  69     template <typename C>
  70     struct IntegralConstant;
  71 BOOST_HANA_NAMESPACE_END
  72
  73 #endif // !BOOST_HANA_FWD_CONCEPT_INTEGRAL_CONSTANT_HPP