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/*!
@file
Forward declares `boost::hana::IntegralConstant`.
@copyright Louis Dionne 2013-2017
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
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