boost/hana/zero.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::zero`.
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_ZERO_HPP
0011 #define BOOST_HANA_ZERO_HPP
0012
0013 #include <boost/hana/fwd/zero.hpp>
0014
0015 #include <boost/hana/concept/constant.hpp>
0016 #include <boost/hana/concept/monoid.hpp>
0017 #include <boost/hana/config.hpp>
0018 #include <boost/hana/core/to.hpp>
0019 #include <boost/hana/core/dispatch.hpp>
0020 #include <boost/hana/detail/canonical_constant.hpp>
0021
0022 #include <type_traits>
0023
0024
0025 namespace boost { namespace hana {
0026     //! @cond
0027     template <typename M>
0028     constexpr decltype(auto) zero_t<M>::operator()() const {
0029     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
0030         static_assert(hana::Monoid<M>::value,
0031         "hana::zero<M>() requires 'M' to be a Monoid");
0032     #endif
0033
0034         using Zero = BOOST_HANA_DISPATCH_IF(zero_impl<M>,
0035             hana::Monoid<M>::value
0036         );
0037
0038         return Zero::apply();
0039     }
0040     //! @endcond
0041
0042     template <typename M, bool condition>
0043     struct zero_impl<M, when<condition>> : default_ {
0044         template <typename ...Args>
0045         static constexpr auto apply(Args&& ...) = delete;
0046     };
0047
0048     //////////////////////////////////////////////////////////////////////////
0049     // Model for non-boolean arithmetic data types
0050     //////////////////////////////////////////////////////////////////////////
0051     template <typename T>
0052     struct zero_impl<T, when<
0053         std::is_arithmetic<T>::value &&
0054         !std::is_same<T, bool>::value
0055     >> {
0056         static constexpr T apply()
0057         { return static_cast<T>(0); }
0058     };
0059
0060     //////////////////////////////////////////////////////////////////////////
0061     // Model for Constants over a Monoid
0062     //////////////////////////////////////////////////////////////////////////
0063     namespace detail {
0064         template <typename C>
0065         struct constant_from_zero {
0066             static constexpr auto value = hana::zero<typename C::value_type>();
0067             using hana_tag = detail::CanonicalConstant<typename C::value_type>;
0068         };
0069     }
0070
0071     template <typename C>
0072     struct zero_impl<C, when<
0073         hana::Constant<C>::value &&
0074         Monoid<typename C::value_type>::value
0075     >> {
0076         static constexpr decltype(auto) apply()
0077         { return hana::to<C>(detail::constant_from_zero<C>{}); }
0078     };
0079 }} // end namespace boost::hana
0080
0081 #endif // !BOOST_HANA_ZERO_HPP