1/*!
2@file
3Defines `boost::hana::zero`.
4
5Copyright Louis Dionne 2013-2022
6Distributed 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_ZERO_HPP
11#define BOOST_HANA_ZERO_HPP
12
13#include <boost/hana/fwd/zero.hpp>
14
15#include <boost/hana/concept/constant.hpp>
16#include <boost/hana/concept/monoid.hpp>
17#include <boost/hana/config.hpp>
18#include <boost/hana/core/to.hpp>
19#include <boost/hana/core/dispatch.hpp>
20#include <boost/hana/detail/canonical_constant.hpp>
21
22#include <type_traits>
23
24
25namespace boost { namespace hana {
26 //! @cond
27 template <typename M>
28 constexpr decltype(auto) zero_t<M>::operator()() const {
29 #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
30 static_assert(hana::Monoid<M>::value,
31 "hana::zero<M>() requires 'M' to be a Monoid");
32 #endif
33
34 using Zero = BOOST_HANA_DISPATCH_IF(zero_impl<M>,
35 hana::Monoid<M>::value
36 );
37
38 return Zero::apply();
39 }
40 //! @endcond
41
42 template <typename M, bool condition>
43 struct zero_impl<M, when<condition>> : default_ {
44 template <typename ...Args>
45 static constexpr auto apply(Args&& ...) = delete;
46 };
47
48 //////////////////////////////////////////////////////////////////////////
49 // Model for non-boolean arithmetic data types
50 //////////////////////////////////////////////////////////////////////////
51 template <typename T>
52 struct zero_impl<T, when<
53 std::is_arithmetic<T>::value &&
54 !std::is_same<T, bool>::value
55 >> {
56 static constexpr T apply()
57 { return static_cast<T>(0); }
58 };
59
60 //////////////////////////////////////////////////////////////////////////
61 // Model for Constants over a Monoid
62 //////////////////////////////////////////////////////////////////////////
63 namespace detail {
64 template <typename C>
65 struct constant_from_zero {
66 static constexpr auto value = hana::zero<typename C::value_type>();
67 using hana_tag = detail::CanonicalConstant<typename C::value_type>;
68 };
69 }
70
71 template <typename C>
72 struct zero_impl<C, when<
73 hana::Constant<C>::value &&
74 Monoid<typename C::value_type>::value
75 >> {
76 static constexpr decltype(auto) apply()
77 { return hana::to<C>(detail::constant_from_zero<C>{}); }
78 };
79}} // end namespace boost::hana
80
81#endif // !BOOST_HANA_ZERO_HPP
82

source code of boost/libs/hana/include/boost/hana/zero.hpp