63e88f80
 Hu Chunming
提交三方库
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/*!
@file
Forward declares `boost::hana::zero`.
Copyright Louis Dionne 2013-2022
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_ZERO_HPP
#define BOOST_HANA_FWD_ZERO_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
namespace boost { namespace hana {
//! Identity of `plus`.
//! @ingroup group-Monoid
//!
//! @tparam M
//! The tag (a `Monoid`) of the returned identity.
//!
//!
//! Example
//! -------
//! @include example/zero.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
template <typename M>
constexpr auto zero = []() -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename M, typename = void>
struct zero_impl : zero_impl<M, when<true>> { };
template <typename M>
struct zero_t {
constexpr decltype(auto) operator()() const;
};
template <typename M>
BOOST_HANA_INLINE_VARIABLE constexpr zero_t<M> zero{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_ZERO_HPP
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