63e88f80
 Hu Chunming
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/*!
@file
Forward declares `boost::hana::first`.
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_FIRST_HPP
#define BOOST_HANA_FWD_FIRST_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
namespace boost { namespace hana {
//! Returns the first element of a pair.
//! @ingroup group-Product
//!
//! Note that if the `Product` actually stores the elements it contains,
//! `hana::first` is required to return a lvalue reference, a lvalue
//! reference to const or a rvalue reference to the first element, where
//! the type of reference must match that of the pair passed to `first`.
//! If the `Product` does not store the elements it contains (i.e. it
//! generates them on demand), this requirement is dropped.
//!
//!
//! Example
//! -------
//! @include example/first.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto first = [](auto&& product) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename P, typename = void>
struct first_impl : first_impl<P, when<true>> { };
struct first_t {
template <typename Pair>
constexpr decltype(auto) operator()(Pair&& pair) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr first_t first{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_FIRST_HPP
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