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 Hu Chunming
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/*!
@file
Forward declares `boost::hana::front`.
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_FRONT_HPP
#define BOOST_HANA_FWD_FRONT_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
namespace boost { namespace hana {
//! Returns the first element of a non-empty iterable.
//! @ingroup group-Iterable
//!
//! Given a non-empty Iterable `xs` with a linearization of `[x1, ..., xN]`,
//! `front(xs)` is equal to `x1`. If `xs` is empty, it is an error to
//! use this function. Equivalently, `front(xs)` must be equivalent to
//! `at_c<0>(xs)`, and that regardless of the value category of `xs`
//! (`front` must respect the reference semantics of `at`).
//!
//!
//! Example
//! -------
//! @include example/front.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto front = [](auto&& xs) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename It, typename = void>
struct front_impl : front_impl<It, when<true>> { };
struct front_t {
template <typename Xs>
constexpr decltype(auto) operator()(Xs&& xs) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr front_t front{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_FRONT_HPP
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