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/*!
@file
Forward declares `boost::hana::at` and `boost::hana::at_c`.
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_AT_HPP
#define BOOST_HANA_FWD_AT_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
#include <cstddef>
namespace boost { namespace hana {
//! Returns the `n`th element of an iterable.
//! @ingroup group-Iterable
//!
//! Given an `Iterable` and an `IntegralConstant` index, `at` returns the
//! element located at the index in the linearization of the iterable.
//! Specifically, given an iterable `xs` with a linearization of
//! `[x1, ..., xN]`, `at(xs, k)` is equivalent to `xk`.
//!
//! If the `Iterable` actually stores the elements it contains, `at` is
//! required to return a lvalue reference, a lvalue reference to const
//! or a rvalue reference to the matching element, where the type of
//! reference must match that of the iterable passed to `at`. If the
//! `Iterable` does not store the elements it contains (i.e. it generates
//! them on demand), this requirement is dropped.
//!
//!
//! @param xs
//! The iterable in which an element is retrieved. The iterable must
//! contain at least `n + 1` elements.
//!
//! @param n
//! A non-negative `IntegralConstant` representing the 0-based index of
//! the element to return. It is an error to call `at` with an index that
//! out of bounds of the iterable.
//!
//!
//! Example
//! -------
//! @include example/at.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto at = [](auto&& xs, auto const& n) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename It, typename = void>
struct at_impl : at_impl<It, when<true>> { };
struct at_t {
template <typename Xs, typename N>
constexpr decltype(auto) operator()(Xs&& xs, N const& n) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr at_t at{};
#endif
//! Equivalent to `at`; provided for convenience.
//! @ingroup group-Iterable
//!
//!
//! @note
//! `hana::at_c<n>` is an overloaded function, not a function object.
//! Hence, it can't be passed to higher-order algorithms. This is done
//! for compile-time performance reasons.
//!
//!
//! Example
//! -------
//! @include example/at_c.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
template <std::size_t n>
constexpr auto at_c = [](auto&& xs) {
return hana::at(forwarded(xs), hana::size_c<n>);
};
#else
template <std::size_t n, typename Xs>
constexpr decltype(auto) at_c(Xs&& xs);
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_AT_HPP
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