90 lines
2.8 KiB
C++
90 lines
2.8 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::at` and `boost::hana::at_c`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FWD_AT_HPP
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#define BOOST_HANA_FWD_AT_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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#include <cstddef>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Returns the `n`th element of an iterable.
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//! @ingroup group-Iterable
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//!
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//! Given an `Iterable` and an `IntegralConstant` index, `at` returns the
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//! element located at the index in the linearization of the iterable.
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//! Specifically, given an iterable `xs` with a linearization of
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//! `[x1, ..., xN]`, `at(xs, k)` is equivalent to `xk`.
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//!
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//! If the `Iterable` actually stores the elements it contains, `at` is
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//! required to return a lvalue reference, a lvalue reference to const
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//! or a rvalue reference to the matching element, where the type of
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//! reference must match that of the iterable passed to `at`. If the
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//! `Iterable` does not store the elements it contains (i.e. it generates
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//! them on demand), this requirement is dropped.
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//!
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//!
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//! @param xs
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//! The iterable in which an element is retrieved. The iterable must
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//! contain at least `n + 1` elements.
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//!
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//! @param n
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//! A non-negative `IntegralConstant` representing the 0-based index of
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//! the element to return. It is an error to call `at` with an index that
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//! out of bounds of the iterable.
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//!
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//!
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//! Example
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//! -------
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//! @include example/at.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto at = [](auto&& xs, auto const& n) -> decltype(auto) {
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return tag-dispatched;
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};
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#else
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template <typename It, typename = void>
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struct at_impl : at_impl<It, when<true>> { };
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struct at_t {
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template <typename Xs, typename N>
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constexpr decltype(auto) operator()(Xs&& xs, N const& n) const;
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};
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constexpr at_t at{};
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#endif
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//! Equivalent to `at`; provided for convenience.
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//! @ingroup group-Iterable
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//!
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//!
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//! @note
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//! `hana::at_c<n>` is an overloaded function, not a function object.
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//! Hence, it can't be passed to higher-order algorithms. This is done
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//! for compile-time performance reasons.
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//!
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//!
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//! Example
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//! -------
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//! @include example/at_c.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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template <std::size_t n>
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constexpr auto at_c = [](auto&& xs) {
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return hana::at(forwarded(xs), hana::size_c<n>);
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};
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#else
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template <std::size_t n, typename Xs>
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constexpr decltype(auto) at_c(Xs&& xs);
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_AT_HPP
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