diff options
Diffstat (limited to 'boost/hana/fwd/at.hpp')
-rw-r--r-- | boost/hana/fwd/at.hpp | 97 |
1 files changed, 97 insertions, 0 deletions
diff --git a/boost/hana/fwd/at.hpp b/boost/hana/fwd/at.hpp new file mode 100644 index 0000000000..d069143c4a --- /dev/null +++ b/boost/hana/fwd/at.hpp @@ -0,0 +1,97 @@ +/*! +@file +Forward declares `boost::hana::at` and `boost::hana::at_c`. + +@copyright Louis Dionne 2013-2016 +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> + + +BOOST_HANA_NAMESPACE_BEGIN + //! 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 + //! + //! + //! Benchmarks + //! ---------- + //! <div class="benchmark-chart" + //! style="min-width: 310px; height: 400px; margin: 0 auto" + //! data-dataset="benchmark.at.compile.json"> + //! </div> +#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; + }; + + 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 +BOOST_HANA_NAMESPACE_END + +#endif // !BOOST_HANA_FWD_AT_HPP |