63e88f80
 Hu Chunming
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/*!
@file
Forward declares `boost::hana::length`.
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_LENGTH_HPP
#define BOOST_HANA_FWD_LENGTH_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
namespace boost { namespace hana {
//! Return the number of elements in a foldable structure.
//! @ingroup group-Foldable
//!
//! Given a `Foldable` `xs`, `length(xs)` must return an object of an
//! unsigned integral type, or an `IntegralConstant` holding such an
//! object, which represents the number of elements in the structure.
//!
//! @note
//! Since only compile-time `Foldable`s are supported in the library
//! right now, `length` must always return an `IntegralConstant`.
//!
//!
//! Example
//! -------
//! @include example/length.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto length = [](auto const& xs) {
return tag-dispatched;
};
#else
template <typename T, typename = void>
struct length_impl : length_impl<T, when<true>> { };
struct length_t {
template <typename Xs>
constexpr auto operator()(Xs const& xs) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr length_t length{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_LENGTH_HPP
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