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/*!
@file
Forward declares `boost::hana::is_subset`.
@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_IS_SUBSET_HPP
#define BOOST_HANA_FWD_IS_SUBSET_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
#include <boost/hana/functional/infix.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! Returns whether a structure contains a subset of the keys of
//! another structure.
//! @ingroup group-Searchable
//!
//! Given two `Searchable`s `xs` and `ys`, `is_subset` returns a `Logical`
//! representing whether `xs` is a subset of `ys`. In other words, it
//! returns whether all the keys of `xs` are also present in `ys`. This
//! method does not return whether `xs` is a _strict_ subset of `ys`; if
//! `xs` and `ys` are equal, all the keys of `xs` are also present in
//! `ys`, and `is_subset` returns true.
//!
//! @note
//! For convenience, `is_subset` can also be applied in infix notation.
//!
//!
//! Cross-type version of the method
//! --------------------------------
//! This method is tag-dispatched using the tags of both arguments.
//! It can be called with any two `Searchable`s sharing a common
//! `Searchable` embedding, as defined in the main documentation
//! of the `Searchable` concept. When `Searchable`s with two different
//! tags but sharing a common embedding are sent to `is_subset`, they
//! are first converted to this common `Searchable` and the `is_subset`
//! method of the common embedding is then used. Of course, the method
//! can be overriden for custom `Searchable`s for efficieny.
//!
//! @note
//! While cross-type dispatching for `is_subset` is supported, it is
//! not currently used by the library because there are no models
//! of `Searchable` with a common embedding.
//!
//!
//! @param xs
//! The structure to check whether it is a subset of `ys`.
//!
//! @param ys
//! The structure to check whether it is a superset of `xs`.
//!
//!
//! Example
//! -------
//! @include example/is_subset.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto is_subset = [](auto&& xs, auto&& ys) {
return tag-dispatched;
};
#else
template <typename S1, typename S2, typename = void>
struct is_subset_impl : is_subset_impl<S1, S2, when<true>> { };
struct is_subset_t {
template <typename Xs, typename Ys>
constexpr auto operator()(Xs&& xs, Ys&& ys) const;
};
constexpr auto is_subset = hana::infix(is_subset_t{});
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_IS_SUBSET_HPP
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