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/*!
@file
Forward declares `boost::hana::difference`.
@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_DIFFERENCE_HPP
#define BOOST_HANA_FWD_DIFFERENCE_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! Returns the set-theoretic difference of two sets.
//! @relates hana::set
//!
//! Given two sets `xs` and `ys`, `difference(xs, ys)` is a new set
//! containing all the elements of `xs` that are _not_ contained in `ys`.
//! For any object `x`, the following holds:
//! @code
//! x ^in^ difference(xs, ys) if and only if x ^in^ xs && !(x ^in^ ys)
//! @endcode
//!
//!
//! @note
//! This operation is not commutative, i.e. `difference(xs, ys)` is not
//! necessarily the same as `difference(ys, xs)`. Indeed, consider the
//! case where `xs` is empty and `ys` isn't. Then, `difference(xs, ys)`
//! is empty but `difference(ys, xs)` is equal to `ys`. For the symmetric
//! version of this operation, see `symmetric_difference`.
//!
//!
//! @param xs
//! A set to remove values from.
//!
//! @param ys
//! The set whose values are removed from `xs`.
//!
//!
//! Example
//! -------
//! @include example/difference.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto difference = [](auto&& xs, auto&& ys) {
return tag-dispatched;
};
#else
template <typename S, typename = void>
struct difference_impl : difference_impl<S, when<true>> { };
struct difference_t {
template <typename Xs, typename Ys>
constexpr auto operator()(Xs&& xs, Ys&& ys) const;
};
constexpr difference_t difference{};
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_DIFFERENCE_HPP
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