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|---|---|---|
| 7c673cae FG |
1 | /*! |
| 2 | @file | |
| 3 | Forward declares `boost::hana::difference`. | |
| 4 | ||
| 5 | @copyright Louis Dionne 2013-2016 | |
| 6 | Distributed under the Boost Software License, Version 1.0. | |
| 7 | (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) | |
| 8 | */ | |
| 9 | ||
| 10 | #ifndef BOOST_HANA_FWD_DIFFERENCE_HPP | |
| 11 | #define BOOST_HANA_FWD_DIFFERENCE_HPP | |
| 12 | ||
| 13 | #include <boost/hana/config.hpp> | |
| 14 | #include <boost/hana/core/when.hpp> | |
| 15 | ||
| 16 | ||
| 17 | BOOST_HANA_NAMESPACE_BEGIN | |
| 18 | //! Returns the set-theoretic difference of two sets. | |
| 19 | //! @relates hana::set | |
| 20 | //! | |
| 21 | //! Given two sets `xs` and `ys`, `difference(xs, ys)` is a new set | |
| 22 | //! containing all the elements of `xs` that are _not_ contained in `ys`. | |
| 23 | //! For any object `x`, the following holds: | |
| 24 | //! @code | |
| 25 | //! x ^in^ difference(xs, ys) if and only if x ^in^ xs && !(x ^in^ ys) | |
| 26 | //! @endcode | |
| 27 | //! | |
| 28 | //! | |
| 29 | //! @note | |
| 30 | //! This operation is not commutative, i.e. `difference(xs, ys)` is not | |
| 31 | //! necessarily the same as `difference(ys, xs)`. Indeed, consider the | |
| 32 | //! case where `xs` is empty and `ys` isn't. Then, `difference(xs, ys)` | |
| 33 | //! is empty but `difference(ys, xs)` is equal to `ys`. For the symmetric | |
| 34 | //! version of this operation, see `symmetric_difference`. | |
| 35 | //! | |
| 36 | //! | |
| 37 | //! @param xs | |
| 38 | //! A set to remove values from. | |
| 39 | //! | |
| 40 | //! @param ys | |
| 41 | //! The set whose values are removed from `xs`. | |
| 42 | //! | |
| 43 | //! | |
| 44 | //! Example | |
| 45 | //! ------- | |
| 46 | //! @include example/difference.cpp | |
| 47 | #ifdef BOOST_HANA_DOXYGEN_INVOKED | |
| 48 | constexpr auto difference = [](auto&& xs, auto&& ys) { | |
| 49 | return tag-dispatched; | |
| 50 | }; | |
| 51 | #else | |
| 52 | template <typename S, typename = void> | |
| 53 | struct difference_impl : difference_impl<S, when<true>> { }; | |
| 54 | ||
| 55 | struct difference_t { | |
| 56 | template <typename Xs, typename Ys> | |
| 57 | constexpr auto operator()(Xs&& xs, Ys&& ys) const; | |
| 58 | }; | |
| 59 | ||
| 60 | constexpr difference_t difference{}; | |
| 61 | #endif | |
| 62 | BOOST_HANA_NAMESPACE_END | |
| 63 | ||
| 64 | #endif // !BOOST_HANA_FWD_DIFFERENCE_HPP |