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 /*!
 @file
 Forward declares `boost::hana::set`.
 
 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_SET_HPP
 #define BOOST_HANA_FWD_SET_HPP
 
 #include <boost/hana/config.hpp>
 #include <boost/hana/fwd/core/to.hpp>
 #include <boost/hana/fwd/core/make.hpp>
 #include <boost/hana/fwd/erase_key.hpp>
 
 
 namespace boost { namespace hana {
     //! @ingroup group-datatypes
     //! Basic unordered container requiring unique, `Comparable` and
     //! `Hashable` keys.
     //!
     //! A set is an unordered container that can hold heterogeneous keys.
     //! A set requires (and ensures) that no duplicates are present when
     //! inserting new keys.
     //!
     //! @note
     //! The actual representation of a `hana::set` is implementation-defined.
     //! In particular, one should not take for granted the order of the
     //! template parameters and the presence of any additional constructors
     //! or assignment operators than what is documented. The canonical way of
     //! creating a `hana::set` is through `hana::make_set`. More details
     //! [in the tutorial](@ref tutorial-containers-types).
     //!
     //!
     //! Modeled concepts
     //! ----------------
     //! 1. `Comparable`\n
     //! Two sets are equal iff they contain the same elements, regardless of
     //! the order.
     //! @include example/set/comparable.cpp
     //!
     //! 2. Foldable\n
     //! Folding a set is equivalent to folding the sequence of its values.
     //! However, note that the values are not required to be in any specific
     //! order, so using the folds provided here with an operation that is not
     //! both commutative and associative will yield non-deterministic behavior.
     //! @include example/set/foldable.cpp
     //!
     //! 3. Searchable\n
     //! The elements in a set act as both its keys and its values. Since the
     //! elements of a set are unique, searching for an element will return
     //! either the only element which is equal to the searched value, or
     //! `nothing`. Also note that `operator[]` can be used instead of the
     //! `at_key` function.
     //! @include example/set/searchable.cpp
     //!
     //!
     //! Conversion from any `Foldable`
     //! ------------------------------
     //! Any `Foldable` structure can be converted into a `hana::set` with
     //! `to<set_tag>`. The elements of the structure must all be compile-time
     //! `Comparable`. If the structure contains duplicate elements, only
     //! the first occurence will appear in the resulting set. More
     //! specifically, conversion from a `Foldable` is equivalent to
     //! @code
     //!     to<set_tag>(xs) == fold_left(xs, make_set(), insert)
     //! @endcode
     //!
     //! __Example__
     //! @include example/set/to.cpp
 #ifdef BOOST_HANA_DOXYGEN_INVOKED
     template <typename implementation_defined>
     struct set {
         //! Default-construct a set. This constructor only exists when all the
         //! elements of the set are default-constructible.
         constexpr set() = default;
 
         //! Copy-construct a set from another set. This constructor only
         //! exists when all the elements of the set are copy-constructible.
         constexpr set(set const& other) = default;
 
         //! Move-construct a set from another set. This constructor only
         //! exists when all the elements of the set are move-constructible.
         constexpr set(set&& other) = default;
 
         //! Equivalent to `hana::equal`
         template <typename X, typename Y>
         friend constexpr auto operator==(X&& x, Y&& y);
 
         //! Equivalent to `hana::not_equal`
         template <typename X, typename Y>
         friend constexpr auto operator!=(X&& x, Y&& y);
 
         //! Equivalent to `hana::at_key`
         template <typename Key>
         constexpr decltype(auto) operator[](Key&& key);
     };
 #else
     template <typename ...Xs>
     struct set;
 #endif
 
     //! Tag representing the `hana::set` container.
     //! @relates hana::set
     struct set_tag { };
 
     //! Function object for creating a `hana::set`.
     //! @relates hana::set
     //!
     //! Given zero or more values `xs...`, `make<set_tag>` returns a `set`
     //! containing those values. The values must all be compile-time
     //! `Comparable`, and no duplicate values may be provided. To create
     //! a `set` from a sequence with possible duplicates, use `to<set_tag>`
     //! instead.
     //!
     //!
     //! Example
     //! -------
     //! @include example/set/make.cpp
 #ifdef BOOST_HANA_DOXYGEN_INVOKED
     template <>
     constexpr auto make<set_tag> = [](auto&& ...xs) {
         return set<implementation_defined>{forwarded(xs)...};
     };
 #endif
 
     //! Equivalent to `make<set_tag>`; provided for convenience.
     //! @relates hana::set
     //!
     //!
     //! Example
     //! -------
     //! @include example/set/make.cpp
     BOOST_HANA_INLINE_VARIABLE constexpr auto make_set = make<set_tag>;
 
     //! Insert an element in a `hana::set`.
     //! @relates hana::set
     //!
     //! If the set already contains an element that compares equal, then
     //! nothing is done and the set is returned as is.
     //!
     //!
     //! @param set
     //! The set in which to insert a value.
     //!
     //! @param element
     //! The value to insert. It must be compile-time `Comparable`.
     //!
     //!
     //! Example
     //! -------
     //! @include example/set/insert.cpp
 #ifdef BOOST_HANA_DOXYGEN_INVOKED
     constexpr auto insert = [](auto&& set, auto&& element) {
         return tag-dispatched;
     };
 #endif
 
     //! Remove an element from a `hana::set`.
     //! @relates hana::set
     //!
     //! Returns a new set containing all the elements of the original,
     //! except the one comparing `equal` to the given element. If the set
     //! does not contain such an element, a new set equal to the original
     //! set is returned.
     //!
     //!
     //! @param set
     //! The set in which to remove a value.
     //!
     //! @param element
     //! The value to remove. It must be compile-time `Comparable`.
     //!
     //!
     //! Example
     //! -------
     //! @include example/set/erase_key.cpp
 #ifdef BOOST_HANA_DOXYGEN_INVOKED
     constexpr auto erase_key = [](auto&& set, auto&& element) {
         return tag-dispatched;
     };
 #endif
 
     //! Returns the union of two sets.
     //! @relates hana::set
     //!
     //! Given two sets `xs` and `ys`, `union_(xs, ys)` is a new set containing
     //! all the elements of `xs` and all the elements of `ys`, without
     //! duplicates. For any object `x`, the following holds: `x` is in
     //! `hana::union_(xs, ys)` if and only if `x` is in `xs` or `x` is in `ys`.
     //!
     //!
     //! @param xs, ys
     //! Two sets to compute the union of.
     //!
     //!
     //! Example
     //! -------
     //! @include example/set/union.cpp
 #ifdef BOOST_HANA_DOXYGEN_INVOKED
     constexpr auto union_ = [](auto&& xs, auto&& ys) {
         return tag-dispatched;
     };
 #endif
 
     //! Returns the intersection of two sets.
     //! @relates hana::set
     //!
     //! Given two sets `xs` and `ys`, `intersection(xs, ys)` is a new set
     //! containing exactly those elements that are present both in `xs` and
     //! in `ys`.
     //! In other words, the following holds for any object `x`:
     //! @code
     //!     x ^in^ intersection(xs, ys) if and only if x ^in^ xs && x ^in^ ys
     //! @endcode
     //!
     //!
     //! @param xs, ys
     //! Two sets to intersect.
     //!
     //!
     //! Example
     //! -------
     //! @include example/set/intersection.cpp
 #ifdef BOOST_HANA_DOXYGEN_INVOKED
     constexpr auto intersection = [](auto&& xs, auto&& ys) {
         return tag-dispatched;
     };
 #endif
     //! Equivalent to `to<set_tag>`; provided for convenience.
     //! @relates hana::set
     constexpr auto to_set = to<set_tag>;
 
     //! 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
     //!
     //!
     //! 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 param to remove values from.
     //!
     //! @param ys
     //! The set whose values are removed from `xs`.
     //!
     //!
     //! Example
     //! -------
     //! @include example/set/difference.cpp
 #ifdef BOOST_HANA_DOXYGEN_INVOKED
     constexpr auto difference = [](auto&& xs, auto&& ys) {
         return tag-dispatched;
 };
 #endif
 
     //! Returns the symmetric set-theoretic difference of two sets.
     //! @relates hana::set
     //!
     //! Given two sets `xs` and `ys`, `symmetric_difference(xs, ys)` is a new
     //! set containing all the elements of `xs` that are not contained in `ys`,
     //! and all the elements of `ys` that are not contained in `xs`. The
     //! symmetric difference of two sets satisfies the following:
     //! @code
     //!     symmetric_difference(xs, ys) == union_(difference(xs, ys), difference(ys, xs))
     //! @endcode
     //!
     //!
     //! @param xs, ys
     //! Two sets to compute the symmetric difference of.
     //!
     //!
     //! Example
     //! -------
     //! @include example/set/symmetric_difference.cpp
 #ifdef BOOST_HANA_DOXYGEN_INVOKED
 constexpr auto symmetric_difference = [](auto&& xs, auto&& ys) {
         return tag-dispatched;
     };
 #endif
 
 
 }} // end namespace boost::hana
 
 #endif // !BOOST_HANA_FWD_SET_HPP

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