67 lines
2.2 KiB
C++
67 lines
2.2 KiB
C++
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/*
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* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/.
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*
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* Copyright 2021, Danny Robson <danny@nerdcruft.net>
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*/
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#pragma once
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#include <iterator>
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#include <tuple>
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#include <type_traits>
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#include <compare>
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///////////////////////////////////////////////////////////////////////////////
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namespace cruft::iterator {
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/// An iterator adapter that returns the tuple_element at IndexV for the
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/// underlying iterator when dereferenced.
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template <std::size_t IndexV, typename IteratorT>
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class tuple_picker {
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public:
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using iterator_category = typename std::iterator_traits<IteratorT>::iterator_category;
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using value_type = std::tuple_element_t<
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IndexV,
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// Ideally we'd use iterator_traits::value_type but that removes
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// constness from value_type for pointers to const, and we need to
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// retain const semantics to support some existing cases (like
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// map::keys queries).
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std::remove_reference_t<decltype(*std::declval<IteratorT> ())>
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>;
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using difference_type = typename std::iterator_traits<IteratorT>::difference_type;
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using pointer = value_type*;
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using reference = value_type&;
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tuple_picker (IteratorT _cursor)
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: m_cursor (_cursor)
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{ ; }
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reference operator* () { return std::get<IndexV> (*m_cursor); }
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tuple_picker& operator++ () { ++m_cursor; return *this; }
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auto operator<=> (tuple_picker const &rhs) const = default;
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private:
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IteratorT m_cursor;
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};
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/// A convenience constructor for tuple_picker.
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///
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/// Provided in-lieu of a user deduction guide so that partial template
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/// parameters can be provided. ie, make_tuple_picker<1> (iter).
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template <std::size_t IndexV, typename IteratorT>
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auto
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make_tuple_picker (IteratorT &&iterator)
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{
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return tuple_picker<
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IndexV,
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std::remove_reference_t<IteratorT>
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> (
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std::forward<IteratorT> (iterator)
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);
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}
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}
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