iterator: add tuple_picker adapter

CMakeLists.txt

@@ -426,6 +426,7 @@ list (
     iterator/placement_output.hpp
     iterator/referencing.hpp
     iterator/transform.hpp
+    iterator/tuple_picker.hpp
     iterator/unequal.hpp
     iterator/unordered_insert.hpp
     iterator/zip.hpp

iterator/tuple_picker.hpp

@@ -0,0 +1,67 @@
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * Copyright 2021, Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include <iterator>
+#include <tuple>
+#include <type_traits>
+#include <compare>
+
+
+///////////////////////////////////////////////////////////////////////////////
+namespace cruft::iterator {
+    /// An iterator adapter that returns the tuple_element at IndexV for the
+    /// underlying iterator when dereferenced.
+    template <std::size_t IndexV, typename IteratorT>
+    class tuple_picker {
+    public:
+        using iterator_category = typename std::iterator_traits<IteratorT>::iterator_category;
+        using value_type = std::tuple_element_t<
+            IndexV,
+            // Ideally we'd use iterator_traits::value_type but that removes
+            // constness from value_type for pointers to const, and we need to
+            // retain const semantics to support some existing cases (like
+            // map::keys queries).
+            std::remove_reference_t<decltype(*std::declval<IteratorT> ())>
+        >;
+
+        using difference_type = typename std::iterator_traits<IteratorT>::difference_type;
+        using pointer = value_type*;
+        using reference = value_type&;
+
+        tuple_picker (IteratorT _cursor)
+            : m_cursor (_cursor)
+        { ; }
+
+        reference operator* () { return std::get<IndexV> (*m_cursor); }
+        tuple_picker& operator++ () { ++m_cursor; return *this; }
+
+        auto operator<=> (tuple_picker const &rhs) const = default;
+
+    private:
+        IteratorT m_cursor;
+    };
+
+
+    /// A convenience constructor for tuple_picker.
+    ///
+    /// Provided in-lieu of a user deduction guide so that partial template
+    /// parameters can be provided. ie, make_tuple_picker<1> (iter).
+    template <std::size_t IndexV, typename IteratorT>
+    auto
+    make_tuple_picker (IteratorT &&iterator)
+    {
+        return tuple_picker<
+            IndexV,
+            std::remove_reference_t<IteratorT>
+        > (
+            std::forward<IteratorT> (iterator)
+        );
+    }
+}