util/DereferenceIterator: new utility class

src/util/DereferenceIterator.hxx

+/*
+ * Copyright 2012-2020 Max Kellermann <max.kellermann@gmail.com>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the
+ * distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
+ * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef DEREFERENCE_ITERATOR_HXX
+#define DEREFERENCE_ITERATOR_HXX
+
+#include <iterator>
+#include <type_traits>
+
+/**
+ * An iterator wrapper that dereferences the values returned by the
+ * original iterator.
+ */
+template<typename IT,
+	 typename VT=typename std::remove_pointer<typename IT::value_type>::type>
+class DereferenceIterator {
+	using Traits = std::iterator_traits<IT>;
+
+	IT original;
+
+public:
+	using iterator_category = typename Traits::iterator_category;
+	using difference_type = typename Traits::difference_type;
+	using value_type = VT;
+	using pointer = VT *;
+	using reference = VT &;
+
+	DereferenceIterator() = default;
+
+	constexpr DereferenceIterator(const IT _original) noexcept
+		:original(_original) {}
+
+	reference operator*() const noexcept {
+		return static_cast<reference>(**original);
+	}
+
+	pointer operator->() const noexcept {
+		return static_cast<reference>(*original.IT::operator->());
+	}
+
+	auto &operator++() noexcept {
+		++original;
+		return *this;
+	}
+
+	auto operator++(int) noexcept {
+		auto old = *this;
+		original++;
+		return old;
+	}
+
+	auto &operator+=(difference_type n) noexcept {
+		original += n;
+		return *this;
+	}
+
+	auto &operator+(difference_type n) noexcept {
+		return original + n;
+	}
+
+	auto &operator--() noexcept {
+		original = --original;
+		return *this;
+	}
+
+	auto operator--(int) noexcept {
+		auto old = *this;
+		original--;
+		return old;
+	}
+
+	auto &operator-=(difference_type n) noexcept {
+		original -= n;
+		return *this;
+	}
+
+	auto &operator-(difference_type n) noexcept {
+		return original - n;
+	}
+
+	bool operator==(const DereferenceIterator<IT,VT> &other) const noexcept {
+		return original == other.original;
+	}
+
+	bool operator!=(const DereferenceIterator<IT,VT> &other) const noexcept {
+		return original != other.original;
+	}
+};
+
+#endif