Squashed 'third_party/immer/' content from commit ad3e3556d

git-subtree-dir: third_party/immer
git-subtree-split: ad3e3556d38bb75966dd24c61a774970a7c7957e

immer/heap/with_data.hpp

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+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include <cstdio>
+
+namespace immer {
+
+/*!
+ * Appends a default constructed extra object of type `T` at the
+ * *before* the requested region.
+ *
+ * @tparam T Type of the appended data.
+ * @tparam Base Type of the parent heap.
+ */
+template <typename T, typename Base>
+struct with_data : Base
+{
+    using base_t = Base;
+
+    template <typename... Tags>
+    static void* allocate(std::size_t size, Tags... tags)
+    {
+        auto p = base_t::allocate(size + sizeof(T), tags...);
+        return new (p) T{} + 1;
+    }
+
+    template <typename... Tags>
+    static void deallocate(std::size_t size, void* p, Tags... tags)
+    {
+        auto dp = static_cast<T*>(p) - 1;
+        dp->~T();
+        base_t::deallocate(size + sizeof(T), dp, tags...);
+    }
+};
+
+} // namespace immer