SERVER-17816 Implementation of a generic decoration facility.

1 files changed, 131 insertions, 0 deletions

diff --git a/src/mongo/util/decoration_container.h b/src/mongo/util/decoration_container.h
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+++ b/src/mongo/util/decoration_container.h
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+/**
+ *    Copyright (C) 2015 MongoDB Inc.
+ *
+ *    This program is free software: you can redistribute it and/or  modify
+ *    it under the terms of the GNU Affero General Public License, version 3,
+ *    as published by the Free Software Foundation.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    GNU Affero General Public License for more details.
+ *
+ *    You should have received a copy of the GNU Affero General Public License
+ *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the GNU Affero General Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+#include <memory>
+
+#include "mongo/base/disallow_copying.h"
+#include "mongo/platform/cstdint.h"
+
+namespace mongo {
+
+    class DecorationRegistry;
+
+    /**
+     * An container for decorations.
+     */
+    class DecorationContainer {
+        MONGO_DISALLOW_COPYING(DecorationContainer);
+    public:
+
+        /**
+         * Opaque descriptor of a decoration.  It is an identifier to a field on the
+         * DecorationContainer that is private to those modules that have access to the descriptor.
+         */
+        class DecorationDescriptor {
+        public:
+            DecorationDescriptor() = default;
+
+        private:
+            friend class DecorationContainer;
+            friend class DecorationRegistry;
+
+            explicit DecorationDescriptor(size_t index) : _index(index) {}
+
+            size_t _index;
+        };
+
+        /**
+         * Opaque description of a decoration of specified type T.  It is an identifier to a field
+         * on the DecorationContainer that is private to those modules that have access to the
+         * descriptor.
+         */
+        template <typename T>
+        class DecorationDescriptorWithType {
+        public:
+            DecorationDescriptorWithType() = default;
+
+        private:
+            friend class DecorationContainer;
+            friend class DecorationRegistry;
+
+            explicit DecorationDescriptorWithType(DecorationDescriptor raw) :
+                _raw(std::move(raw)) {}
+
+            DecorationDescriptor _raw;
+        };
+
+        /**
+         * Constructs a decorable built based on the given "registry."
+         *
+         * The registry must stay in scope for the lifetime of the DecorationContainer, and must not
+         * have any declareDecoration() calls made on it while a DecorationContainer dependent on it
+         * is in scope.
+         */
+        explicit DecorationContainer(const DecorationRegistry* registry);
+        ~DecorationContainer();
+
+        /**
+         * Gets the decorated value for the given descriptor.
+         *
+         * The descriptor must be one returned from this DecorationContainer's associated _registry.
+         */
+        void* getDecoration(DecorationDescriptor descriptor) {
+            return _decorationData.get() + descriptor._index;
+        }
+
+        /**
+         * Same as the non-const form above, but returns a const result.
+         */
+        const void* getDecoration(DecorationDescriptor descriptor) const {
+            return _decorationData.get() + descriptor._index;
+        }
+
+        /**
+         * Gets the decorated value or the given typed descriptor.
+         */
+        template <typename T>
+        T& getDecoration(DecorationDescriptorWithType<T> descriptor) {
+            return *static_cast<T*>(getDecoration(descriptor._raw));
+        }
+
+        /**
+         * Same as the non-const form above, but returns a const result.
+         */
+        template <typename T>
+        const T& getDecoration(DecorationDescriptorWithType<T> descriptor) const {
+            return *static_cast<const T*>(getDecoration(descriptor._raw));
+        }
+
+    private:
+        const DecorationRegistry* const _registry;
+        const std::unique_ptr<unsigned char[]> _decorationData;
+    };
+
+}  // namespace mongo