Add WeakValueHashMap class

Author: TheMostDiligent

Common/CMakeLists.txt

@@ -50,6 +50,7 @@ set(INTERFACE
     interface/Cast.hpp
     interface/CompilerDefinitions.h
     interface/CallbackWrapper.hpp
+    interface/WeakValueHashMap.hpp
 )
 
 set(SOURCE

Common/interface/WeakValueHashMap.hpp

@@ -0,0 +1,270 @@
+/*
+ *  Copyright 2025 Diligent Graphics LLC
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ *
+ *  In no event and under no legal theory, whether in tort (including negligence),
+ *  contract, or otherwise, unless required by applicable law (such as deliberate
+ *  and grossly negligent acts) or agreed to in writing, shall any Contributor be
+ *  liable for any damages, including any direct, indirect, special, incidental,
+ *  or consequential damages of any character arising as a result of this License or
+ *  out of the use or inability to use the software (including but not limited to damages
+ *  for loss of goodwill, work stoppage, computer failure or malfunction, or any and
+ *  all other commercial damages or losses), even if such Contributor has been advised
+ *  of the possibility of such damages.
+ */
+
+#pragma once
+
+/// \file
+/// Defines WeakValueHashMap class
+
+#include <unordered_map>
+#include <memory>
+#include <mutex>
+
+#include "../../Platforms/Basic/interface/DebugUtilities.hpp"
+
+namespace Diligent
+{
+
+/// WeakValueHashMap is a thread-safe hash map that holds weak pointers to its values.
+
+/// \tparam KeyType   - Type of the keys in the map. Must be hashable and comparable.
+/// \tparam ValueType - Type of the values in the map.
+///
+/// When a value is requested via GetOrInsert(), a strong pointer (shared_ptr) to the value is returned
+/// wrapped in a ValueHandle object. The ValueHandle object is responsible for removing the entry from the map
+/// when it is destroyed. If there are no more strong pointers to the value, the entry is removed from the map.
+///
+/// If a value is requested via Get(), a strong pointer to the value is returned wrapped in a ValueHandle object
+/// if the entry exists and the value has not expired. Otherwise, an empty ValueHandle is returned.
+///
+/// The map is thread-safe and can be accessed from multiple threads simultaneously.
+///
+/// Example usage:
+///
+///     WeakValueHashMap<int, std::string> Map;
+///     auto Handle = Map.GetOrInsert(1, "Value");
+///     std::cout << *Handle << std::endl; // Outputs "Value"
+///
+template <typename KeyType, typename ValueType>
+class WeakValueHashMap
+{
+private:
+    using MapType     = std::unordered_map<KeyType, std::weak_ptr<ValueType>>;
+    using MapIterType = typename MapType::iterator;
+    class Impl;
+
+public:
+    /// ValueHandle is a handle to a value in the WeakValueHashMap.
+
+    /// It holds a strong pointer to the value and removes the entry from the map when it is destroyed.
+    class ValueHandle
+    {
+    public:
+        ValueHandle() = default;
+
+        ~ValueHandle()
+        {
+            Release();
+        }
+
+        // Disable copy semantics
+        ValueHandle(const ValueHandle&) = delete;
+        ValueHandle& operator=(const ValueHandle&) = delete;
+
+        ValueHandle(ValueHandle&& rhs) noexcept :
+            m_pMap{std::move(rhs.m_pMap)},
+            m_pValue{std::move(rhs.m_pValue)},
+            m_Key{std::move(rhs.m_Key)}
+        {
+            rhs.m_pMap.reset();
+            rhs.m_pValue.reset();
+        }
+
+        ValueHandle& operator=(ValueHandle&& rhs) noexcept
+        {
+            if (this != &rhs)
+            {
+                Release();
+
+                m_pMap   = std::move(rhs.m_pMap);
+                m_pValue = std::move(rhs.m_pValue);
+                m_Key    = std::move(rhs.m_Key);
+
+                rhs.m_pMap.reset();
+                rhs.m_pValue.reset();
+            }
+            return *this;
+        }
+
+        ValueType*       Get() { return m_pValue.get(); }
+        const ValueType* Get() const { return m_pValue.get(); }
+
+        ValueType&       operator*() { return *m_pValue; }
+        const ValueType& operator*() const { return *m_pValue; }
+        ValueType*       operator->() { return m_pValue.get(); }
+        const ValueType* operator->() const { return m_pValue.get(); }
+
+        explicit operator bool() const { return m_pMap && m_pValue; }
+
+    private:
+        friend class WeakValueHashMap<KeyType, ValueType>;
+
+        void Release()
+        {
+            if (m_pMap)
+            {
+                // Release the shared pointer first so that Remove() can check if
+                // any other shared pointers exist
+                m_pValue.reset();
+
+                m_pMap->Remove(m_Key);
+                m_pMap.reset();
+            }
+        }
+
+        explicit ValueHandle(Impl&                      Map,
+                             std::shared_ptr<ValueType> pValue,
+                             KeyType                    Key) :
+            m_pMap{Map.shared_from_this()},
+            m_pValue{std::move(pValue)},
+            m_Key{std::move(Key)}
+        {}
+
+    private:
+        std::shared_ptr<Impl>      m_pMap;
+        std::shared_ptr<ValueType> m_pValue;
+        KeyType                    m_Key;
+    };
+
+    ValueHandle Get(const KeyType& Key) const
+    {
+        return m_pImpl->Get(Key);
+    }
+
+    template <typename... ArgsType>
+    ValueHandle GetOrInsert(const KeyType& Key, ArgsType&&... Args) const
+    {
+        return m_pImpl->GetOrInsert(Key, std::forward<ArgsType>(Args)...);
+    }
+
+private:
+    class Impl : public std::enable_shared_from_this<Impl>
+    {
+    public:
+        ValueHandle Get(const KeyType& Key)
+        {
+            std::lock_guard<std::mutex> Lock{m_Mtx};
+
+            auto it = m_Map.find(Key);
+            if (it != m_Map.end())
+            {
+                auto pValue = it->second.lock();
+                if (pValue)
+                {
+                    return ValueHandle{*this, std::move(pValue), Key};
+                }
+                else
+                {
+                    // Since ValueHandle::Release() resets the shared_ptr before calling Remove(),
+                    // we may find expired weak pointers in the map. Remove them.
+                    m_Map.erase(it);
+                }
+            }
+
+            return ValueHandle{};
+        }
+
+        template <typename... ArgsType>
+        ValueHandle GetOrInsert(const KeyType& Key, ArgsType&&... Args)
+        {
+            {
+                std::lock_guard<std::mutex> Lock{m_Mtx};
+
+                auto it = m_Map.find(Key);
+                if (it != m_Map.end())
+                {
+                    if (auto pValue = it->second.lock())
+                    {
+                        return ValueHandle{*this, std::move(pValue), Key};
+                    }
+                    else
+                    {
+                        // Since ValueHandle::Release() resets the shared_ptr before calling Remove(),
+                        // we may find expired weak pointers in the map. Remove them.
+                        m_Map.erase(it);
+                    }
+                }
+            }
+
+            // Create the new value outside of the lock
+            auto pNewValue = std::make_shared<ValueType>(std::forward<ArgsType>(Args)...);
+
+            std::lock_guard<std::mutex> Lock{m_Mtx};
+
+            // Check again in case another thread inserted the value while we were creating it
+            if (auto it = m_Map.find(Key); it != m_Map.end())
+            {
+                if (auto pValue = it->second.lock())
+                {
+                    // Discard the newly created value and use the one created by the other thread
+                    return ValueHandle{*this, std::move(pValue), Key};
+                }
+
+                // Replace the expired weak pointer with the newly created value
+                it->second = pNewValue;
+                return ValueHandle{*this, std::move(pNewValue), Key};
+            }
+
+            // Insert the new value
+            m_Map.emplace(Key, pNewValue);
+            return ValueHandle{*this, std::move(pNewValue), Key};
+        }
+
+        void Remove(const KeyType& Key)
+        {
+            std::lock_guard<std::mutex> Lock{m_Mtx};
+
+            auto Iter = m_Map.find(Key);
+            if (Iter == m_Map.end())
+            {
+                // If two ValueHandles are destroyed simultaneously from different threads,
+                // both may try to remove the same entry. In this case, just return.
+                return;
+            }
+
+            // If the weak pointer is not expired, it means that another ValueHandle instance exists,
+            // which will remove the entry when it is destroyed.
+            if (Iter->second.expired())
+            {
+                m_Map.erase(Iter);
+            }
+        }
+
+        ~Impl()
+        {
+            VERIFY(m_Map.empty(), "Map is not empty upon destruction. This should never happen because all entries should have been "
+                                  "removed by destructors of ValueHandle objects, and the map can't be destroyed while any ValueHandle "
+                                  "instances are alive.");
+        }
+
+    private:
+        std::mutex m_Mtx;
+        MapType    m_Map;
+    };
+    std::shared_ptr<Impl> m_pImpl = std::make_shared<Impl>();
+};
+
+} // namespace Diligent