Add ReferenceCountedThreadSafe<T>.

CesiumUtility/include/CesiumUtility/ReferenceCountedThreadSafe.h

@@ -0,0 +1,52 @@
+#pragma once
+
+#include <atomic>
+#include <cstdint>
+
+namespace CesiumUtility {
+
+/**
+ * @brief A reference-counted base class, meant to be used with
+ * {@link IntrusivePointer}. The reference count is thread-safe, so references
+ * may be added and removed from any thread at any time. The object will be
+ * destroyed in the thread that releases the last reference.
+ *
+ * @tparam T The type that is _deriving_ from this class. For example, you
+ * should declare your class as
+ * `class MyClass : public ReferenceCountedThreadSafe<MyClass> { ... };`
+ */
+template <typename T> class ReferenceCountedThreadSafe {
+public:
+  /**
+   * @brief Adds a counted reference to this object. Use
+   * {@link CesiumUtility::IntrusivePointer} instead of calling this method
+   * directly.
+   */
+  void addReference() const noexcept { ++this->_referenceCount; }
+
+  /**
+   * @brief Removes a counted reference from this object. When the last
+   * reference is removed, this method will delete this instance. Use
+   * {@link CesiumUtility::IntrusivePointer} instead of calling this method
+   * directly.
+   */
+  void releaseReference() const noexcept {
+    assert(this->_referenceCount > 0);
+    const int32_t references = --this->_referenceCount;
+    if (references == 0) {
+      delete static_cast<const T*>(this);
+    }
+  }
+
+  /**
+   * @brief Returns the current reference count of this instance.
+   */
+  std::int32_t getReferenceCount() const noexcept {
+    return this->_referenceCount;
+  }
+
+private:
+  mutable std::atomic<std::int32_t> _referenceCount{0};
+};
+
+} // namespace CesiumUtility