1 files changed, 120 insertions, 0 deletions
diff --git a/common/intrusive_ptr.h b/common/intrusive_ptr.h
new file mode 100644
index 00000000..595c831d
--- /dev/null
+++ b/common/intrusive_ptr.h
@@ -0,0 +1,120 @@
+#ifndef INTRUSIVE_PTR_H
+#define INTRUSIVE_PTR_H
+
+#include "atomic.h"
+#include "opthelpers.h"
+
+
+namespace al {
+
+template<typename T>
+class intrusive_ref {
+    RefCount mRef{1u};
+
+public:
+    unsigned int add_ref() noexcept { return IncrementRef(mRef); }
+    unsigned int release() noexcept
+    {
+        auto ref = DecrementRef(mRef);
+        if UNLIKELY(ref == 0)
+            delete static_cast<T*>(this);
+        return ref;
+    }
+
+    /**
+     * Release only if doing so would not bring the object to 0 references and
+     * delete it. Returns false if the object could not be released.
+     *
+     * NOTE: The caller is responsible for handling a failed release, as it
+     * means the object has no other references and needs to be be deleted
+     * somehow.
+     */
+    bool releaseIfNoDelete() noexcept
+    {
+        auto val = mRef.load(std::memory_order_acquire);
+        while(val > 1 && !mRef.compare_exchange_strong(val, val-1, std::memory_order_acq_rel))
+        {
+            /* val was updated with the current value on failure, so just try
+             * again.
+             */
+        }
+
+        return val >= 2;
+    }
+};
+
+
+template<typename T>
+class intrusive_ptr {
+    T *mPtr{nullptr};
+
+public:
+    intrusive_ptr() noexcept = default;
+    intrusive_ptr(const intrusive_ptr &rhs) noexcept : mPtr{rhs.mPtr}
+    { if(mPtr) mPtr->add_ref(); }
+    intrusive_ptr(intrusive_ptr&& rhs) noexcept : mPtr{rhs.mPtr}
+    { rhs.mPtr = nullptr; }
+    intrusive_ptr(std::nullptr_t) noexcept { }
+    explicit intrusive_ptr(T *ptr) noexcept : mPtr{ptr} { }
+    ~intrusive_ptr() { if(mPtr) mPtr->release(); }
+
+    intrusive_ptr& operator=(const intrusive_ptr &rhs) noexcept
+    {
+        if(rhs.mPtr) rhs.mPtr->add_ref();
+        if(mPtr) mPtr->release();
+        mPtr = rhs.mPtr;
+        return *this;
+    }
+    intrusive_ptr& operator=(intrusive_ptr&& rhs) noexcept
+    {
+        if(mPtr)
+            mPtr->release();
+        mPtr = rhs.mPtr;
+        rhs.mPtr = nullptr;
+        return *this;
+    }
+
+    operator bool() const noexcept { return mPtr != nullptr; }
+
+    T& operator*() const noexcept { return *mPtr; }
+    T* operator->() const noexcept { return mPtr; }
+    T* get() const noexcept { return mPtr; }
+
+    void reset() noexcept
+    {
+        if(mPtr)
+            mPtr->release();
+        mPtr = nullptr;
+    }
+
+    T* release() noexcept
+    {
+        T *ret{mPtr};
+        mPtr = nullptr;
+        return ret;
+    }
+
+    void swap(intrusive_ptr &rhs) noexcept { std::swap(mPtr, rhs.mPtr); }
+    void swap(intrusive_ptr&& rhs) noexcept { std::swap(mPtr, rhs.mPtr); }
+};
+
+#define AL_DECL_OP(op)                                                        \
+template<typename T>                                                          \
+inline bool operator op(const intrusive_ptr<T> &lhs, const T *rhs) noexcept   \
+{ return lhs.get() op rhs; }                                                  \
+template<typename T>                                                          \
+inline bool operator op(const T *lhs, const intrusive_ptr<T> &rhs) noexcept   \
+{ return lhs op rhs.get(); }
+
+AL_DECL_OP(==)
+AL_DECL_OP(!=)
+AL_DECL_OP(<=)
+AL_DECL_OP(>=)
+AL_DECL_OP(<)
+AL_DECL_OP(>)
+
+#undef AL_DECL_OP
+
+} // namespace al
+
+#endif /* INTRUSIVE_PTR_H */