common/intrusive_ptr.h

   1 #ifndef INTRUSIVE_PTR_H
   2 #define INTRUSIVE_PTR_H
   3
   4 #include <atomic>
   5 #include <cstddef>
   6 #include <utility>
   7
   8 #include "atomic.h"
   9 #include "opthelpers.h"
  10
  11
  12 namespace al {
  13
  14 template<typename T>
  15 class intrusive_ref {
  16     std::atomic<unsigned int> mRef{1u};
  17
  18 public:
  19     unsigned int add_ref() noexcept { return IncrementRef(mRef); }
  20     unsigned int dec_ref() noexcept
  21     {
  22         auto ref = DecrementRef(mRef);
  23         if(ref == 0) UNLIKELY
  24             delete static_cast<T*>(this);
  25         return ref;
  26     }
  27
  28     /**
  29      * Release only if doing so would not bring the object to 0 references and
  30      * delete it. Returns false if the object could not be released.
  31      *
  32      * NOTE: The caller is responsible for handling a failed release, as it
  33      * means the object has no other references and needs to be be deleted
  34      * somehow.
  35      */
  36     bool releaseIfNoDelete() noexcept
  37     {
  38         auto val = mRef.load(std::memory_order_acquire);
  39         while(val > 1 && !mRef.compare_exchange_strong(val, val-1, std::memory_order_acq_rel))
  40         {
  41             /* val was updated with the current value on failure, so just try
  42              * again.
  43              */
  44         }
  45
  46         return val >= 2;
  47     }
  48 };
  49
  50
  51 template<typename T>
  52 class intrusive_ptr {
  53     T *mPtr{nullptr};
  54
  55 public:
  56     intrusive_ptr() noexcept = default;
  57     intrusive_ptr(const intrusive_ptr &rhs) noexcept : mPtr{rhs.mPtr}
  58     { if(mPtr) mPtr->add_ref(); }
  59     intrusive_ptr(intrusive_ptr&& rhs) noexcept : mPtr{rhs.mPtr}
  60     { rhs.mPtr = nullptr; }
  61     intrusive_ptr(std::nullptr_t) noexcept { }
  62     explicit intrusive_ptr(T *ptr) noexcept : mPtr{ptr} { }
  63     ~intrusive_ptr() { if(mPtr) mPtr->dec_ref(); }
  64
  65     /* NOLINTBEGIN(bugprone-unhandled-self-assignment)
  66      * Self-assignment is handled properly here.
  67      */
  68     intrusive_ptr& operator=(const intrusive_ptr &rhs) noexcept
  69     {
  70         static_assert(noexcept(std::declval<T*>()->dec_ref()), "dec_ref must be noexcept");
  71
  72         if(rhs.mPtr) rhs.mPtr->add_ref();
  73         if(mPtr) mPtr->dec_ref();
  74         mPtr = rhs.mPtr;
  75         return *this;
  76     }
  77     /* NOLINTEND(bugprone-unhandled-self-assignment) */
  78     intrusive_ptr& operator=(intrusive_ptr&& rhs) noexcept
  79     {
  80         if(&rhs != this) LIKELY
  81         {
  82             if(mPtr) mPtr->dec_ref();
  83             mPtr = std::exchange(rhs.mPtr, nullptr);
  84         }
  85         return *this;
  86     }
  87
  88     explicit operator bool() const noexcept { return mPtr != nullptr; }
  89
  90     [[nodiscard]] auto operator*() const noexcept -> T& { return *mPtr; }
  91     [[nodiscard]] auto operator->() const noexcept -> T* { return mPtr; }
  92     [[nodiscard]] auto get() const noexcept -> T* { return mPtr; }
  93
  94     void reset(T *ptr=nullptr) noexcept
  95     {
  96         if(mPtr)
  97             mPtr->dec_ref();
  98         mPtr = ptr;
  99     }
 100
 101     T* release() noexcept { return std::exchange(mPtr, nullptr); }
 102
 103     void swap(intrusive_ptr &rhs) noexcept { std::swap(mPtr, rhs.mPtr); }
 104     void swap(intrusive_ptr&& rhs) noexcept { std::swap(mPtr, rhs.mPtr); }
 105 };
 106
 107 } // namespace al
 108
 109 #endif /* INTRUSIVE_PTR_H */