common/intrusive_ptr.h

   1 #ifndef INTRUSIVE_PTR_H
   2 #define INTRUSIVE_PTR_H
   3
   4 #include <utility>
   5
   6 #include "atomic.h"
   7 #include "opthelpers.h"
   8
   9
  10 namespace al {
  11
  12 template<typename T>
  13 class intrusive_ref {
  14     RefCount mRef{1u};
  15
  16 public:
  17     unsigned int add_ref() noexcept { return IncrementRef(mRef); }
  18     unsigned int dec_ref() noexcept
  19     {
  20         auto ref = DecrementRef(mRef);
  21         if(ref == 0) [[unlikely]]
  22             delete static_cast<T*>(this);
  23         return ref;
  24     }
  25
  26     /**
  27      * Release only if doing so would not bring the object to 0 references and
  28      * delete it. Returns false if the object could not be released.
  29      *
  30      * NOTE: The caller is responsible for handling a failed release, as it
  31      * means the object has no other references and needs to be be deleted
  32      * somehow.
  33      */
  34     bool releaseIfNoDelete() noexcept
  35     {
  36         auto val = mRef.load(std::memory_order_acquire);
  37         while(val > 1 && !mRef.compare_exchange_strong(val, val-1, std::memory_order_acq_rel))
  38         {
  39             /* val was updated with the current value on failure, so just try
  40              * again.
  41              */
  42         }
  43
  44         return val >= 2;
  45     }
  46 };
  47
  48
  49 template<typename T>
  50 class intrusive_ptr {
  51     T *mPtr{nullptr};
  52
  53 public:
  54     intrusive_ptr() noexcept = default;
  55     intrusive_ptr(const intrusive_ptr &rhs) noexcept : mPtr{rhs.mPtr}
  56     { if(mPtr) mPtr->add_ref(); }
  57     intrusive_ptr(intrusive_ptr&& rhs) noexcept : mPtr{rhs.mPtr}
  58     { rhs.mPtr = nullptr; }
  59     intrusive_ptr(std::nullptr_t) noexcept { }
  60     explicit intrusive_ptr(T *ptr) noexcept : mPtr{ptr} { }
  61     ~intrusive_ptr() { if(mPtr) mPtr->dec_ref(); }
  62
  63     intrusive_ptr& operator=(const intrusive_ptr &rhs) noexcept
  64     {
  65         static_assert(noexcept(std::declval<T*>()->dec_ref()), "dec_ref must be noexcept");
  66
  67         if(rhs.mPtr) rhs.mPtr->add_ref();
  68         if(mPtr) mPtr->dec_ref();
  69         mPtr = rhs.mPtr;
  70         return *this;
  71     }
  72     intrusive_ptr& operator=(intrusive_ptr&& rhs) noexcept
  73     {
  74         if(&rhs != this) [[likely]]
  75         {
  76             if(mPtr) mPtr->dec_ref();
  77             mPtr = std::exchange(rhs.mPtr, nullptr);
  78         }
  79         return *this;
  80     }
  81
  82     explicit operator bool() const noexcept { return mPtr != nullptr; }
  83
  84     T& operator*() const noexcept { return *mPtr; }
  85     T* operator->() const noexcept { return mPtr; }
  86     T* get() const noexcept { return mPtr; }
  87
  88     void reset(T *ptr=nullptr) noexcept
  89     {
  90         if(mPtr)
  91             mPtr->dec_ref();
  92         mPtr = ptr;
  93     }
  94
  95     T* release() noexcept { return std::exchange(mPtr, nullptr); }
  96
  97     void swap(intrusive_ptr &rhs) noexcept { std::swap(mPtr, rhs.mPtr); }
  98     void swap(intrusive_ptr&& rhs) noexcept { std::swap(mPtr, rhs.mPtr); }
  99 };
 100
 101 #define AL_DECL_OP(op)                                                        \
 102 template<typename T>                                                          \
 103 inline bool operator op(const intrusive_ptr<T> &lhs, const T *rhs) noexcept   \
 104 { return lhs.get() op rhs; }                                                  \
 105 template<typename T>                                                          \
 106 inline bool operator op(const T *lhs, const intrusive_ptr<T> &rhs) noexcept   \
 107 { return lhs op rhs.get(); }
 108
 109 AL_DECL_OP(==)
 110 AL_DECL_OP(!=)
 111 AL_DECL_OP(<=)
 112 AL_DECL_OP(>=)
 113 AL_DECL_OP(<)
 114 AL_DECL_OP(>)
 115
 116 #undef AL_DECL_OP
 117
 118 } // namespace al
 119
 120 #endif /* INTRUSIVE_PTR_H */