refBase.h

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//
// Copyright 2016 Pixar
//
// Licensed under the terms set forth in the LICENSE.txt file available at
// https://openusd.org/license.
//
#ifndef PXR_BASE_TF_REF_BASE_H
#define PXR_BASE_TF_REF_BASE_H
 
 
#include "pxr/pxr.h"
 
#include "pxr/base/tf/api.h"
 
#include <atomic>
#include <cmath>
 
PXR_NAMESPACE_OPEN_SCOPE
 
template <class T> class TfRefPtr;
template <class T> class TfWeakPtr;
 
class TfRefBase {
public:
 
    typedef void (*UniqueChangedFuncPtr)(TfRefBase const *, bool);
    struct UniqueChangedListener {
        void (*lock)();
        UniqueChangedFuncPtr func;
        void (*unlock)();
    };
 
    // This mimics the old TfRefCount's default ctor behavior, which set
    // _refCount to 1.
    TfRefBase() : _refCount(1) {}
 
    // This mimics the old TfRefCount's copy ctor behavior, which set _refCount
    // to 1 on copy construction.
    TfRefBase(TfRefBase const &) : _refCount(1) {}
 
    // This mimics the old TfRefCount's copy assignment behavior, which took no
    // action.
    TfRefBase &operator=(TfRefBase const &) {
        return *this;
    }
 
    size_t GetCurrentCount() const {
        // Return the absolute value since the sign encodes whether or not this
        // TfRefBase invokes the UniqueChangedListener.
        return std::abs(_refCount.load(std::memory_order_relaxed));
    }
 
    bool IsUnique() const {
        return GetCurrentCount() == 1;
    }
 
    void SetShouldInvokeUniqueChangedListener(bool shouldCall) {
        int curValue = _refCount.load(std::memory_order_relaxed);
        while ((curValue > 0 && shouldCall) ||
               (curValue < 0 && !shouldCall)) {
            if (_refCount.compare_exchange_weak(curValue, -curValue)) {
                return;
            }                    
        }
    }
 
    TF_API static void SetUniqueChangedListener(UniqueChangedListener listener);
 
protected:
    /*
     * Prohibit deletion through a TfRefBase pointer.
     */
    TF_API virtual ~TfRefBase();
 
private:
    // For TfRefPtr's use.
    std::atomic_int &_GetRefCount() const {
        return _refCount;
    }
    
    // Note! Counts can be both positive or negative.  Negative counts indicate
    // that we must invoke the _uniqueChangedListener if the count goes 1 -> 2
    // or 2 -> 1 (which is really -1 -> -2 or -2 -> -1).
    mutable std::atomic_int _refCount;
 
    static UniqueChangedListener _uniqueChangedListener;
    template <typename T> friend class TfRefPtr;
    friend struct Tf_RefPtr_UniqueChangedCounter;
    friend struct Tf_RefPtr_Counter;
 
    template <typename T> friend TfRefPtr<T>
    TfCreateRefPtrFromProtectedWeakPtr(TfWeakPtr<T> const &);
};
 
class TfSimpleRefBase : public TfRefBase {
public:
    TF_API virtual ~TfSimpleRefBase();
};
 
PXR_NAMESPACE_CLOSE_SCOPE
 
#endif // PXR_BASE_TF_REF_BASE_H