src/butil/shared_object.h

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// to you under the Apache License, Version 2.0 (the
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// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
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#ifndef BUTIL_SHARED_OBJECT_H
#define BUTIL_SHARED_OBJECT_H

#include "butil/intrusive_ptr.hpp"                   // butil::intrusive_ptr
#include "butil/atomicops.h"


namespace butil {

// Inherit this class to be intrusively shared. Comparing to shared_ptr,
// intrusive_ptr saves one malloc (for shared_count) and gets better cache
// locality when the ref/deref are frequent, in the cost of lack of weak_ptr
// and worse interface.
class SharedObject {
friend void intrusive_ptr_add_ref(SharedObject*);
friend void intrusive_ptr_release(SharedObject*);

public:
    SharedObject() : _nref(0) { }
    int ref_count() const { return _nref.load(butil::memory_order_relaxed); }
    
    // Add ref and returns the ref_count seen before added.
    // The effect is basically same as butil::intrusive_ptr<T>(obj).detach()
    // except that the latter one does not return the seen ref_count which is
    // useful in some scenarios.
    int AddRefManually()
    { return _nref.fetch_add(1, butil::memory_order_relaxed); }

    // Remove one ref, if the ref_count hit zero, delete this object.
    // Same as butil::intrusive_ptr<T>(obj, false).reset(NULL)
    void RemoveRefManually() {
        if (_nref.fetch_sub(1, butil::memory_order_release) == 1) {
            butil::atomic_thread_fence(butil::memory_order_acquire);
            delete this;
        }
    }

protected:
    virtual ~SharedObject() { }
private:
    butil::atomic<int> _nref;
};

inline void intrusive_ptr_add_ref(SharedObject* obj) {
    obj->AddRefManually();
}

inline void intrusive_ptr_release(SharedObject* obj) {
    obj->RemoveRefManually();
}

} // namespace butil


#endif // BUTIL_SHARED_OBJECT_H