E.9 Shared and SimpleShared

IceUtil::Shared and IceUtil::SimpleShared are base classes that implement the reference-counting mechanism for smart pointers (see Section 6.14.6). The two classes provide identical interfaces; the difference between Shared and SimpleShared is that SimpleShared is not thread-safe and, therefore, can only be used if the corresponding class instances are accessed only by a single thread. (SimpleShared is marginally faster than Shared because it avoids the locking overhead that is incurred by Shared.)

The interface of Shared looks as follows. (Because SimpleShared has the same interface, we do not show it separately here.)

class Shared {
public:
    Shared();
    Shared(const Shared&);
    virtual ~Shared();

    Shared& operator=(const Shared&);

    virtual void __incRef();
    virtual void __decRef();
    virtual int __getRef() const;
    virtual void __setNoDelete(bool);
};

The class maintains a reference that is initialized to zero by the constructor. __incRef increments the reference count and __decRef decrements it. If, during a call to __decRef, after decrementing the reference count, the reference count drops to zero, __decRef calls delete this, which causes the corresponding class instance to delete itself. The copy constructor increments the reference count of the copied instance, and the assignment operator increments the reference count of the source and decrements the reference count of the target.

The __getRef member function returns the value of the reference count and is useful mainly for debugging.

The __setNoDelete member function can be used to temporarily disable self-deletion and re-enable it again. This is useful mainly if you initialize a smart pointer with the this pointer of a class instance during construction—see page 254 for a detailed explanation.

To create a class that is reference-counted, you simply derive the class from Shared and define a smart pointer type for the class, for example:

class MyClass : public IceUtil::Shared {
    // ...
};

typedef IceUtil::Handle<MyClass> MyClassPtr;