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Tweet Automatic global object iterator

2012-05-29

Have you ever had following dillema: how to iterate through all existing instances of given class? I had. And the solution is really easy. For example, when resetting DirectX 9 device you may want to broadcast "after/before reset" signals to all rendertargets and dynamic buffers, so they may properly destroy and recreate. Also in my framework I'm using a lot of global objects (always keep things simple!) that represent rendertargets, shaders and other stuff and these objects want to know when device is created so they may create themselves. Usually I ended making singleton manager that keeps track of these objects, but you can automate and generalize it a bit using templates...

The code below makes this bit of magic true. :)

template<class T>
class Tracked {
public:
 
    class iterator {
    public:
 
        iterator() : _ptr(0) {}
 
        T &operator *()  const { return *_ptr; }
        T *operator ->() const { return _ptr; }
 
        iterator &operator ++() { _ptr = _ptr->_next; return *this; }
        iterator &operator --() { _ptr = (_ptr ? _ptr->_prev : T::Tail()); return *this; }
 
        iterator operator ++(int) { iterator pp(_ptr); _ptr = _ptr->_next; return pp; }
        iterator operator --(int) { iterator pp(_ptr); _ptr = (_ptr ? _ptr->_prev : T::Tail()); return pp; }
 
        bool operator ==(const iterator &i) const { return _ptr == i._ptr; }
        bool operator !=(const iterator &i) const { return _ptr != i._ptr; }
 
    private:
        friend class Tracked;
 
        T *_ptr;
 
        iterator(T *p) : _ptr(p) {}
    };
 
    static iterator begin() { return iterator(_head()); }
    static iterator end()   { return iterator(); }
 
 
protected:
    Tracked()                                       { _register(); }
    Tracked(const Tracked<T> &t)                   { _register(); }
    Tracked<T> &operator =(const Tracked<T> &t)   {}
 
    ~Tracked() { _unregister(); }
 
private:
    friend class Tracked::iterator;
 
    T  *_prev;
    T  *_next;
 
    void _register()
    {
        _prev = Tail();
        _next = 0;
        Tail() = (T*)this;
        if(_prev)   _prev->_next = (T*)this;
        else        _head() = (T*)this;
    }
 
    void _unregister()
    {
        if(_prev) _prev->_next = _next;
        else      _head() = _next;
        if(_next) _next->_prev = _prev;
        else      Tail() = _prev;
    }
 
    static T *&_head()
    {
        static T *head = 0;
        return head;
    }
 
    static T *&Tail()
    {
        static T *tail = 0;
        return tail;
    }
};

Now, you can derive any class from this template to make instances of this class iterable.

class MyClass : public Tracked<MyClass> {
  ...
};

And whenever you want to iterate all live objects (including derived ones), you can:

for( MyClass::iterator p = MyClass::begin(); p != MyClass::end(); ++p )
    p->DoSomething();

Or just use one of algorithms from STL.

Simple and effective. :)

Links:

Comments for this article

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