weak_reference.h

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/*! \file
  \brief  weak reference and related classes

  \par Copyright Information

  Copyright (c) 2016 by The DiSTI Corporation.<br>
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  NO WARRANTY. THE SOFTWARE IS PROVIDED "AS-IS," WITHOUT WARRANTY OF ANY KIND,
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  LIMITATION OF LIABILITY. TO THE MAXIMUM EXTENT PERMITTED BY APPLICABLE LAW,
IN NO EVENT SHALL DISTI OR ITS SUPPLIERS BE LIABLE FOR ANY SPECIAL, INCIDENTAL,
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*/
#ifndef INCLUDED_WEAK_REFERENCE_H
#define INCLUDED_WEAK_REFERENCE_H

#include "disti_assert.h"
#include "scoped_ptr.h"

namespace disti
{
class WeakReferenceable;

/** Interface for holding a weak reference to an object.  This class should generally not be used directly.  Use WeakRef instead */
class WeakReference
{
public:
    /** Get a pointer to the object.  Returns NULL if the object was deleted */
    virtual WeakReferenceable* GetReferent() const = 0;

    /** Notify this weak reference that the object destroyed */
    virtual void NotifyReferentDestroyed() = 0;

    virtual ~WeakReference() {}
};

/** Interface for getting a weak reference to an object.  To implement this class, inherit from WeakReferenceableMixin (see DisplayObject as an example) */
class WeakReferenceable
{
public:
    /** Add a weak reference to this object, that gets notified when this object is destroyed.  Does not take ownership of weakRef.  
          * \param weakRef the weak reference to add.
          */
    virtual void AddWeakReference( WeakReference* weakRef ) = 0;

    /** Notify this object that its weak reference is being destroyed */
    virtual void NotifyWeakReferenceDestroyed( WeakReference* weakReference ) = 0;

    virtual ~WeakReferenceable() {}
};

/** helper class to simplify working with a weak reference 
      * Ex Usage:
      *   DisplayObject* obj = GetSomeDisplayObject();
      *   WeakRef< DisplayObject > weakRef = obj;
      *   ...
      *   ... some code that may or may not have deleted obj
      *   ...
      *   if( !weakRef.IsNull() )
      *   {
      *       weakRef.Get()->Location( 0, 0, 0 );
      *   }
      */
template<class T>
class WeakRef
{
private:
    /** Implementation for a weak reference */
    class WeakReferenceImpl : virtual public WeakReference
    {
    public:
        /** constructor
              * \param referent a pointer to the WeakReferenceable object
              */
        WeakReferenceImpl( WeakReferenceable* referent )
            : _referent( referent )
        {
            if( referent )
            {
                referent->AddWeakReference( this );
            }
        }

        /** destructor.  Notifies the WeakReferenceable that this WeakReference is being deleted */
        ~WeakReferenceImpl()
        {
            if( _referent )
            {
                _referent->NotifyWeakReferenceDestroyed( this );
            }
        }

        /** \see WeakReference */
        WeakReferenceable* GetReferent() const DISTI_METHOD_OVERRIDE { return _referent; }

        /** notifies the WeakReference that the WeakReferenceable object is being destroyed so it can set its referent to NULL */
        void NotifyReferentDestroyed()
        {
            _referent = NULL;
        }

    private:
        /** pointer to the WeakReferenceable object */
        WeakReferenceable* _referent;
    };

public:
    /** default ctor -- initializes weak reference to NULL
          */
    WeakRef()
    {
        GLS_ASSERT( IsNull() );
    }

    /** construct from a WeakReferenceable pointer
          * \param[in] weakReferenceable pointer to an WeakReferenceable interface ( can be NULL )
          */
    WeakRef( WeakReferenceable* weakReferenceable )
    {
        SetWeakReference( weakReferenceable );
    }

    /** construct from an WeakRef< T >
          * \param[in] w WeakRef< T > ( can be NULL )
          */
    WeakRef( const WeakRef& w )
    {
        SetWeakReference( w.GetWeakReferenceable() );
    }

    /** construct from an WeakRef< U >
          * \param[in] w WeakRef< U > ( can be NULL )
          */
    template<class U>
    WeakRef( const WeakRef<U>& w )
    {
        SetWeakReference( w.GetWeakReferenceable() );
    }

    /** assign this weak ref to reference the given WeakReferenceable*
          * \param weakReferenceable WeakReferenceable* in question (can be NULL)
          * \return a ref to this
          */
    WeakRef& operator=( WeakReferenceable* weakReferenceable )
    {
        SetWeakReference( weakReferenceable );
        return *this;
    }

    /** assign this pointer to point to the given WeakRef< T >
          * \param w WeakRef< T > in question (can be NULL)
          * \return a ref to this
          */
    WeakRef& operator=( const WeakRef& w )
    {
        SetWeakReference( w.GetWeakReferenceable() );
        return *this;
    }

    /** assign this pointer to point to the given WeakRef< U >
          * \param w WeakRef< U > in question (can be NULL)
          * \return a ref to this
          */
    template<class U>
    WeakRef& operator=( const WeakRef<U>& w )
    {
        SetWeakReference( w.GetWeakReferenceable() );
        return *this;
    }

    /** determine if weak reference is NULL
          * \return true if weak ref is NULL else false
          */
    bool IsNull() const
    {
        return ( _weakReference.IsNull() || NULL == _weakReference->GetReferent() );
    }

    /** Get a T*
          * \return if referent exists and supports interface T then return a pointer to T, else return NULL
          */
    T* Get() const
    {
        T* ret = NULL;

        // if there is still a weak ref, see if it has a referent that supports interface T
        if( !IsNull() )
        {
            ret = dynamic_cast<T*>( _weakReference->GetReferent() );
        }

        return ret;
    }

    /** get the WeakReferenceable* contained in this weak ref
          * \return the WeakReferenceable pointer contained in this weak ref ( can be NULL )
          */
    WeakReferenceable* GetWeakReferenceable() const
    {
        return !_weakReference.IsNull() ? _weakReference->GetReferent() : NULL;
    }

private:
    void SetWeakReference( WeakReferenceable* weakReferenceable )
    {
        WeakReference* weakRef = NULL;
        if( weakReferenceable )
        {
            // Important - this weak reference must be allocated in the same unit that will delete it, which is
            // why its only allocated here, right before we store it in the object that will delete it.
            weakRef = new WeakReferenceImpl( weakReferenceable );
            GLS_ASSERT( NULL != weakRef->GetReferent() );
        }

        _weakReference.Reset( weakRef );
    }

    ScopedPtr<WeakReference> _weakReference; /**< weak reference else NULL */
};

} // namespace disti

#endif