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Coin::ReferencedObject Class Reference

ReferencedObject class. More...

#include <CoinSmartPtr.hpp>

Public Member Functions

 ReferencedObject ()
 
virtual ~ReferencedObject ()
 
int ReferenceCount () const
 
void AddRef () const
 
void ReleaseRef () const
 

Private Attributes

int reference_count_
 

Detailed Description

ReferencedObject class.

This is part of the implementation of an intrusive smart pointer design. This class stores the reference count of all the smart pointers that currently reference it. See the documentation for the SmartPtr class for more details.

A SmartPtr behaves much like a raw pointer, but manages the lifetime of an object, deleting the object automatically. This class implements a reference-counting, intrusive smart pointer design, where all objects pointed to must inherit off of ReferencedObject, which stores the reference count. Although this is intrusive (native types and externally authored classes require wrappers to be referenced by smart pointers), it is a safer design. A more detailed discussion of these issues follows after the usage information.

Usage Example: Note: to use the SmartPtr, all objects to which you point MUST inherit off of ReferencedObject.

* 
* In MyClass.hpp...
* 
* #include "CoinSmartPtr.hpp"

* 
* class MyClass : public Coin::ReferencedObject // must derive from ReferencedObject
*    {
*      ...
*    }
* 
* In my_usage.cpp...
* 
* #include "CoinSmartPtr.hpp"
* #include "MyClass.hpp"
* 
* void func(AnyObject& obj)
*  {
*    Coin::SmartPtr<MyClass> ptr_to_myclass = new MyClass(...);
*    // ptr_to_myclass now points to a new MyClass,
*    // and the reference count is 1
*    
*    ...
* 
*    obj.SetMyClass(ptr_to_myclass);
*    // Here, let's assume that AnyObject uses a
*    // SmartPtr<MyClass> internally here.
*    // Now, both ptr_to_myclass and the internal
*    // SmartPtr in obj point to the same MyClass object
*    // and its reference count is 2.
* 
*    ...
* 
*    // No need to delete ptr_to_myclass, this
*    // will be done automatically when the
*    // reference count drops to zero.
* 
*  }   
*  
* 

Other Notes: The SmartPtr implements both dereference operators -> & *. The SmartPtr does NOT implement a conversion operator to the raw pointer. Use the GetRawPtr() method when this is necessary. Make sure that the raw pointer is NOT deleted. The SmartPtr implements the comparison operators == & != for a variety of types. Use these instead of

*    if (GetRawPtr(smrt_ptr) == ptr) // Don't use this
*    

SmartPtr's, as currently implemented, do NOT handle circular references. For example: consider a higher level object using SmartPtrs to point to A and B, but A and B also point to each other (i.e. A has a SmartPtr to B and B has a SmartPtr to A). In this scenario, when the higher level object is finished with A and B, their reference counts will never drop to zero (since they reference each other) and they will not be deleted. This can be detected by memory leak tools like valgrind. If the circular reference is necessary, the problem can be overcome by a number of techniques:

1) A and B can have a method that "releases" each other, that is they set their internal SmartPtrs to NULL.

*        void AClass::ReleaseCircularReferences()
*          {
*          smart_ptr_to_B = NULL;
*          }
*        

Then, the higher level class can call these methods before it is done using A & B.

2) Raw pointers can be used in A and B to reference each other. Here, an implicit assumption is made that the lifetime is controlled by the higher level object and that A and B will both exist in a controlled manner. Although this seems dangerous, in many situations, this type of referencing is very controlled and this is reasonably safe.

3) This SmartPtr class could be redesigned with the Weak/Strong design concept. Here, the SmartPtr is identified as being Strong (controls lifetime of the object) or Weak (merely referencing the object). The Strong SmartPtr increments (and decrements) the reference count in ReferencedObject but the Weak SmartPtr does not. In the example above, the higher level object would have Strong SmartPtrs to A and B, but A and B would have Weak SmartPtrs to each other. Then, when the higher level object was done with A and B, they would be deleted. The Weak SmartPtrs in A and B would not decrement the reference count and would, of course, not delete the object. This idea is very similar to item (2), where it is implied that the sequence of events is controlled such that A and B will not call anything using their pointers following the higher level delete (i.e. in their destructors!). This is somehow safer, however, because code can be written (however expensive) to perform run-time detection of this situation. For example, the ReferencedObject could store pointers to all Weak SmartPtrs that are referencing it and, in its destructor, tell these pointers that it is dying. They could then set themselves to NULL, or set an internal flag to detect usage past this point.

Comments on Non-Intrusive Design: In a non-intrusive design, the reference count is stored somewhere other than the object being referenced. This means, unless the reference counting pointer is the first referencer, it must get a pointer to the referenced object from another smart pointer (so it has access to the reference count location). In this non-intrusive design, if we are pointing to an object with a smart pointer (or a number of smart pointers), and we then give another smart pointer the address through a RAW pointer, we will have two independent, AND INCORRECT, reference counts. To avoid this pitfall, we use an intrusive reference counting technique where the reference count is stored in the object being referenced.

Definition at line 157 of file CoinSmartPtr.hpp.

Constructor & Destructor Documentation

Coin::ReferencedObject::ReferencedObject ( )
inline

Definition at line 159 of file CoinSmartPtr.hpp.

virtual Coin::ReferencedObject::~ReferencedObject ( )
inlinevirtual

Definition at line 160 of file CoinSmartPtr.hpp.

Member Function Documentation

int Coin::ReferencedObject::ReferenceCount ( ) const
inline

Definition at line 161 of file CoinSmartPtr.hpp.

void Coin::ReferencedObject::AddRef ( ) const
inline

Definition at line 162 of file CoinSmartPtr.hpp.

void Coin::ReferencedObject::ReleaseRef ( ) const
inline

Definition at line 163 of file CoinSmartPtr.hpp.

Member Data Documentation

int Coin::ReferencedObject::reference_count_
mutableprivate

Definition at line 166 of file CoinSmartPtr.hpp.


The documentation for this class was generated from the following file: