C++ Smart pointer own implementation

Question

Let's say that you're on a system where you cant use TR1, boost, etc. You just have the straight-up standard library C++.

If you were stuck with this scenario, what's the simplest reference-counting smart-pointer you could use? (our system only has auto_ptr which is not overly useful).

I'm happy with a link to something somewhat well-established or a simple "it's too complicated to implement yourself" if that's the answer (which I'm half expecting :( ).

Can't you go to boost and copy and paste their smart pointer into your code base? — Loki Astari
– Loki Astari, Commented Aug 25, 2011 at 18:58
Is your application guaranteed to be single-threaded, for ever and eternity? — Kerrek SB
– Kerrek SB, Commented Aug 25, 2011 at 18:59
In which scenario can't you use boost? isnt't the shared_ptr header-only? — KillianDS
– KillianDS, Commented Aug 25, 2011 at 19:01
Yeah just write a script that lists all the dependencies for shared_ptr and be done with it. Though that obviously begs the question why the hell you can't use boost header files in the first place. — Voo
– Voo, Commented Aug 25, 2011 at 19:43

Matthew Flaschen · Accepted Answer · 2011-08-25 18:58:22Z

4

I would probably go with std::shared_ptr, which was approved in C++0x two weeks ago (so no TR1 needed). In all seriousness, I would recommend boost (or upgrading). You certainly can implement it yourself, but the cost-benefit doesn't make sense, since constructs like this are complicated to do right.

answered Aug 25, 2011 at 18:58

Matthew Flaschen

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legion · Accepted Answer · 2011-08-25 19:44:54Z

2

Well some time ago I just made one for studding interest. It's not so "smart" but anyway better then raw pointer.

class CReferenceCount
{
private:
    unsigned int m_count;

public:
   CReferenceCount() : m_count(0) { }
   virtual ~CReferenceCount() { }

   void increseRef()
   {
      ++m_count;
   }

   unsigned int decreseRef()
   {
      return --m_count;
   }
};

class CCustomDeleter
{
public:
  template<typename T>
  void operator()(const T* ptr) const
  {
      delete ptr; ptr = NULL;
  }

  void operator()(const char* ptr) const
  {
     delete[] ptr; ptr = NULL;
  }
};

template <typename T>
class CWrapPtr
{
private:
 void makeRefCountObj()
 {
    try
    {
        m_rcPtr = new CReferenceCount();
    }
    catch (std::bad_alloc &err)
    {
        std::cout<<"-- CWrapPtr : "<<err.what()<<std::endl;
        // should do something about failed CWrap object...
    }

    m_rcPtr->increseRef();
 }

 public:
  T *m_objPtr;
  CReferenceCount *m_rcPtr;


  CWrapPtr() : m_objPtr(NULL), m_rcPtr(NULL)
  {
     makeRefCountObj();
  }

  CWrapPtr(T *obj) : m_objPtr(obj), m_rcPtr(NULL)
  {
     makeRefCountObj();
  }

  virtual ~CWrapPtr()
  {
    if (m_rcPtr && m_rcPtr->decreseRef() == 0)
    {
        CCustomDeleter dd;
        dd(m_objPtr);

        delete m_rcPtr; m_rcPtr = NULL;
    }
  }

  CWrapPtr(const CWrapPtr<T> &other) : m_objPtr(other.m_objPtr),
                                     m_rcPtr(other.m_rcPtr)
  {
      m_rcPtr->increseRef();
  }


T& operator*()
{
    assert(m_objPtr != NULL);
    return *m_objPtr;
}

T* operator->()
{
    assert(m_objPtr != NULL);
    return m_objPtr;
}

CWrapPtr<T>& operator=(const CWrapPtr<T> &other)
{
    if (this != &other)
    {

        if (m_rcPtr && m_rcPtr->decreseRef() == 0)
        {
            CCustomDeleter dd;
            dd(m_objPtr);

            delete m_rcPtr; m_rcPtr = NULL;
        }

        m_objPtr = other.m_objPtr;
        m_rcPtr = other.m_rcPtr;

        m_rcPtr->increseRef();
    }

    return *this;
 }
};

And yeah it's just demo..

answered Aug 25, 2011 at 19:44

legion

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3 Comments

Jagannath Over a year ago

Do we need virtual destructor ? I would add "reset" as well. If not it's usage is a bit restricted IMO.

legion Over a year ago

@jagansai. Probably you are right. You know it's was only for learning purpose not for usage :)

John Humphreys Over a year ago

Well, its just a learning solution but I appreciate the effort. I found a nice experts-exchange article by evilrix @ experts-exchange.com/Programming/Languages/CPP/… with a very detailed smart pointer implementation thats like this if you're interested. Anyway I accepted :) it's a good starting place for a non-library smart pointer. Thanks!

Ajeet Ganga · Accepted Answer · 2011-08-25 19:47:22Z

If you are lucky, then you havent designed the classes that need shared pointer but dont have a readymade one. If you are lucky and your entire program will run in single thread ...

Then you have chance of having a really cheap shared pointer.

Use a base class SharedPtr. Derive all the object from that. Objects of type SharedPtr will contain some free-store memory for counting. When you are copying it, do increment When you are destroying it decrement and free the object if necessary. Et cetra et cetra of the shared pointer semantics.

Store pointers as SharedPtr. and do downcast whenever you need some operation.

I know this is cheapo solution, but... 1. Multithreaded will require locks. Costly + involved. 2. Template programming. Will probably require a half-day and another half day to debug and fix your issues, which you will get when you try to replicate shared_ptr from BOOST

If yo uhave just couple of classes whose existence is more important and not with hundreds of operations on them, then you can try this approach.

Btw, this is kind of "template-pattern"

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