I am implementing standart template tree structure, and I came across small problem.
Idea that each node will hold pointer to some data. Later in order to delete element correctly I need to know if its a single pointer or pointer to an array.
Inside of my tree->ReleaseMemory() method I have this code:
if (node->_data != NULL) {
switch (_destructionMethod){
case tree_delete:
delete node->_data; break;
case tree_deleteList:
delete [] node->_data; break;
case tree_free:
free(node->_data); break;
}
}
where _destructionMethod has been set during initialisation of the node.
Is there any way I can choose correct destructor without predefining it in special variable for it during initialisation ?
Thanks!
First basic:
delete is used when you allocate memory with new:
int *p = new int;
int *a = new int[10];
//...
delete p;
delete []a; //note the difference!
free is used when you allocate memory with malloc:
int *p = (int*)malloc(sizeof(int) * 1);
int *a = (int*)malloc(sizeof(int) * 10);
//...
free(p);
free(a); //no difference!
Now your problem:
Is there any way I can choose correct destructor without predefining it in special variable for it during initialisation
Consider policy-based design. That means, write allocator that will encapsulate allocation and deallocation in a class and use that consistently in your code.
No, there is no portable way to find out what allocator a particular pointer originally came from.
There's no way to interrogate a pointer to find out how it was allocated, but a common idiom is to give the allocated object itself responsibility for its own destruction. It looks like your object types are not all class types, though, so you would need to wrap them in order to do this. For example:
class destroyable_node
{
virtual void destroy() = 0;
};
template <typename T> class object_node : public destroyable_node
{
private:
T * value_;
public:
// Presumes T is copy-constructable.
object_node(T value) : value_( new T(value) ) {}
operator T&() {return value_;}
operator T const &() const {return value_;}
void destroy() {delete value_;}
};
template<typename T> class array_node : public destroyable_node
{
private:
T * value_;
public:
array_node(T[] value)
: value_( new T[ sizeof(value)/sizeof(T) ] )
{
std::copy(value, value + sizeof(value)/sizeof(T), value_);
}
operator T*() {return value_;}
operator T const *() const {return value_;}
void destroy() {delete[] value_;}
};
...and so on.
Don't do this at all. Use a smart pointer, like shared_ptr from C++0x or Boost or if that is not an option, auto_ptr from C++. If you could have more than one object, consider using std::vector.
Manual resource management is messy and difficult to get right.
Maybe a better design is to implement an abstract interface used by your container with three concrete subclasses that know what kind of thing they hold pointers to. Your container would simply call a destroy() method in the base class and let the derived classes worry about calling the correct destructor.
Related
In book C++ Primer 13.5.1, it implement a Smart Pointer Class using a Use-Count Class. Their implementation is as follows:
Use-Count Class
// private class for use by HasPtr only
class U_Ptr {
friend class HasPtr;
int *ip;
size_t use;
U_Ptr(int *p): ip(p), use(1) { }
~U_Ptr() { delete ip; }
};
Smart Pointer Class
/*
smart pointer class: takes ownership of the dynamically allocated
object to which it is bound
User code must dynamically allocate an object to initialize a HasPtr
and must not delete that object; the HasPtr class will delete it
*/
class HasPtr {
public:
// HasPtr owns the pointer; p must have been dynamically allocated
HasPtr(int *p, int i)
: ptr(new U_Ptr(p)), val(i) { }
// copy members and increment the use count
HasPtr(const HasPtr &orig)
: ptr(orig.ptr), val(orig.val) { ++ptr->use; }
HasPtr& operator=(const HasPtr&);
// if use count goes to zero, delete the U_Ptr object
~HasPtr() { if (--ptr->use == 0) delete ptr; }
friend ostream& operator<<(ostream&, const HasPtr&);
// copy control and constructors as before
// accessors must change to fetch value from U_Ptr object
int *get_ptr() const { return ptr->ip; }
int get_int() const { return val; }
// change the appropriate data member
void set_ptr(int *p) { ptr->ip = p; }
void set_int(int i) { val = i; }
// return or change the value pointed to, so ok for const objects
// Note: *ptr->ip is equivalent to *(ptr->ip)
int get_ptr_val() const { return *ptr->ip; }
void set_ptr_val(int i) { *ptr->ip = i; }
private:
U_Ptr *ptr; // points to use-counted U_Ptr class
int val;
};
Wonder: I am curious about why not simply using a int * to act like the Use-Count Class, just like the int* countPtr; used in the following new Smart Pointer Class:
class T
{
private:
int* countPtr; //
int* p;
int val;
public:
T(){
p = new int();
countPtr = new int();
*countPtr = 1;
val = 0;
}
T(T& t){
p = t.p;
countPtr = t.countPtr;
val = t.val;
*countPtr += 1;
}
T& operator = ( const T& rT){
if(*countPtr>1){
*countPtr -= 1;
}
else{
delete p;
delete countPtr;
}
p = rT.p;
countPtr = rT.countPtr;
val = rT.val;
*countPtr += 1;
return *this;
}
~T(){
if(*countPtr>1){
*countPtr -= 1;
}
else{
delete p;
delete countPtr;
}
}
int *get_ptr() const { return p; }
int get_int() const { return val; }
// change the appropriate data member
void set_ptr(int *ptr) { p = ptr; }
void set_int(int i) { val = i; }
};
Test: I tested the above Smart Pointer Class using code like the following and it seems working well.
int main()
{
T t1;
T t2(t1);
T t3(t1);
T t4;
t4 = t1;
return 0;
}
Real question: Is this new Smart Pointer Class with simply a int *countPtr sufficient enough? If yes, why bother to use an extra Use-Count Class like in the book? If no, what do I miss?
One property of the original implementation is that the delete is performed, in the control block object, with the original pointer type. This is a partial type erasure. No matter how much the smart pointer objects are copied, with somewhat different types, the original control block remains the same, with delete via the original pointer type.
However, since the original code you show is not templated, one must assume that it is an early example, followed later by similar templated code.
Converting a pointer up in a base class hierarchy, as can happen with copying of a smart pointer, means that delete on the new pointer type is only valid if the statically known new type has a virtual destructor.
For example, std::shared_ptr also deletes (guaranteed) via the original pointer type, unless one explicitly supplies a deleter functor that does something else.
My guess is that the author - whether consciously or subconsciously - is aware that having a separate class is useful in real-world smart pointers, e.g.:
a count of weak pointers (not sure if you'll have heard of them yet - they track an object without extending its lifetime, such that you can try to convert one into a (normal) shared pointer later, but it only works if there's at least one shared pointer to the object around to have kept it alive)
a mutex to make the shared pointer thread safe (though atomic operations may be better when available),
debugging information (e.g. boost::shared_ptr has a #ifdef to include an shared counter id)
virtual dispatch table, used by e.g. boost shared pointers to dispatch to OS-appropriate code (see boost/smart_ptr/detail/sp_counted_base_*.hpp headers)
I don't know the book, but perhaps they'll go on to explain what else might go into U_Ptr....
Your code is equivalent to the standard code reported by the book. However it is worst in some respects:
you need two allocations/deallocations instead of one (two ints instead of a single object). This might be slower and a little bit more difficult to manage.
you have a copy of the pointer duplicated in every object. So: duplicated information which you should guarantee to keep valid.
your object is larger (two pointers instead of one)
You only have one positive note:
the access to the pointer is direct instead of having one indirection. This could mean that the access to the referred object is slightly faster with your implementation...
(Updated 14.4.2017)
I by myself tried out unique_ptr and shared_ptr and was bit surprised to see that those classes does not make your life easier. I had function in one API where function would pick up Object*& - fill it out (pointer) and after that you need to delete that object. It's possible to use c++11, but you need to add extra temporary pointer for that purpose. (So use of *_ptr classes does not makes my life easier)
How complex would it be to implement smart pointer ?
Just by quickly glancing auto_ptr class implementation, I've quickly coded simple smart point container class, but then I've noticed that I need to support multiple smart pointers referencing the same object pointer.
Ok, so how complex could it be to code reference counting - I through, and went to google - and found one interesting article about it:
http://www.codingwisdom.com/codingwisdom/2012/09/reference-counted-smart-pointers-are-for-retards.html
Somehow I tend to agree with author of that article and comments written in that article that reference counting makes life even more complex, but still trying to stick to plain C also sounds bit dumb.
I'll now add code snippet here of my own class, and if you want to get newest version, you can check in this svn repository: https://sourceforge.net/p/testcppreflect/code/HEAD/tree/SmartPtr.h
Below is older version.
#pragma once
//
// If you're using multithreading, please make sure that two threads are not accessing
// SmartPtr<> pointers which are cross linked.
//
template <class T>
class SmartPtr
{
public:
SmartPtr() : ptr( nullptr ), next( nullptr )
{
}
SmartPtr( T* pt ) : ptr( pt ), next( nullptr )
{
}
SmartPtr( SmartPtr<T>& sp ) : ptr( nullptr ), next( nullptr )
{
operator=(sp);
}
~SmartPtr()
{
release();
}
// Reference to pointer - assumed to be filled out by user.
T*& refptr()
{
release();
return ptr;
}
// Pointer itself, assumed to be used.
T* get()
{
return ptr;
}
T* operator->() const
{
return ptr;
}
T* operator=( T* _ptr )
{
release();
ptr = _ptr;
return ptr;
}
SmartPtr<T>& operator=( SmartPtr<T>& sp )
{
release();
ptr = sp.ptr;
if ( ptr ) // If we have valid pointer, share ownership.
{
if( sp.next == nullptr )
{
next = &sp;
sp.next = this;
} else {
SmartPtr<T>* it = &sp;
while( it->next != &sp )
it = it->next;
next = &sp;
it->next = this;
}
}
return *this;
}
void release()
{
if ( !ptr )
return;
// Shared ownership.
if( next != nullptr )
{
// Remove myself from shared pointer list.
SmartPtr<T>* it = next;
while( it->next != this )
it = it->next;
if( it == it->next->next )
it->next = nullptr;
else
it->next = next;
next = nullptr;
ptr = nullptr;
return;
}
// Single user.
delete ptr;
ptr = nullptr;
}
T* ptr; // pointer to object
SmartPtr<T>* next; // nullptr if pointer is not shared with anyone,
// otherwise cyclic linked list of all SmartPtr referencing that pointer.
};
I've replaced reference counting with simple linked list - linked list keeps all class instances referenced, each destructor removes one reference away.
I've decided to rename operator* to refptr() function just to avoid developers writing extra fancy code. ("C++ jewels")
So in general I agree with article above - please don't make smart pointers too smart. :-)
I'm free to any improvement suggestions concerning this class and potential bugfixes.
And I wanted also to answer to original author's questions:
Real question: Is this new Smart Pointer Class with simply a int *countPtr
sufficient enough? If yes, why bother to use an extra Use-Count Class
like in the book? If no, what do I miss?
You're using separate mechanics for count management, like article link above mentions - it will becomes non-trivial to follow and debug reference counting. In my code snippet I use linked list of smart pointer instances, which does not perform any allocation ( so implementation above is faster than any other existing smart pointer implementation ), also it easier debugging of smart pointer itself - you can check by link (next) who locks down your memory from being collected.
But in overall - if you are experiencing memory leaks, I would say that it's highly non-trivial to find where they are if you're not originally made that code. Smart class pointer does not help in that sense to figure out who and how much has leaked out. Better to code once and properly that to fight with your own beast later on.
For memory leaks I recommend to find existing tools and use them - for example this one:
https://sourceforge.net/projects/diagnostic/
(There are plenty of them, but none of them works reliably/good enough).
I know that you're eager to put dislike to this implementation, but really - please tell me what obstacles you see in this implementation ?!
I have a simple C++ class for which I need to know whether an object should be delete'd or not at a certain point in my program. The mechanism should be portable across platforms and modern C++ compilers.
One way of doing it I can think of is: have a member field which is not initialized by the constructor but instead is assigned by the overloaded operator new, like so:
class message
{
protected:
int id;
bool dynamic;
public:
message(int _id): id(_id)
{
// don't touch `dynamic` in the constructor
}
void* operator new(size_t size)
{
message* m = (message*)::operator new(size);
m->dynamic = true;
return m;
}
void operator delete(void* m)
{
if (((message*)m)->dynamic)
::operator delete(m);
}
};
Apart form that it "feels" wrong, what is wrong with this method?
Edit: should have mentioned that the object is either dynamic or static (and never stack-local) and thus is guaranteed to be either zeroed or initialized with new.
The constructor needs to set dynamic to false, and then instead of overriding new, you need a static method like:
static message *createMessage(int _id)
{
message *ret = new message(_id);
ret->dynamic = true;
return ret;
}
And then call that method instead of newing a message.
Don’t do this. Apart from the fact that it won’t work, an object shouldn’t be managing anything about its own lifetime. You can use a unique_ptr or shared_ptr with a custom deleter, and if the object is stack-allocated, you know at its allocation site; in that case, you can supply a no-op deleter such as the following:
struct null_deleter {
template<class T>
void operator()(const T*) const {}
};
How the push_back of stl::vector is implemented so it can make copy of any datatype .. may be pointer, double pointer and so on ...
I'm implementing a template class having a function push_back almost similar to vector. Within this method a copy of argument should be inserted in internal allocated memory.
In case the argument is a pointer or a chain of pointers (an object pointer); the copy should be made of actual data pointed. [updated as per comment]
Can you pls tell how to create copy from pointer. so that if i delete the pointer in caller still the copy exists in my template class?
Code base is as follows:
template<typename T>
class Vector
{
public:
void push_back(const T& val_in)
{
T a (val_in); // It copies pointer, NOT data.
m_pData[SIZE++] = a;
}
}
Caller:
// Initialize my custom Vector class.
Vector<MyClass*> v(3);
MyClass* a = new MyClass();
a->a = 0;
a->b = .5;
// push MyClass object pointer
// now push_back method should create a copy of data
// pointed by 'a' and insert it to internal allocated memory.
// 'a' can be a chain of pointers also.
// how to achieve this functionality?
v.push_back(a);
delete a;
I can simply use STL vector to accomplish the tasks but for experiment purposes i'm writing a template class which does exactly the same.
Thanks.
if you have polymorphic object ( the pointed object may be more specialized than the variable ), I suggest you creating a virtual method called clone() that allocate a new pointer with a copy of your object:
Base* A::clone() {
A* toReturn = new A();
//copy stuff
return toReturn;
}
If you can't modify your Base class, you can use RTTI, but I will not approach this solution in this answer. ( If you want more details in this solution, please make a question regarding polymorphic cloning with RTTI).
If you have not a polymorphic object, you may allocate a new object by calling the copy constructor.
void YourVector::push_back(Base* obj) {
Base* copy = new Base(obj);
}
But it smells that what you are really needing is shared_ptr, avaliable in <tr1/memory> ( or <memory> if you use C++0x ).
Update based on comments
You may also have a two template parameters list:
template <typename T>
struct CopyConstructorCloner {
T* operator()(const T& t) {
return new T(t);
}
}
template <typename T, typename CLONER=CopyConstructorCloner<T> >
class MyList {
CLONER cloneObj;
public:
// ...
void push_back(const T& t) {
T* newElement = cloneObj(t);
// save newElemenet somewhere, dont forget to delete it later
}
}
With this approach it is possible to define new cloning politics for things like pointers.
Still, I recommend you to use shared_ptrs.
I think for this kind of problems it is better to use smart pointers ex: boost::shared_ptr or any other equivalent implementation.
There is no need to call new for the given datatype T. The push_back implementation should (must) call the copy-constructor or the assignment operator. The memory should have been allocated to hold those elemnets that are being pushed. The intial memory allocation should not call CTOR of type T. Something like:
T* pArray;
pArray = (T*) new BYTE[sizeof(T) * INITIAL_SIZE);
And then just put new object into pArray, calling the assignment operator.
One solution is to make a copy construction:
MyClass *p = new MyClass();
MyVector<MyClass*> v;
v.push_back(new MyClass(*p));
Update: From you updated question, you can definitely override push_back
template<typename T>
class MyVector {
public:
void push_back (T obj); // general push_back
template<typename TYPE> // T can already be a pointer, so declare TYPE again
void push_back (TYPE *pFrom)
{
TYPE *pNew = new TYPE(*pFrom);
// use pNew in your logic...
}
};
Something like this:
template<typename T>
class MyVector
{
T* data; // Pointer to internal memory
size_t count; // Number of items of T stored in data
size_t allocated; // Total space that is available in data
// (available space is => allocated - count)
void push_back(std::auto_ptr<T> item) // Use auto pointer to indicate transfer of ownership
/*void push_back(T* item) The dangerous version of the interface */
{
if ((allocated - count) == 0)
{ reallocateSomeMemory();
}
T* dest = &data[count]; // location to store item
new (dest) T(*item); // Use placement new and copy constructor.
++count;
}
// All the other stuff you will need.
};
Edit based on comments:
To call it you need to do this:
MyVector<Plop> data;
std::auto_ptr<Plop> item(new Plop()); // ALWAYS put dynamically allocated objects
// into a smart pointer. Not doing this is bad
// practice.
data.push_back(item);
I use auto_ptr because RAW pointers are bad (ie in real C++ code (unlike C) you rarely see pointers, they are hidden inside smart pointers).
Now that I've got SmartPointers, I decided to try DumbPointers, which do the terribly difficult job of calling "delete" in a destructor. Deciding to continue my tests of storing arrays in SmartPointer, using a DumbPointer, I recalled a previous error and tried this:
DumbPointer<char> dumbPointer = "Hello World\0";
For undefined reasons of "Perhaps that's not a pointer or proper pointer?", it causes the program to hang. Naturally, I'd like to know why, with an eye toward solving said problem. I will now provide the terribly complex DumbPointer code (maybe I should go to sleep) below: (Let me re-assure you: The destructor is called)
#pragma once
#include "stdafx.h"
template <typename T> class DumbPointer
{
private:
T* myPtr;
public:
T* Value() { return myPtr; }
DumbPointer(const DumbPointer<T>& a)
{ throw new "No assignments between DumbPointer."; }
DumbPointer(T* ptr) { myPtr = ptr; }
DumbPointer(T value) { myPtr = &value; }
~DumbPointer() { delete myPtr; }
operator T*() { return myPtr; }
T* operator ->() { return myPtr; }
};
You must only delete pointers that you allocated with new. You pass a string constant to your DumbPointer, which has static allocation.
In addition to Frederik's comment, I would just add that there's absolutely no reason to assign a string constant to a smart pointer.
Since there's absolutely nothing to delete here, just assign it to a plain old char* pointer. Again, there's nothing that needs deleting or cleaning up.
I'm in the process of writing a smart pointer countedptr and I've hit a speed bump. The basic function of countedptr is to work like any other smart pointer and also have a count of how many pointers are pointing to a single object. So far, the code is:
[SOLVED]
#include "std_lib_facilities.h"
template <class T>
class counted_ptr{
private:
T* pointer;
int* count;
public:
counted_ptr(T* p = 0, int* c = new int(1)) : pointer(p), count(c) {} // default constructor
explicit counted_ptr(const counted_ptr& p) : pointer(p.pointer), count(p.count) { ++*count; } // copy constructor
~counted_ptr() { --*count; delete pointer; }
counted_ptr& operator=(const counted_ptr& p)
{
pointer = p.pointer;
count = p.count;
++*count;
return *this;
}
T* operator->() const{ return pointer; }
T& operator*() const { return *pointer; }
int Get_count() const { return *count; }
};
int main()
{
counted_ptr<double> one;
counted_ptr<double>two(one);
int a = one.Get_count();
cout << a << endl;
}
When I try to do something like
one->pointer = new double(5);
then I get a compiler error saying "request for member 'pointer' in '*(&one)->counted_ptr::operator->with T = double' which is of non-class type double".
I considered making a function to do this, and while I could make a function to allocate an array of T's, I can't think of a way of making one for allocating actual objects. Any help is appreciated, thanks.
Old Solution
What about another assignment operator?
counted_ptr& counted_ptr::operator=(T* p)
{
if (! --*count) { delete count; }
pointer = p;
count = new int(1);
return *this;
}
...
one = new double(5);
Also, your destructor always deletes a shared pointer, which is probably what caused *one to be a random nomber. Perhaps you want something like:
counted_ptr::~counted_ptr() { if (! --*count) { delete pointer; delete count; } }
New Solution
As you want repointing a counted_ptr (eg one = new double(5)) to update all related counted_ptrs, place both the pointer and the count in a helper class, and have your pointer class hold a pointer to the helper class (you might already be headed down this path). You could go two ways in filling out this design:
Make the helper class a simple struct (and a private inner class) and place all the logic in the outer class methods
Make counted_ptr the helper class. counted_ptr maintains a reference count but doesn't automatically update the count; it's not a smart pointer, it only responds to release and retain messages. If you're at all familiar with Objective-C, this is basically its traditional memory management (autoreleasing aside). counted_ptr may or may not delete itself when the reference count reaches 0 (another potential difference from Obj-C). counted_ptrs shouldn't be copyable. The intent is that for any plain pointer, there should be at most one counted_ptr.
Create a smart_ptr class that has a pointer to a counted_ptr, which is shared among smart_ptr instances that are supposed to hold the same plain pointer. smart_ptr is responsible for automatically updating the count by sending its counted_ptr release and retain methods.
counted_ptr may or may not be a private inner class of shared_ptr.
Here's an interface for option two. Since you're doing this as an exercise, I'll let you fill out the method definitions. Potential implementations would be similar to what's already been posted except that you don't need a copy constructor and copy assignment operator for counted_ptr, counted_ptr::~counted_ptr doesn't call counted_ptr::release (that's smart_ptr::~smart_ptr's job) and counted_ptr::release might not free counted_ptr::_pointer (you might leave that up to the destructor).
// counted_ptr owns its pointer an will free it when appropriate.
template <typename T>
class counted_ptr {
private:
T *_pointer;
size_t _count;
// Make copying illegal
explicit counted_ptr(const counted_ptr&);
counted_ptr& operator=(const counted_ptr<T>& p);
public:
counted_ptr(T* p=0, size_t c=1);
~counted_ptr();
void retain(); // increase reference count.
bool release(); // decrease reference count. Return true iff count is 0
void reassign(T *p); // point to something else.
size_t count() const;
counted_ptr& operator=(T* p);
T& operator*() const;
T* operator->() const;
};
template <typename T>
class smart_ptr {
private:
counted_ptr<T> *_shared;
void release(); // release the shared pointer
void retain(); // retain the shared pointer
public:
smart_ptr(T* p=0, int c=1); // make a smart_ptr that points to p
explicit smart_ptr(counted_ptr<T>& p); // make a smart_ptr that shares p
explicit smart_ptr(smart_ptr& p); // copy constructor
~smart_ptr();
// note: a smart_ptr's brethren are the smart_ptrs that share a counted_ptr.
smart_ptr& operator=(smart_ptr& p); /* Join p's brethren. Doesn't alter pre-call
* brethren. p is non-const because this->_shared can't be const. */
smart_ptr& operator=(counted_ptr<T>& p); /* Share p. Doesn't alter brethren.
* p is non-const because *this isn't const. */
smart_ptr& operator=(T* p); // repoint this pointer. Alters brethren
size_t count() const; // reference count
T& operator*() const; // delegate these to _shared
T* operator->() const;
};
Hopefully, the only ambiguous points above are the intentional ones.
(Sorry, newbie here, and can't leave comments). What Adatapost added, "one=new double(5);" should work. One other change needed, though: the reference counting needs a little help.
...
~counted_ptr() {
--*count;
// deallocate objects whose last reference is gone.
if (!*count)
{
delete pointer;
delete count;
}
}
counted_ptr& operator=(const counted_ptr& p)
{
// be careful to accommodate self assignment
++*p.count;
// may lose a reference here
--*count;
if (!*count)
{
delete pointer;
delete count;
}
count=p.count;
pointer=p.pointer;
return *this;
}
Of course, there's some code repetition here. It might make sense to refactor that code into its own function, e.g.
private:
/** remove our reference */
void release()
{
--*count;
if (!*count)
{
delete pointer;
delete count;
}
}
Did you, perhaps, mean "one.pointer=new double(5);"? Writing "one->pointer=new double(5);" invokes counted_ptr<double>::operator->. That is, it is approximately equivalent to:
double *tmp = one.operator->(); // returns one.pointer
tmp->pointer = new double(5);
But a double pointer isn't a structure, and so it doesn't have a pointer member.
Since the immediate problem has already been solved, I want to offer something more long term:
As you continue to develop this code, you'll definitely want to offer it up for full review by experienced programmers, whether here or elsewhere. There were a few obvious problems with your code as you posted it, though outis has helped correct them. But even once your code all compiles and seems to work in your own tests, there may be tests and situations which you haven't yet learned to think about. Smart pointers can easily have subtle problems that don't show up until very specific situations. So you'll want others to look over your code to find anything which you may have missed.
Please don't take this as any kind of insult towards your current code. I'm just offering this as friendly advice to ensure you learn the most you can out of this project.
Unless you are not doing this for academic reasons, you might want to use consider using the use_count() member of boost::shared_ptr. It's not entirely efficient, but it does work and you're better off using something well tested, mature, and thread safe. If you are doing this for learning purposes, be sure to check out the treatment of Reference Counting and Smart Pointers in More Effective C++.
You need to decrement the count and possibly delete the pointer to the old value in operator = before you overwrite it. You also need 'delete count' everywhere you have 'delete pointer' to avoid leaking memory