Mega Code Archive
Class for counted reference semantics
<![CDATA[
/* The following code example is taken from the book
* "The C++ Standard Library - A Tutorial and Reference"
* by Nicolai M. Josuttis, Addison-Wesley, 1999
*
* (C) Copyright Nicolai M. Josuttis 1999.
* Permission to copy, use, modify, sell and distribute this software
* is granted provided this copyright notice appears in all copies.
* This software is provided "as is" without express or implied
* warranty, and with no claim as to its suitability for any purpose.
*/
#include <iostream>
#include <list>
#include <deque>
#include <algorithm>
using namespace std;
/* class for counted reference semantics
* - deletes the object to which it refers when the last CountedPtr
* that refers to it is destroyed
*/
template <class T>
class CountedPtr {
private:
T* ptr; // pointer to the value
long* count; // shared number of owners
public:
// initialize pointer with existing pointer
// - requires that the pointer p is a return value of new
explicit CountedPtr (T* p=0)
: ptr(p), count(new long(1)) {
}
// copy pointer (one more owner)
CountedPtr (const CountedPtr<T>& p) throw()
: ptr(p.ptr), count(p.count) {
++*count;
}
// destructor (delete value if this was the last owner)
~CountedPtr () throw() {
dispose();
}
// assignment (unshare old and share new value)
CountedPtr<T>& operator= (const CountedPtr<T>& p) throw() {
if (this != &p) {
dispose();
ptr = p.ptr;
count = p.count;
++*count;
}
return *this;
}
// access the value to which the pointer refers
T& operator*() const throw() {
return *ptr;
}
T* operator->() const throw() {
return ptr;
}
private:
void dispose() {
if (--*count == 0) {
delete count;
delete ptr;
}
}
};
void printCountedPtr (CountedPtr<int> elem)
{
cout << *elem << ' ';
}
int main()
{
// array of integers (to share in different containers)
static int values[] = { 3, 5, 9, 1, 6, 4 };
// two different collections
typedef CountedPtr<int> IntPtr;
deque<IntPtr> coll1;
list<IntPtr> coll2;
/* insert shared objects into the collections
* - same order in coll1
* - reverse order in coll2
*/
for (int i=0; i<sizeof(values)/sizeof(values[0]); ++i) {
IntPtr ptr(new int(values[i]));
coll1.push_back(ptr);
coll2.push_front(ptr);
}
// print contents of both collections
for_each (coll1.begin(), coll1.end(),printCountedPtr);
cout << endl;
for_each (coll2.begin(), coll2.end(),printCountedPtr);
cout << endl << endl;
/* modify values at different places
* - square third value in coll1
* - negate first value in coll1
* - set first value in coll2 to 0
*/
*coll1[2] *= *coll1[2];
(**coll1.begin()) *= -1;
(**coll2.begin()) = 0;
// print contents of both collections again
for_each (coll1.begin(), coll1.end(),printCountedPtr);
cout << endl;
for_each (coll2.begin(), coll2.end(),printCountedPtr);
cout << endl;
}
3 5 9 1 6 4
4 6 1 9 5 3
-3 5 81 1 6 0
0 6 1 81 5 -3
]]>