class for counted reference semantics : reference « Data Types « C++ Tutorial

1. C++ Tutorial
2. Data Types
3. reference

/* 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