[boost] [shared_ptr] dealing with shared_ptr and cycles

From: Lorenzo Bettini (bettini_at_[hidden])
Date: 2005-06-06 08:58:41

Hi

I have this simple program, that has a class with a shared pointer to 
another object of the same type:

#include <boost/shared_ptr.hpp>
#include <string>
#include <iostream>

using namespace std;

class C;

typedef boost::shared_ptr<C> CP;

class C
{
private:
   string s;
   CP cp;

public:
   C(const string &ss, CP c = CP()) : s(ss), cp(c) {}
   void set(CP c) { cp = c; }
   ~C() { cout << "deleting C" << endl; }
};

int
main()
{
   CP cp = CP(new C("C"));
   CP cp2 = CP(new C("C2"));
   cp->set(cp2);
   //cp->set(cp);
   cout << "use_count: " << cp.use_count() << endl;;

   return 0;
}

if I remove the comment for cp->set(cp), i.e., I set the shared_pointer 
to itself, there's a cycle, and thus use_count returns 2, and the object 
  cp is never deleted (shown also by using valgrind).

This is a simple program just to reproduce a much more complex situation 
with a data structure that deals with mutual recursive references.

I do not know in advance whether the shared pointer refers to the same 
object or to another different object (for instance I perform also 
cp->set(cp2)).

Now, the documentation 
(http://www.boost.org/libs/smart_ptr/shared_ptr.htm) says that

"Because the implementation uses reference counting, cycles of 
shared_ptr instances will not be reclaimed. For example, if main() holds 
a shared_ptr to A, which directly or indirectly holds a shared_ptr back 
to A, A's use count will be 2. Destruction of the original shared_ptr 
will leave A dangling with a use count of 1. Use weak_ptr to "break 
cycles.""

However, it is not clear to me how to use weak_ptr in this context...

can some one provide some clue, please?

many thanks in advance
        Lorenzo