[shared_ptr] dealing with shared_ptr and cycles

6 Jun 2005

      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

Lorenzo Bettini

Larry Evans

Jonathan Wakely

Lorenzo Bettini

David Abrahams

Lorenzo Bettini

Larry Evans

David Abrahams

Lorenzo Bettini

David Abrahams

Eric Niebler

Larry Evans

Peter Dimov

Lorenzo Bettini

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