boost::shared_ptr cycle break with weak_ptr

Question

I am currently in a situation like:
struct A
{
shared_ptr< B > b;
}
struct B
{
shared_ptr< A > a;
}
shared_ptr< A > a(new A()); shared_ptr< B > b(new B();
a->b(b); b->a(a);
I know this won't work, because the references would continue to point to each other. I've also been told that weak_ptr solves this issue.

However, weak ptr has no get or -> overload. I've heard mentions of 'use lock()', but can anyone give code examples of how to do this correctly?

Thanks.

Answer 1

Have you checked the boost reference on weak_ptr?

shared_ptr<int> p(new int(5));
weak_ptr<int> q(p);

// some time later

if(shared_ptr<int> r = q.lock())
{
    // use *r
}

The idea is that you lock the weak_ptr thereby obtaining a shared_ptr which does have the operators.

First check, whether the obtained pointer points to something. A weak_ptr does not determine the life-time of the resource, but it lets you check whether the resource has already been destroyed.

Answer 2

#include <boost/shared_ptr.hpp>
#include <boost/weak_ptr.hpp>

struct B;

struct A
{
    boost::weak_ptr< B > b;
};

struct B
{
    boost::weak_ptr< A > a;
};

int
main()
{
    boost::shared_ptr<A> a(new A());
    boost::shared_ptr<B> b(new B());
    a->b = b;
    b->a = a; 
    boost::shared_ptr<A> another_a( b->a.lock() );
}

you can promote a weak_ptr to a shared_ptr using weak_ptr::lock.

Answer 3

Come on now.

http://boost.org/doc/libs/1_42_0/libs/smart_ptr/weak_ptr.htm

^^^^^ EXAMPLE IS RIGHT THERE ^^^^^^

DAMN!

Answer 4

I think the bigger issue here is one of ambiguous ownership. You'd be better off to decide whether A encapsulates B or the other way around. If that isn't possible then you're still better off introducing another class C that owns both A and B.

Assuming that A owns B, you can write:

classs B;

classs A
{
private:
    boost::scoped_ptr<B> b_;
public:
    A() : b_(new B()) {}
}

class B
{
private:
    A* a_;
public:
    B(A* a) : a_(a) {}
}

And so on. You can even get rid of scoped_ptr by making B a local variable or deleting it manually in the destructor.

The Google C++ Style Guide has more to say on this in the section titled "Smart Pointers."

HTH