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477

answers:

1

Consider the following code.

using boost::shared_ptr;
struct B;
struct A{
    ~A() { std::cout << "~A" << std::endl; }
    shared_ptr<B> b; 
};
struct B {
    ~B() { std::cout << "~B" << std::endl; }
    shared_ptr<A> a;
};

void main() {
    shared_ptr<A> a (new A);
    shared_ptr<B> b (new B);
    a->b = b;
    b->a = a;
}

There is no output. No desctructor is called. Memory leak. I have always believed that the smart pointer helps avoid memory leaks.

What should I do if I need cross-references in the classes?

+25  A: 

If you have circular references like this, one object should hold a weak_ptr to the other, not a shared_ptr.

From the shared_ptr introduction:

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."

Thanks, Glen, for the link.

James McNellis
@Alexey, here's a link to the docs where it explicitly warns of this problem in the introduction. http://www.boost.org/doc/libs/1_41_0/libs/smart_ptr/shared_ptr.htm
Glen