Question

Suppose I have two boost::shared_ptr‘s pointing to two different objects of class A:

boost::shared_ptr<A> x = boost::make_shared<A>();
boost::shared_ptr<A> y = boost::make_shared<A>();

At some point, I need x to discard ownership of the object it’s owning and share ownership of the y‘s object with y. How can this be achieved (note that both shared_ptr’s are constructed at that point, so no chance to use the copy constructor)?

Thanks!

Answer 1

You can simply assign it:

x = y;

See the assignment operators for std::shared_ptr and boost::shared_ptr assignment. You can verify this by checking the reference count before and after the assignment. This example uses C++11’s std::shared_ptr but boost::shared_ptr would yield the same results:

#include <memory>
int main()
{
    std::shared_ptr<int> x(new int);
    std::cout << x.use_count() << "\n"; // 1
    std::shared_ptr<int> y(new int);
    std::cout << x.use_count() << "\n"; // still 1
    y = x;
    std::cout << x.use_count() << "\n"; // 2
}