From: Edward Diener [mailto:eddielee@tropicsoft.com]
I don't understand what the practical use of Boost weak_ptr<> is ? Anyone like to clue me in ? I understand that Boost weak_ptr<> creates an internal reference to a Boost shared_ptr<> but I don't understand what that is supposed to accomplish.
A weak_ptr is an observer of the pointee, i.e it doesn't take part of the lifetime management of the resource. One use for it is to break cyclic dependencies; another is to avoid dangling pointers. When a shared_ptr deallocates its pointee, it notifies the weak_ptrs. When the weak_ptr is subsequently used, it knows that the resource is gone (compare this with storing raw pointers, where there is no way to tell whether the pointer is still valid or not). Example: A Company class has a container that holds elements of type shared_ptr<Employee>. Now, the Supervisor class needs to know things about certain Employees, but it cannot/must not control their lifetime (in the context of the Company class, that is). Rather than asking the Company to search through each Employee (for example by name) each time Supervisor needs to access/update some Employee information, it caches information in weak_ptr<Employee> for efficient access. If an Employee quits (is removed from the container of shared_ptr<Employee>), the Supervisor class will be informed of this the next time it tries to access that weak_ptr<Employee>. At the time of access, a (temporary) shared_ptr is created from a weak_ptr, to ensure that the pointee isn't deleted in the midst of using it. Without weak_ptr, the above would be hard(er) to accomplish: * Asking Company to find an Employee each time it is needed may be too inefficient. * Storing shared_ptr<Employee> in the Supervisor class interferes with lifetime management and can cause inconsistent state. * Storing a raw pointer to an Employee is extremely dangerous (known as the dangling Employee problem); there's no way of telling if the pointer is valid. Bjorn Karlsson