C++11 Smart Pointer: auto_ptr is deprecated. Use unique_ptr instead. Here also come shared_ptr and weak_ptr.

我们在 C++ Exception Handling / auto_ptr 里介绍了 RAII wrapper for pointers–auto_ptr，然后 C++11 又 deprecated 了……对应的 replacement 是 unique_ptr。顺带还推出了 shared_ptr 和 weak_ptr，它们的特性是：

shared_ptr allows multiple pointers to refer to the same object.
- shared_ptrs automatically destroy their objects and free the associated memory when its reference count gets 0.
- destroy 的时候一般是调用 delete，也可以自己提供一个 deleter 函数并用类似的逻辑来管理我们自定义的资源类，比如 shared_ptr<connection> p(&conn, close_connection);
unique_ptr “owns” the object to which it points. Only one unique_ptr at a time can point to a given object.
- The object to which a unique_ptr points is destroyed when the unique_ptr is destroyed.
weak_ptr is a weak reference to an object managed by a shared_ptr.
- Binding a weak_ptr to a shared_ptr does not change the reference count of that shared_ptr. Once the last shared_ptr pointing to the object goes away, the object itself will be deleted, even if there are weak_ptrs pointing to it—hence the name weak_ptr, which captures the idea that a weak_ptr shares its object “weakly.”
- Because the object might no longer exist, we cannot use a weak_ptr to access its object directly. To access that object, we must call lock() function to check whether the object to which the weak_ptr points still exists. If so, lock() returns a shared_ptr to the shared object.
- 参 Understanding Weak References