A weak_ptr is a smart pointer in C++ that provides a way to reference an object managed by a shared_ptr without affecting its reference count. This means that a weak_ptr can observe an object but does not keep it alive, preventing circular references that can lead to memory leaks. When the last shared_ptr to an object is destroyed, the object is deleted, and any associated weak_ptr becomes invalid. To use a weak_ptr, you typically create it from a shared_ptr. You can check if the weak_ptr is still valid by converting it to a shared_ptr using the `lock()` method. If the object is still alive, `lock()` returns a valid shared_ptr; otherwise, it returns a null pointer. This mechanism allows for safe and efficient memory management in C++.