12.1. Dynamic Memory and Smart Pointers

In C++, dynamic memory is managed through a pair of operators: new , which allocates, and optionally initializes, an object in dynamic memory and returns a pointer to that object; and delete , which takes a pointer to a dynamic object, destroys that object, and frees the associated memory.

Dynamic memory is problematic because it is surprisingly hard to ensure that we free memory at the right time. Either we forget to free the memory—in which case we have a memory leak—or we free the memory when there are still pointers referring to that memory—in which case we have a pointer that refers to memory that is no longer valid.

To make using dynamic memory easier (and safer), the new library provides two smart pointer types that manage dynamic objects. A smart pointer acts like a regular pointer with the important exception that it automatically deletes the object to which it points. The new library defines two kinds of smart pointers that differ in how they manage their underlying pointers: shared_ptr , which allows multiple pointers to refer to the same object, and unique_ptr , which “owns” the object to which it points. The library also defines a companion class named weak_ptr that is a weak reference to an object managed by a shared_ptr. All three are defined in the memory header.