In computer science, a smart pointer is an abstract data type that simulates a pointer while providing additional features, such as automatic garbage collection or bounds checking. These additional features are intended to reduce bugs caused by the misuse of pointers while retaining efficiency. Smart pointers typically keep track of the objects they point to for the purpose of memory management. They may also be used to manage other resources, such as network connections and file handles.
The misuse of pointers is a major source of bugs: the constant allocation, deallocation and referencing that must be performed by a program written using pointers introduces the risk that memory leaks will occur. Smart pointers try to prevent memory leaks by making the resource deallocation automatic: when the pointer (or the last in a series of pointers) to an object is destroyed, for example because it goes out of scope, the pointed object is destroyed too.
Several types of smart pointers exist. Some work with reference counting, others by assigning ownership of the object to a single pointer. If the language supports automatic garbage collection (for instance, Java or C#), then smart pointers are unnecessary for memory management, but may still be useful in managing other resources.
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In C++, smart pointers may be implemented as a template class that mimics, by means of operator overloading, the behaviour of traditional (raw) pointers, (e.g.: dereferencing, assignment) while providing additional memory management algorithms.
Smart pointers can facilitate intentional programming by expressing the use of a pointer in the type itself. For example, if a C++ function returns a pointer, there is no way to know whether the caller should delete the memory pointed to when the caller is finished with the information.
some_type* ambiguous_function(); // What should be done with the result?
Traditionally, this has been solved with comments, but this can be error-prone. By returning an auto_ptr,
auto_ptr<some_type> obvious_function1();
the function makes explicit that the caller will take ownership of the result, and furthermore, that if the caller does nothing, no memory will be leaked.
C++11 provides std::unique_ptr, defined in the header <memory>.
The copy constructor and assignment operators of std::auto_ptr do not actually copy the stored pointer. Instead, they transfer it, leaving the previous std::auto_ptr object empty. This was one way to implement strict ownership, so that only one auto_ptr object could own the pointer at any given time. This means that auto_ptr should not be used where copy semantics are needed.[1]
C++11 provides support for move semantics; it allows for the explicit support of transferring values as a different operation from copying them. C++11 also provided support for explicitly preventing an object from being copied. Since std::auto_ptr already existed with its copy semantics, it could not be upgraded to be a move-only pointer without breaking backwards compatibility with existing code. Therefore, C++11 introduced a new pointer type: std::unique_ptr.
This pointer type has its copy constructor and assignment operator explicitly deleted; it cannot be copied. It can be moved using std::move, which allows one unique_ptr object to transfer ownership to another.
std::unique_ptr<int> p1(new int(5)); std::unique_ptr<int> p2 = p1; //Compile error. std::unique_ptr<int> p3 = std::move(p1); //Transfers ownership. p3 now owns the memory and p1 is rendered invalid. p3.reset(); //Deletes the memory. p1.reset(); //Does nothing.
std::auto_ptr is still available, but it is deprecated under C++11.
C++11 incorporates shared_ptr and weak_ptr, based on versions used by the Boost libraries. TR1 first introduced them to the standard, but C++11 gives them additional functionality in line with the Boost version.
std::shared_ptr represents reference counted ownership of a pointer. Each copy of the same shared_ptr owns the same pointer. That pointer will only be freed if all instances of the shared_ptr in the program are destroyed.
std::shared_ptr<int> p1(new int(5)); std::shared_ptr<int> p2 = p1; //Both now own the memory. p1.reset(); //Memory still exists, due to p2. p2.reset(); //Deletes the memory, since no one else owns the memory.
A std::shared_ptr uses reference counting, so circular references are potentially a problem. To break up cycles, std::weak_ptr can be used to access the stored object. The stored object will be deleted if the only references to the object are weak_ptr references. weak_ptr therefore does not ensure that the object will continue to exist, but it can ask for the resource.
std::shared_ptr<int> p1(new int(5)); std::weak_ptr<int> wp1 = p1; //p1 owns the memory. { std::shared_ptr<int> p2 = wp1.lock(); //Now p1 and p2 own the memory. if(p2) //Always check to see if the memory still exists { //Do something with p2 } } //p2 is destroyed. Memory is owned by p1. p1.reset(); //Memory is deleted. std::shared_ptr<int> p3 = wp1.lock(); //Memory is gone, so we get an empty shared_ptr. if(p3) { //Will not execute this. }
Operations that change the reference count, due to copying or destroying shared_ptr or weak_ptr objects, do not provoke data race conditions. This means that multiple threads can safely store shared_ptr or weak_ptr objects that reference the same object. This only protects the reference count itself; it does not protect the object being stored by the smart pointer.
The above only applies when multiple threads have their own shared_ptr instances that are referring to the same object. In cases where multiple threads are accessing the same shared_ptr instance, C++11 provides a number of atomic functions for accessing and manipulating the shared_ptr.