auto_ptr

In the C++ programming language, auto_ptr is an obsolete smart pointer class template that was available in previous versions of the C++ standard library (declared in the <memory> header file), which provides some basic RAII features for C++ raw pointers. It has been replaced by the unique_ptr class.

The auto_ptr template class describes an object that stores a pointer to a single allocated object that ensures that the object to which it points gets destroyed automatically when control leaves a scope.^[1]

The characteristics of auto_ptr are now considered unsatisfactory: it was introduced before C++11's move semantics, so it uses copying for what should be done with moves (and confusingly sets the copied-from auto_ptr to a NULL pointer). These copy semantics mean that it cannot be used in STL containers.^[2]

The C++11 standard made auto_ptr deprecated, replacing it with the unique_ptr class template.^[3][4] auto_ptr was fully removed in C++17.^[5] For shared ownership, the shared_ptr template class can be used. shared_ptr was defined in C++11 and is also available in the Boost library for use with previous C++ versions.^[6]

Declaration[edit]

The auto_ptr class is declared in ISO/IEC 14882, section 20.4.5 as:

namespace std {      template <class Y> struct auto_ptr_ref {};      template <class X>     class auto_ptr {     public:         typedef X element_type;          // 20.4.5.1 construct/copy/destroy:         explicit           auto_ptr(X* p =0) throw();                            auto_ptr(auto_ptr&) throw();         template <class Y> auto_ptr(auto_ptr<Y>&) throw();          auto_ptr&                      operator=(auto_ptr&) throw();         template <class Y> auto_ptr&   operator=(auto_ptr<Y>&) throw();         auto_ptr&                      operator=(auto_ptr_ref<X>) throw();          ~auto_ptr() throw();          // 20.4.5.2 members:         X&     operator*() const throw();         X*     operator->() const throw();         X*     get() const throw();         X*     release() throw();         void   reset(X* p =0) throw();          // 20.4.5.3 conversions:                                     auto_ptr(auto_ptr_ref<X>) throw();         template <class Y> operator auto_ptr_ref<Y>() throw();         template <class Y> operator auto_ptr<Y>() throw();     };  } 

Semantics[edit]

The auto_ptr has semantics of strict ownership, meaning that the auto_ptr instance is the sole entity responsible for the object's lifetime. If an auto_ptr is copied, the source loses the reference. For example:

#include <iostream> #include <memory> using namespace std;   int main(int argc, char **argv) {     int *i = new int;     auto_ptr<int> x(i);     auto_ptr<int> y;          y = x;          cout << x.get() << endl; // Print NULL     cout << y.get() << endl; // Print non-NULL address i      return 0; } 

This code will print a NULL address for the first auto_ptr object and some non-NULL address for the second, showing that the source object lost the reference during the assignment (=). The raw pointer i in the example should not be deleted, as it will be deleted by the auto_ptr that owns the reference. In fact, new int could be passed directly into x, eliminating the need for i.

Notice that the object pointed by an auto_ptr is destroyed using operator delete; this means that you should only use auto_ptr for pointers obtained with operator new. This excludes pointers returned by malloc/calloc/realloc, and pointers to arrays (because arrays are allocated by operator new[] and must be deallocated by operator delete[]).

Because of its copy semantics, auto_ptr may not be used in STL containers that may perform element copies in their operations.

See also[edit]

• Smart pointer
• Generic programming

References[edit]

External links[edit]

• Using auto_ptr effectively
• Avoiding Memory Leaks with auto_ptr
• Article "Using the auto_ptr Class Template to Facilitate Dynamic Memory Management" by Danny Kalev
• Article "Container of auto_ptr" by Zeeshan Amjad
• Article "Update on auto_ptr" by Scott Meyers
• auto_ptr Class Template Reference from GNU libstdc++
• auto_ptr reference from Rogue Wave