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 It has been replaced by the unique_ptr class.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

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

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

• Smart pointer
• Generic programming

References

1. "auto_ptr Class". Microsoft. Retrieved 2006-09-27.
2. Meyers, Scott (2014). Effective Modern C++: 42 specific ways to improve your use of C++11 and C++14. Sebastopol, CA. p. 118. ISBN 978-1-491-90399-5.{{cite book}}: CS1 maint: location missing publisher (link)
3. "Working Draft, Standard for Programming Language C++ N3242" (PDF). 28 February 2011. p. 1233. Retrieved 2013-02-17.
4. Kalev, Danny. "Using unique_ptr, Part I". informIT. Retrieved 30 September 2010.
5. "Programming Language C++, Library Evolution Working Group JTC1/SC22/WG21 N4190". 2014-10-09. Retrieved 2016-08-29.
6. "Collecting Shared Objects". Dr. Dobb's. 2004-07-01. Retrieved 2006-09-27.

External links

• Using auto_ptr effectively
• Avoiding Memory Leaks with auto_ptr
• Article "" 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++