Auto_ptr — Stl | CodeCogs

smart pointer that deletes the instance it points to when going out of scope

Definition

The auto_ptr class template is defined in the standard header <memory>, and in the nonstandard backward-compatibility header <memory.h>.

template <class Type>
class auto_ptr

Parameter	Description
`Type`	The type of object for which storage is being allocated or deallocated

Description

The auto_ptr is provided by the C++ standard library as a kind of a smart pointer that helps to avoid resource leaks when exceptions are thrown.

It manages an object obtained via new and deletes that object when auto_ptr itself is destroyed. It may be used to provide exception safety for dynamically-allocated objects, for passing ownership of dynamically-allocated objects into functions and for returning dynamically-allocated objects from functions.

NOTE:

Copying an auto_ptr changes the object being copied by setting the contents to NULL
auto_ptr objects are not guaranteed to work correctly with the STL containers

Member Functions

Function	Description
(constructor)	Creates a new auto_ptr object
(destructor)	Destroys an auto_ptr and the managed object
get	Obtains a pointer to the managed object
operator*	Dereference object (accesses the managed object)
operator->	Dereference object member (accesses the managed object)
release	Releases ownership of the managed object
reset	Deallocate object pointed and set new value
(conversion operators)	Conversion operators

References

Example 1

Problem

The following example of program illustrates how to reset an auto_ptr.

Workings

#include <iostream>
#include <memory>

struct Resetter
{
  Resetter() { std::cout << "Constructor" << std::endl; }
  ~Resetter() { std::cout << "Destructor" << std::endl; }
};

int main()
{
  std::auto_ptr<Resetter> pReset(new Resetter);
  pReset.reset(new Resetter);
}

Solution

Output:

Constructor
Constructor
Destructor
Destructor

References

http://www.richelbilderbeek.nl/

Example 2

Problem

This example of program shows how auto_ptrs behave regarding the transfer of ownership.

Workings

#include <iostream>
#include <memory>
using namespace std;

Â  Â /* define output operator for auto_ptr
Â  Â  * - print object value or NULL
Â  Â  */
template <class T>
ostream& operator<< (ostream& strm, const auto_ptr<T>& p)
{
Â Â //does p own an object ?
Â  if (p.get() == NULL)
  {
Â  Â  Â strm << "NULL"; Â  Â  Â  Â //NO: print NULL
Â  }
Â  else 
  {
Â  Â  Â strm << *p; Â  Â  Â  Â  Â  //YES: print the object
Â  }

Â  return strm;
}

int main()
{
  auto_ptr<int> p(new int(42));
  auto_ptr<int> q;

  cout << "after initialization:" << endl;
  cout << " p: " << p << endl;
  cout << " q: " << q << endl;

  q = p;
  cout << "after assigning auto pointers:" << endl;
  cout << " p: " << p << endl;
  cout << " q: " << q << endl;

  *q += 13;                   //change value of the object q owns
  p = q;
  cout << "after change and reassignment:" << endl;
  cout << " p: " << p << endl;
  cout << " q: " << q << endl;
}

Solution

Output:

after initialization:
p: 42
q: NULL
after assigning auto pointers:
p: NULL
q: 42
after change and reassignment:
p: 55
q: NULL

References

Nicolai M. Josuttis: "The C++ Standard Library"