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auto_ptr class

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Wraps a smart pointer around a resource that ensures the resource is destroyed automatically when control leaves a block.

Starting in C++11, use unique_ptr instead of auto_ptr. For more information, see unique_ptr class. auto_ptr was deprecated in C++11 and removed in C++17.

For more information about throw() and exception handling, see Exception Specifications (throw).

Syntax

class auto_ptr {
    typedef Type element_type;
    explicit auto_ptr(Type* ptr = 0) throw();
    auto_ptr(auto_ptr<Type>& right) throw()
        ;
    template <class Other>
    operator auto_ptr<Other>() throw();
    template <class Other>
    auto_ptr<Type>& operator=(auto_ptr<Other>& right) throw();
    template <class Other>
    auto_ptr(auto_ptr<Other>& right);
    auto_ptr<Type>& operator=(auto_ptr<Type>& right);
    ~auto_ptr();
    Type& operator*() const throw();
    Type * operator->()const throw();
    Type *get() const throw();
    Type *release()throw();
    void reset(Type* ptr = 0);
};

Parameters

right
The auto_ptr from which to get an existing resource.

ptr
The pointer specified to replace the stored pointer.

Remarks

The class template describes a smart pointer, called an auto_ptr, to an allocated object. The pointer must be either null or designate an object allocated by new. The auto_ptr transfers ownership if its stored value is assigned to another object. (It replaces the stored value after a transfer with a null pointer.) The destructor for auto_ptr<Type> deletes the allocated object. The auto_ptr<Type> ensures that an allocated object is automatically deleted when control leaves a block, even through a thrown exception. You shouldn't construct two auto_ptr<Type> objects that own the same object.

You can pass an auto_ptr<Type> object by value as an argument to a function call. An auto_ptr can't be an element of any Standard Library container. You can't reliably manage a sequence of auto_ptr<Type> objects with a C++ Standard Library container.

Members

Constructors

Name Description
auto_ptr The constructor for objects of type auto_ptr.

Typedefs

Name Description
element_type The type is a synonym for the template parameter Type.

Functions

Name Description
get The member function returns the stored pointer myptr.
release The member replaces the stored pointer myptr with a null pointer and returns the previously stored pointer.
reset The member function evaluates the expression delete myptr, but only if the stored pointer value myptr changes as a result of function call. It then replaces the stored pointer with ptr.

Operators

Name Description
operator= An assignment operator that transfers ownership from one auto_ptr object to another.
operator* The dereferencing operator for objects of type auto_ptr.
operator-> The operator for allowing member access.
operator auto_ptr<Other> Casts from one kind of auto_ptr to another kind of auto_ptr.
operator auto_ptr_ref<Other> Casts from an auto_ptr to an auto_ptr_ref.

auto_ptr

The constructor for objects of type auto_ptr.

explicit auto_ptr(Type* ptr  = 0) throw();

auto_ptr(auto_ptr<Type>& right) throw();

auto_ptr(auto _ptr_ref<Type> right) throw();

template <class Other>
auto _ptr(auto _ptr<Other>& right) throw();

Parameters

ptr
The pointer to the object that auto_ptr encapsulates.

right
The auto_ptr object to be copied by the constructor.

Remarks

The first constructor stores ptr in myptr, the stored pointer to the allocated object. The second constructor transfers ownership of the pointer stored in right, by storing right. release in myptr.

The third constructor behaves the same as the second, except that it stores right. ref. release in myptr, where ref is the reference stored in right.

The template constructor behaves the same as the second constructor, if a pointer to Other can be implicitly converted to a pointer to Type.

Example

// auto_ptr_auto_ptr.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>
#include <vector>

using namespace std;

class Int
{
public:
   Int(int i)
   {
      cout << "Constructing " << ( void* )this  << endl;
      x = i;
      bIsConstructed = true;
   };
   ~Int( )
   {
      cout << "Destructing " << ( void* )this << endl;
      bIsConstructed = false;
   };
   Int &operator++( )
   {
      x++;
      return *this;
   };
   int x;
private:
   bool bIsConstructed;
};

void function ( auto_ptr<Int> &pi )
{
   ++( *pi );
   auto_ptr<Int> pi2( pi );
   ++( *pi2 );
   pi = pi2;
}

int main( )
{
   auto_ptr<Int> pi ( new Int( 5 ) );
   cout << pi->x << endl;
   function( pi );
   cout << pi->x << endl;
}

Constructing 00311AF8
5
7
Destructing 00311AF8

element_type

The type is a synonym for the template parameter Type.

typedef Type element  _type;

get

The member function returns the stored pointer myptr.

Type *get() const throw();

Return Value

The stored pointer myptr.

Example

// auto_ptr_get.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>
#include <vector>
using namespace std;

class Int
{
public:
   Int(int i)
   {
      x = i;
      cout << "Constructing " << ( void* )this  << " Value: " << x << endl;
   };
   ~Int( )
   {
      cout << "Destructing " << ( void* )this << " Value: " << x << endl;
   };

   int x;

};

int main( )
{
   auto_ptr<Int> pi ( new Int( 5 ) );
   pi.reset( new Int( 6 ) );
   Int* pi2 = pi.get ( );
   Int* pi3 = pi.release ( );
   if (pi2 == pi3)
      cout << "pi2 == pi3" << endl;
   delete pi3;
}

Constructing 00311AF8 Value: 5
Constructing 00311B88 Value: 6
Destructing 00311AF8 Value: 5
pi2 == pi3
Destructing 00311B88 Value: 6

operator=

An assignment operator that transfers ownership from one auto_ptr object to another.

template <class Other>
    auto_ptr<Type>& operator=(auto_ptr<Other>& right) throw();
auto_ptr<Type>& operator=(auto_ptr<Type>& right) throw();
auto_ptr<Type>& operator=(auto_ptr_ref<Type> right) throw();

Parameters

right
An object of type auto_ptr.

Return Value

A reference to an object of type auto_ptr<Type>.

Remarks

The assignment evaluates the expression delete myptr, but only if the stored pointer myptr changes as a result of the assignment. It then transfers ownership of the pointer stored in right, by storing right.release in myptr. The function returns *this.

Example

For an example of the use of the member operator, see auto_ptr.

operator*

The dereferencing operator for objects of type auto_ptr.

Type& operator*() const throw();

Return Value

A reference to an object of type Type that the pointer owns.

Remarks

The indirection operator returns *get. Hence, the stored pointer must not be null.

Example

For an example of how to use the member function, see auto_ptr.

operator->

The operator for allowing member access.

Type * operator->() const throw();

Return Value

A member of the object that auto_ptr owns.

Remarks

The selection operator returns get( ), so that the expression ap-> member behaves the same as ( ap. get() )-> member, where ap is an object of class auto_ptr<Type>. Hence, the stored pointer must not be null, and Type must be a class, struct, or union type with a member member.

Example

For an example of how to use the member function, see auto_ptr.

operator auto_ptr<Other>

Casts from one kind of auto_ptr to another kind of auto_ptr.

template <class Other>
operator auto _ptr<Other>() throw();

Return Value

The type cast operator returns auto_ptr<Other>(*this).

Example

// auto_ptr_op_auto_ptr.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>
#include <vector>

using namespace std;
int main()
{
   auto_ptr<int> pi ( new int( 5 ) );
   auto_ptr<const int> pc = ( auto_ptr<const int> )pi;
}

operator auto_ptr_ref<Other>

Casts from an auto_ptr to an auto_ptr_ref.

template <class Other>
operator auto _ptr  _ref<Other>() throw();

Return Value

The type cast operator returns auto_ptr_ref<Other>(*this).

Example

// auto_ptr_op_auto_ptr_ref.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>
#include <vector>

using namespace std;

class C {
public:
    C(int _i) : m_i(_i) {
    }
    ~C() {
        cout << "~C:  " << m_i << "\n";
    }
    C &operator =(const int &x) {
        m_i = x;
        return *this;
    }
    int m_i;
};
void f(auto_ptr<C> arg) {
};
int main()
{
    const auto_ptr<C> ciap(new C(1));
    auto_ptr<C> iap(new C(2));

    // Error: this implies transfer of ownership of iap's pointer
    // f(ciap);
    f(iap); // compiles, but gives up ownership of pointer

            // here, iap owns a destroyed pointer so the following is bad:
            // *iap = 5; // BOOM

    cout << "main exiting\n";
}

~C:  2
main exiting
~C:  1

release

The member replaces the stored pointer myptr with a null pointer and returns the previously stored pointer.

Type *release() throw();

Return Value

The previously stored pointer.

Remarks

The member replaces the stored pointer myptr with a null pointer and returns the previously stored pointer.

Example

// auto_ptr_release.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>
#include <vector>
using namespace std;

class Int
{
public:
    Int(int i)
    {
        x = i;
        cout << "Constructing " << (void*)this << " Value: " << x << endl;
    };
    ~Int() {
        cout << "Destructing " << (void*)this << " Value: " << x << endl;
    };

    int x;

};

int main()
{
    auto_ptr<Int> pi(new Int(5));
    pi.reset(new Int(6));
    Int* pi2 = pi.get();
    Int* pi3 = pi.release();
    if (pi2 == pi3)
        cout << "pi2 == pi3" << endl;
    delete pi3;
}

Constructing 00311AF8 Value: 5
Constructing 00311B88 Value: 6
Destructing 00311AF8 Value: 5
pi2 == pi3
Destructing 00311B88 Value: 6

reset

The member function evaluates the expression delete myptr, but only if the stored pointer value myptr changes as a result of a function call. It then replaces the stored pointer with ptr.

void reset(Type* ptr = 0);

Parameters

ptr
The pointer specified to replace the stored pointer myptr.

Example

// auto_ptr_reset.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>
#include <vector>

using namespace std;

class Int
{
public:
    Int(int i)
    {
        x = i;
        cout << "Constructing " << (void*)this << " Value: " << x << endl;
    };
    ~Int()
    {
        cout << "Destructing " << (void*)this << " Value: " << x << endl;
    };

    int x;
};

int main()
{
    auto_ptr<Int> pi(new Int(5));
    pi.reset(new Int(6));
    Int* pi2 = pi.get();
    Int* pi3 = pi.release();
    if (pi2 == pi3)
        cout << "pi2 == pi3" << endl;
    delete pi3;
}

Constructing 00311AF8 Value: 5
Constructing 00311B88 Value: 6
Destructing 00311AF8 Value: 5
pi2 == pi3
Destructing 00311B88 Value: 6

See also

unique_ptr Class