Question about smart pointer

[John]

Hi all:

Just learn to use smart pointer. I get a question. In the following
code:
void foo()
{
auto_ptr<MyClass> p(new MyClass);
p->DoSomething();
}

By the end of function foo(), the memory that is allocated to p will
be automatically deallocated. But if I have other functions that need
to access the memory allocated to p, and eventually that memory will
be deallocated by one function. So I do not want the memory allocated
to p to be deallocated by the end of function foo(). What should I do?

Thanks a lot.

John

[Xiaobin Yang]

can you give the declaration/implementation of "template <class T> class
auot_ptr {.....}"?

On Mon, 7 Jun 2004, John wrote:

Hi all:

Just learn to use smart pointer. I get a question. In the following
code:
void foo()
{
auto_ptr<MyClass> p(new MyClass);
p->DoSomething();
}

By the end of function foo(), the memory that is allocated to p will
be automatically deallocated. But if I have other functions that need
to access the memory allocated to p, and eventually that memory will
be deallocated by one function. So I do not want the memory allocated
to p to be deallocated by the end of function foo(). What should I do?

Thanks a lot.

John

-- xiaobin

[Jeff Flinn]

"John" <jo*********@yahoo.com> wrote in message
news:c3**************************@posting.google.c om...

Hi all:

Just learn to use smart pointer. I get a question. In the following
code:
[see below]
By the end of function foo(), the memory that is allocated to p will
be automatically deallocated. But if I have other functions that need
to access the memory allocated to p, and eventually that memory will
be deallocated by one function. So I do not want the memory allocated
to p to be deallocated by the end of function foo(). What should I do? void foo()
auto_ptr<MyClass> foo()
{
auto_ptr<MyClass> p(new MyClass);
p->DoSomething();
return p;
}

[Alan Johnson]

John wrote:

Hi all:

Just learn to use smart pointer. I get a question. In the following
code:
void foo()
{
auto_ptr<MyClass> p(new MyClass);
p->DoSomething();
}

By the end of function foo(), the memory that is allocated to p will
be automatically deallocated. But if I have other functions that need
to access the memory allocated to p, and eventually that memory will
be deallocated by one function. So I do not want the memory allocated
to p to be deallocated by the end of function foo(). What should I do?

Thanks a lot.

John

The release() method will cause the auto_ptr to give up ownership of the
memory. For example:

void foo()
{
auto_ptr<MyClass> p(new MyClass);
MyClass *t;

p->DoSomething();

// Other stuff.

t = p.release();

// Now we must manually delete the memory.
delete t;
}

Alan

[Alan Johnson]

Xiaobin Yang wrote:

can you give the declaration/implementation of "template <class T> class
auot_ptr {.....}"?

On Mon, 7 Jun 2004, John wrote:

Hi all:

Just learn to use smart pointer. I get a question. In the following
code:
void foo()
{
auto_ptr<MyClass> p(new MyClass);
p->DoSomething();
}

By the end of function foo(), the memory that is allocated to p will
be automatically deallocated. But if I have other functions that need
to access the memory allocated to p, and eventually that memory will
be deallocated by one function. So I do not want the memory allocated
to p to be deallocated by the end of function foo(). What should I do?

Thanks a lot.

John

-- xiaobin

std::auto_ptr<T> is in <memory>.

Alan

[John]

"Jeff Flinn" <NO****@nowhere.com> wrote in message news:<ca**********@bluegill.adi.com>...

"John" <jo*********@yahoo.com> wrote in message
news:c3**************************@posting.google.c om...

Hi all:

Just learn to use smart pointer. I get a question. In the following
code:

[see below]

By the end of function foo(), the memory that is allocated to p will
be automatically deallocated. But if I have other functions that need
to access the memory allocated to p, and eventually that memory will
be deallocated by one function. So I do not want the memory allocated
to p to be deallocated by the end of function foo(). What should I do?

void foo()

auto_ptr<MyClass> foo()

{
auto_ptr<MyClass> p(new MyClass);
p->DoSomething();

return p;
}

But that allocated memory will still be deallocated by the end of foo(), right?

Thanks.

JOhn

[Jeff Flinn]

"John" <jo*********@yahoo.com> wrote in message
news:c3**************************@posting.google.c om...

"Jeff Flinn" <NO****@nowhere.com> wrote in message

news:<ca**********@bluegill.adi.com>...

"John" <jo*********@yahoo.com> wrote in message
news:c3**************************@posting.google.c om...

Hi all:

Just learn to use smart pointer. I get a question. In the following
code:

[see below]

By the end of function foo(), the memory that is allocated to p will
be automatically deallocated. But if I have other functions that need
to access the memory allocated to p, and eventually that memory will
be deallocated by one function. So I do not want the memory allocated
to p to be deallocated by the end of function foo(). What should I do?

void foo()

auto_ptr<MyClass> foo()

{
auto_ptr<MyClass> p(new MyClass);
p->DoSomething();

return p;
}

But that allocated memory will still be deallocated by the end of foo(),

right?

Wrong.

Jeff F

[Richard Herring]

In message <c3**************************@posting.google.com >, John
<jo*********@yahoo.com> writes

"Jeff Flinn" <NO****@nowhere.com> wrote in message
news:<ca**********@bluegill.adi.com>...

"John" <jo*********@yahoo.com> wrote in message
news:c3**************************@posting.google.c om...

> Hi all:
>
> Just learn to use smart pointer. I get a question. In the following
> code:

[see below]

> By the end of function foo(), the memory that is allocated to p will
> be automatically deallocated. But if I have other functions that need
> to access the memory allocated to p, and eventually that memory will
> be deallocated by one function. So I do not want the memory allocated
> to p to be deallocated by the end of function foo(). What should I do?

> void foo()

auto_ptr<MyClass> foo()

> {
> auto_ptr<MyClass> p(new MyClass);
> p->DoSomething();

return p;
}

But that allocated memory will still be deallocated by the end of foo(), right?

No. Ownership is transferred to the copy of the pointer returned by
foo(). The responsibility for deleting it now rests with the entity to
which it was returned.

More generally, you need to ask yourself about your ownership model. Is
the instance of MyClass solely owned by a single instance of some other
class (not necessarily the same one, but exactly one at any point in
time) or is it shared between several other objects, so it should be
kept in existence until all of them have finished with it?

std::auto_ptr is designed to work with the first case, but if what you
really want is the second, you should be looking at boost::shared_ptr.

--
Richard Herring

[Dietmar Kuehl]

"Jeff Flinn" <NO****@nowhere.com> wrote:

"John" <jo*********@yahoo.com> wrote:

"Jeff Flinn" <NO****@nowhere.com> wrote:

"John" <jo*********@yahoo.com> wrot:
auto_ptr<MyClass> foo()

> {
> auto_ptr<MyClass> p(new MyClass);
> p->DoSomething();

return p;
}

But that allocated memory will still be deallocated by the end of foo(),

right?

Wrong.

To elaborate a little bit: 'std::auto_ptr' transfers ownership of the
pointer when it is assigned or copied. That is, the pointer owned by 'p'
is returned to 'std::auto_ptr' returned from 'foo' and will be deleted
when this auto pointer is destructed - unless, of course, it is
transfered to yet another pointer.

The utility of 'std::auto_ptr' is relatively limited. In situations where
the ownership of a pointer is less clear, it is probably advisable to use
a reference counted pointer like eg. 'boost::shared_ptr' which is coming
up in the library TR as 'std::tr1::shared_ptr'.
--
<mailto:di***********@yahoo.com> <http://www.dietmar-kuehl.de/>
<http://www.contendix.com> - Software Development & Consulting

[John]

Thanks a lot.

John

Richard Herring <ju**@[127.0.0.1]> wrote in message news:<MS**************@baesystems.com>...

In message <c3**************************@posting.google.com >, John
<jo*********@yahoo.com> writes

"Jeff Flinn" <NO****@nowhere.com> wrote in message
news:<ca**********@bluegill.adi.com>...

"John" <jo*********@yahoo.com> wrote in message
news:c3**************************@posting.google.c om...
> Hi all:
>
> Just learn to use smart pointer. I get a question. In the following
> code:

[see below]

> By the end of function foo(), the memory that is allocated to p will
> be automatically deallocated. But if I have other functions that need
> to access the memory allocated to p, and eventually that memory will
> be deallocated by one function. So I do not want the memory allocated
> to p to be deallocated by the end of function foo(). What should I do? void foo()

auto_ptr<MyClass> foo()

> {
> auto_ptr<MyClass> p(new MyClass);
> p->DoSomething();

return p;
}

But that allocated memory will still be deallocated by the end of foo(), right?

No. Ownership is transferred to the copy of the pointer returned by
foo(). The responsibility for deleting it now rests with the entity to
which it was returned.

More generally, you need to ask yourself about your ownership model. Is
the instance of MyClass solely owned by a single instance of some other
class (not necessarily the same one, but exactly one at any point in
time) or is it shared between several other objects, so it should be
kept in existence until all of them have finished with it?

std::auto_ptr is designed to work with the first case, but if what you
really want is the second, you should be looking at boost::shared_ptr.