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delete pointers in destructor

 
 
TBass
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      12-19-2007
Hi,

Switching some code from C to C++, and I have a destructor question:

In the C code, when the program wants to close and free all memory, I
have them call a Close function. The linked-lists get free'd.

In the C++ versions, my class has a linked list items of another
class, which also has a linked list of another class. I was going to
add a Close function to the classes, but is the C++ way to just put
the delete statements in the destructor and let it get handled
automatically?

Thanks in advance,
T
 
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Victor Bazarov
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      12-19-2007
TBass wrote:
> Switching some code from C to C++, and I have a destructor question:
>
> In the C code, when the program wants to close and free all memory, I
> have them call a Close function. The linked-lists get free'd.
>
> In the C++ versions, my class has a linked list items of another
> class, which also has a linked list of another class. I was going to
> add a Close function to the classes, but is the C++ way to just put
> the delete statements in the destructor and let it get handled
> automatically?


Usually, yes. There are exceptions, of course. If you actually use
'std::list' as your member, you don't need to delete anything, the
list will take care of that. The exceptions to that rule is if your
list is of pointers to dynamically created objects. The rule is, if
you obtained the pointer through 'new' (or 'new[]'), you must dispose
of it using 'delete' (or 'delete[]'). The exception to that rule, in
turn, is if you're using some kind of "smart" pointer type to hold
the naked pointer for you.

Perhaps if you're most specific in your question, you'd get a more
specific answer

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask


 
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TBass
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      12-19-2007
Sorry. I didn't mean to be vague.

[snip]
The exceptions to that rule is if your list is of pointers to
dynamically created objects.
[/snip]

I am using std::list, but it is a list of pointers to an instance of
the class, which I created with the new statement.

Here's a quick outline of what I'm doing:

class tag
{
...
}

class device
{
...
std::list<tag *> m_listTags
}


device::AddTag
{
tag *mytag = new tag;
m_listTags.push_back( mytag );
}


class project
{
...
std::list<device *> m_listDevices
}

project::AddDevice
{
device *mydevice = new device;
m_listDevices.push_back( device );
}


So I'm thinking I should just add to the destructors:

device::~device(void)
{
std::list<tag *>::iterator item;
device *mytag;
for ( item = m_listTags.begin(); item != m_listTags.end(); +
+item )
{
mytag = *item;
delete mytag
}

m_listTags.empty()
}

project::~project(void)
{
std::list<device *>::iterator item;
device *mydevice;
for ( item = m_listDevices.begin(); item != m_listDevices.end(); +
+item )
{
mydevice = *item;
delete mydevice
}

m_listDevices.empty()
}

Thanks!
T
 
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Victor Bazarov
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      12-19-2007
TBass wrote:
> Sorry. I didn't mean to be vague.
>
> [snip]
> The exceptions to that rule is if your list is of pointers to
> dynamically created objects.
> [/snip]
>
> I am using std::list, but it is a list of pointers to an instance of
> the class, which I created with the new statement.


Do you really need to? What if you keep a list of tags or list of
devices instead? 'std::list' creates its elements dynamically
anyway. Let your lists manage memory.

If you _have to_ keep a list of pointers, you're doing it correctly,
with a bit of excess, see below.

>
> Here's a quick outline of what I'm doing:
>
> class tag
> {
> ...
> }
>
> class device
> {
> ...
> std::list<tag *> m_listTags


Why not change it to

std::list<tag> m_listTags;

?

> }
>
>
> device::AddTag
> {
> tag *mytag = new tag;
> m_listTags.push_back( mytag );


In case you decide to change, you would just do

m_listTags.push_back(tag());

Isn't it better?

> }
>
>
> class project
> {
> ...
> std::list<device *> m_listDevices


Same here, why not just

std::list<device> m_listDevices;

? I know one possible answer, but I don't want to speculate.

> }
>
> project::AddDevice
> {
> device *mydevice = new device;
> m_listDevices.push_back( device );
> }
>
>
> So I'm thinking I should just add to the destructors:


Below my comments assume you still go with the lists of pointers.

>
> device::~device(void)
> {
> std::list<tag *>::iterator item;
> device *mytag;

^^^^^^^^^^^^^
You don't need this.

> for ( item = m_listTags.begin(); item != m_listTags.end(); +
> +item )
> {
> mytag = *item;

^^^^^^^^^^^^^^
You don't need this either.

> delete mytag


Change it to

delete *item;

> }
>
> m_listTags.empty()



No need to empty, it's about to be destructed.

> }
>
> project::~project(void)
> {
> std::list<device *>::iterator item;
> device *mydevice;


Same thing. No need for this local var.

> for ( item = m_listDevices.begin(); item != m_listDevices.end(); +
> +item )
> {
> mydevice = *item;
> delete mydevice


Just do

delete *item;

instead.

> }
>
> m_listDevices.empty()


No need to empty, it's about to be destructed.

> }
>
> Thanks!
> T


V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask


 
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David Harmon
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      12-19-2007
On Wed, 19 Dec 2007 07:33:38 -0800 (PST) in comp.lang.c++, TBass
<(E-Mail Removed)> wrote,
>I am using std::list, but it is a list of pointers to an instance of
>the class, which I created with the new statement.


You should look carefully at whether you really need to do that, or
if you can just use a list of the instances.
 
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TBass
Guest
Posts: n/a
 
      12-19-2007
> Do you really need to? What if you keep a list of tags or list of
> devices instead? 'std::list' creates its elements dynamically
> anyway. Let your lists manage memory.


Agreed, which is what I originally intended. However, I have to pass
out pointers to instances of the class to forms for display and
editing. I wasn't sure how this would affect the list, or if changes
to the list in other functions could cause an address I had previously
passed out to become invalid, so I figured I would play it safe.


> > device *mytag;

>
> ^^^^^^^^^^^^^
> You don't need this.
>
> > for ( item = m_listTags.begin(); item != m_listTags.end(); +
> > +item )
> > {
> > mytag = *item;

>
> ^^^^^^^^^^^^^^
> You don't need this either.
>
> > delete mytag

>
> Change it to
>
> delete *item;
>
> > }

>
> > m_listTags.empty()

>
> No need to empty, it's about to be destructed.
>


Great. Thanks for the tips.

Very much appreciated,
T
 
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Erik Wikström
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Posts: n/a
 
      12-19-2007
On 2007-12-19 17:32, TBass wrote:
>> Do you really need to? What if you keep a list of tags or list of
>> devices instead? 'std::list' creates its elements dynamically
>> anyway. Let your lists manage memory.

>
> Agreed, which is what I originally intended. However, I have to pass
> out pointers to instances of the class to forms for display and
> editing. I wasn't sure how this would affect the list, or if changes
> to the list in other functions could cause an address I had previously
> passed out to become invalid, so I figured I would play it safe.


The list is node-based, a pointer to an element will be valid as long as
you do not delete that element, regardless of what you do with the rest
of the elements in the list. This is not true for all containers though.

--
Erik Wikström
 
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Andre Kostur
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Posts: n/a
 
      12-19-2007
TBass <(E-Mail Removed)> wrote in news:7712834f-c002-421c-9189-
http://www.velocityreviews.com/forums/(E-Mail Removed):

> Hi,
>
> Switching some code from C to C++, and I have a destructor question:
>
> In the C code, when the program wants to close and free all memory, I
> have them call a Close function. The linked-lists get free'd.
>
> In the C++ versions, my class has a linked list items of another
> class, which also has a linked list of another class. I was going to
> add a Close function to the classes, but is the C++ way to just put
> the delete statements in the destructor and let it get handled
> automatically?


If I understand correctly, yes. Your class initiates the deletion of your
linked list (which I'd hope is in some appropriate class itself, like
std::list<>). Your linked list will know how to dispose of itself. Your
"other object" would know to dispose of the linked list it contains.
 
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Yakov Gerlovin
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Posts: n/a
 
      12-20-2007
Indeed, generally, it is better to store the objects, rather than the
pointer to the object in the container.
However, in some cases this is not the solution. For example, when you
want to store instances of different classes with common base class.
Another example ( although very rare ) is when copying the object
( i.e. copy constructor ) is very expensive in means of time or
allocated resources.
In those cases, it could be useful to store smart pointers in your
container.
This way you get the advantage of using pointers, without the need to
delete the instances in the container.

My 2 cents
 
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James Kanze
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Posts: n/a
 
      12-20-2007
On Dec 19, 6:53 pm, Erik Wikstrm <(E-Mail Removed)> wrote:
> On 2007-12-19 17:32, TBass wrote:


> >> Do you really need to? What if you keep a list of tags or
> >> list of devices instead? 'std::list' creates its elements
> >> dynamically anyway. Let your lists manage memory.


> > Agreed, which is what I originally intended. However, I have
> > to pass out pointers to instances of the class to forms for
> > display and editing. I wasn't sure how this would affect the
> > list, or if changes to the list in other functions could
> > cause an address I had previously passed out to become
> > invalid, so I figured I would play it safe.


> The list is node-based, a pointer to an element will be valid
> as long as you do not delete that element, regardless of what
> you do with the rest of the elements in the list. This is not
> true for all containers though.


He does have to support copy on the objects, however. If the
objects have identity, then they usually don't support copy or
assignment. And if they don't have identity, he probably should
be returning them by value anyway, rather than by pointers (but
there are exceptions to both cases).

--
James Kanze (GABI Software) email:(E-Mail Removed)
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9 place Smard, 78210 St.-Cyr-l'cole, France, +33 (0)1 30 23 00 34
 
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