Re: [PLUG] [ON TOPIC] passing a pointer to an object to an object (c++)

[Jason <jason@nocks.com>]

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It's a little unclear what you are trying to do here. You have to be careful 
about when you create an object, when other objects access that object, and 
when you *destroy* the object. Make sure you do destroy any object that you 
are responsible for cleaning up (usually, just the ones that you instantiate 
with new).

If you want to modify the object "while in another object", then don't you 
want the following:

void SomeObject::SomeFunction()
{
  QLabel *qBillBoard = new QLabel("unlabeled", 0, 0);
  voxInstallCommon(qBillBoard);
}

voxInstallCommon::voxInstallCommon(QLabel *qlIncoming)
{
  // Now access the qIIncoming pointer
  // It will point to the same object as the qBillBoard pointer
}

Now, it's unclear without the other code, but maybe you really want to be 
doing the following:

void SomeObject::SomeFunction()
{
  QLabel *qBillBoard = NULL;
  voxInstallCommon(qBillBoard);
}

voxInstallCommon::voxInstallCommon(QLabel *&qlIncoming)
{
  qIIncoming = new QLabel("unlabeled", 0, 0);
  // Now access the qIIncoming pointer
  // The qBillBoard pointer will now also point to the qIIncoming object.
}

Depends a little on what you are trying to do here. Depends on what you mean 
by gain access to an object within another object.

Sounds more like you have a member variable, so maybe you should just have an 
accessor function return it like so:

class voxInstallCommon
{
  public:
    voxInstallCommon();
    QLabel *GetLabel();
  private:
    QLabel *m_pLabel;
};

voxInstallCommon::voxInstallCommon()
{
  m_pLabel = new QLabel("unlabeled", 0, 0);
}

QLabel *voxInstallCommon::GetLabel()
{
  return m_pLabel;
}

If this is the case, and you don't need to worry so much about when 
initialization occurs, you could also implement it like so and avoid the 
pointers somewhat:

class voxInstallCommon
{
  public:
    voxInstallCommon();
    const QLabel &GetLabel();
  private:
    QLabel m_Label;
};

voxInstallCommon::voxInstallCommon()
: m_label(QLabel("unlabeled", 0, 0))
{
}

const QLabel &voxInstallCommon::GetLabel()
{
  return m_Label;
}

Or, something along those lines. May not have the intialization syntax exactly 
right for the QLabel.

HTH.
Jason Nocks

On Sunday 14 July 2002 15H:02, mike.h wrote:
> Fred,
>
> You do not appear to be declaring the local pointer varible in your snip:
>
> voxInstallCommon::voxInstallCommon(QLabel *qlIncoming)
> {
>     QLabel *qlBillBoard=new QLabel("unlabeled", 0, 0);
>     qlBillBoard=qlIncoming;
> ...
> -mike
>
> -----Original Message-----
> From: plug-admin@lists.phillylinux.org
> [mailto:plug-admin@lists.phillylinux.org]On Behalf Of Fred K Ollinger
> Sent: Saturday, July 13, 2002 11:27 AM
> To: plug@lists.phillylinux.org
> Subject: [PLUG] [ON TOPIC] passing a pointer to an object to an object
> (c++)
>
>
> I have this code where I have an object and I'd like to modify it while in
> another object. Is there a way to do this? I get segfaults when I do:
>
> // begin snippet
> voxInstallCommon::voxInstallCommon(QLabel *qlIncoming)
> {
>     qlBillBoard=new QLabel("unlabeled", 0, 0);
>     qlBillBoard=qlIncoming;
> //end snippet
>
> Why?
>
> Is there a better way to gain access to an oject in another object?
>
> I have spent a lot of time reading about this and trying permutations to
> no avail. I want the oject to be persistent in the other object.
>
> I have successfully passed a pointer to an object in the function, but the
> pointer is not persistent so I'm trying to do this via a constructor and
> then make a pointer to the pointer. Since all the pointers are aiming at
> the same patch of memory this should work, but it doesn't.
>
> Fred Ollinger (follinge@sas.upenn.edu)
> CCN sysadmin
>
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