Get copy of object from pointer?

[Mike]

I have a class "Call". I have instantiated a Call object in one class then
passed a pointer to that object to another class.

How do I create a *copy* of that Call object using only the pointer?

[deane_gavin@hotmail.com]

Re: Get copy of object from pointer?

Mike wrote:
[color=blue]
> I have a class "Call". I have instantiated a Call object in one class then
> passed a pointer to that object to another class.
>
> How do I create a *copy* of that Call object using only the pointer?[/color]

The same way you get hold of the object pointed to for any other
purpose. Dereference the pointer.

Call c1;
Call* p = &c1;

p is a pointer to a Call object.

Call c2 = *p;

c2 is created as a copy of the object pointed to by p.

Gavin Deane

[Alf P. Steinbach]

Re: Get copy of object from pointer?

* Mike:[color=blue]
> I have a class "Call". I have instantiated a Call object in one class then
> passed a pointer to that object to another class.
>
> How do I create a *copy* of that Call object using only the pointer?[/color]

If you know that the dynamic type of object pointed to is really Call,
and not some unknown class derived from Call, you can do

Call copyOfCall = *p;

or e.g.

Call* pCopyOfCall = new Call( *p );

Otherwise, if you don't want slicing you can use cloning.

FAQ item 20.8 describes the basics of cloning,
<url: <url:
http://www.parashift.c om/c++-faq-lite/virtual-functions.html# faq-20.8>.

A more in-depth discussion of cloning, especially the use of copy
constructors for cloning in a class hierarchy, is section 1.3.6 of
<url:
http://home.no.net/dubjai/win32cpptut/special/pointers/preview/pointers_01_bet a.doc.pdf>.



Conspiracy theory of the day. How come I gave the same two URLs in this
and my previous posting to clc++? Is there a global conspiracy to ask
questions where the answers naturally list those two URLs?

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Greg]

Re: Get copy of object from pointer?

deane_gavin@hot mail.com wrote:[color=blue]
> Mike wrote:
>[color=green]
> > I have a class "Call". I have instantiated a Call object in one class then
> > passed a pointer to that object to another class.
> >
> > How do I create a *copy* of that Call object using only the pointer?[/color]
>
> The same way you get hold of the object pointed to for any other
> purpose. Dereference the pointer.
>
> Call c1;
> Call* p = &c1;
>
> p is a pointer to a Call object.
>
> Call c2 = *p;
>
> c2 is created as a copy of the object pointed to by p.[/color]

Not necessarily. If p were actually a pointer to a subclass of Call,
then c2 would have ended up with a "sliced" copy of p, and c2 and p
would not be of the same type. For this reason, it is best to use
whatever copy operation the class has defined. In other words, ask p to
copy itself, rather than perform a copy-by-value operation by
dereferencing the pointer and that assumes that p is a pointer to an
exact type.

Greg

[davidrubin@warpmail.net]

Re: Get copy of object from pointer?


Mike wrote:[color=blue]
> I have a class "Call". I have instantiated a Call object in one class then
> passed a pointer to that object to another class.
>
> How do I create a *copy* of that Call object using only the pointer?[/color]

Typically, you don't. Either (a) 'Call' has value semantics, and you
copy the value, or you copy (b) the raw pointer, or (c) a smart pointer
(e.g., one that does reference counting).