Re: question about pointers.

wongjoekmeu_at_yahoo.com
Date: 01/26/05 

Date: 26 Jan 2005 07:44:28 -0800

Dear Karl Heinz Buchegger,

Thank you for your explanation. But if I understood you properly then
why does not the following code does not compile ?

----------

int *p = new int[5];
int *p = new int[10];

-----------
But when I write the following code here below which seems to be the
same for me that I redeclare the pointer to point again to somewhere
else, the compiler does not complain. WHy?? Do I have memory leak in
any of these cases ?
---------------
for (int i = 1; i<10; i++)
{
int *p = new int[i];
}
---------------

Robert

Karl Heinz Buchegger wrote:
> "wongjoekmeu@yahoo.com" wrote:
> >
> > Hello All,
> > I am learning C++ at the moment. Going through the book of SAM of
> > learning C++ in 21 days I have learned about pointers that it is
good
> > custome to always initialise them and to use delete before you try
to
> > give it a new address.
> > Can anyone explain to me why this code below which seems to voilate
> > this rule works ?
> >
> [snip]
>
> > So I initialised pCat to the null pointer and added the lines
between
> > line 28-29. And now the code does not work. The output on line 34
gives
> > only zeros. Can anyone explain me what I am doing wrong. To my
opinion
> > I am doing everything correct and according to the rules. But What
am I
> > doing wrong ??
>
> What the book did wrong: It obviously explained pointers and how to
> deal with them in a way you did not understand.
> What you did wrong: You don't understand pointers or to be more
exact:
> danymic memory allocation by now.
>
> Here is the deal:
> A pointer is a place in memory, just like any other variable that
holds
> the memory address of some other items in memory. As such it has a
name
> just like any other variable.
>
> So when you program does
>
> CAT * Family[500];
> CAT * pCat;
>
> you create 2 of those variables. Here they are:
>
> pCat Family
> +----------+ +------------+
> | | | |
> +----------+ +------------+
> | |
> +------------+
> | |
> +------------+
> | |
> . .
> . .
> . .
> | |
> +------------+
> | |
> +------------+
>
> Each of those rectangles contains a pointer. At the moment they
> are pointing to nowhere in particular, but that will change in
> a moment. When a pointer points to something, I will indicate
> that fact by placing an arrow in it. The arrow starts at the
> rectangle and ends at the thing 'pointed to'. Note: If I want
> to indicate that a pointer points to newhere, I simply write
> a 0 into the rectangle. Since none of the rectangles contain
> a 0 right now, I can conclude that nothing can be said about the
> state of the pointers right now.
>
> You program contines with
>
> pCat = new CAT;
>
> in the loop. OK lets perform the operation at i == 0
>
> new CAT That means that a CAT object is born somewhere in
memory ...
> pCat = new CAt ... and pCat gets a pointer to it
>
> pCat Family
> +----------+ +------------+
> | o | | |
> +----|-----+ +------------+
> | | |
> | +------------+
> | | |
> | +------------+
> | | |
> | . .
> | . .
> | . .
> | | |
> | +------------+
> | | |
> | +------------+
> |
> | +------------+
> +----------------------------------------->| Age: |
> +------------+
>
> Next:
> pCat->SetAge(2*i + 1);
>
> pCat Look up the rectangle called pCat
> pCat-> There must be an arrow originating from it
> Follow that arrow and you will get to an
> object of type CAT
> pCat->SetAge From that object, call the member function
setAge
> pCat->SetAge(2*i+1) and pass it the value 1 (remember, we ar ein
the first
> loop iteration, i has a value of 0)
>
> I guess that SetAge simply will set the Age field, thus
>
> pCat Family
> +----------+ +------------+
> | o | | |
> +----|-----+ +------------+
> | | |
> | +------------+
> | | |
> | +------------+
> | | |
> | . .
> | . .
> | . .
> | | |
> | +------------+
> | | |
> | +------------+
> |
> | +------------+
> +----------------------------------------->| Age: 1 |
> +------------+
>
> Next:
> Family[i] = pCat;
>
> Ok. In the array Family, look up the rectangle which corresponds to
> the 0-th entry. Make that entry point to the same object as pCat
>
> pCat Family
> +----------+ +------------+
> | o | | o----------------+
> +----|-----+ +------------+ |
> | | | |
> | +------------+ |
> | | | |
> | +------------+ |
> | | | |
> | . . |
> | . . |
> | . . |
> | | | |
> | +------------+ |
> | | | |
> | +------------+ |
> | v
> | +------------+
> +----------------------------------------->| Age: 1 |
> +------------+
>
> Loop finished, lets make another turn through the loop, for i == 1
>
> pCat = new CAT;
>
> pCat Family
> +----------+ +------------+
> | o | | o----------------+
> +----|-----+ +------------+ |
> | | | |
> | +------------+ |
> | | | |
> | +------------+ |
> | | | |
> | . . |
> | . . |
> | . . |
> | | | |
> | +------------+ |
> | | | |
> | +------------+ |
> | v
> | +------------+
> | | Age: 1 |
> | +------------+
> |
> |
> | +------------+
> +-------------------------------------->| Age: |
> +------------+
>
> pCat->SetAge( 2*i + 1);
>
> pCat Family
> +----------+ +------------+
> | o | | o----------------+
> +----|-----+ +------------+ |
> | | | |
> | +------------+ |
> | | | |
> | +------------+ |
> | | | |
> | . . |
> | . . |
> | . . |
> | | | |
> | +------------+ |
> | | | |
> | +------------+ |
> | v
> | +------------+
> | | Age: 1 |
> | +------------+
> |
> |
> | +------------+
> +-------------------------------------->| Age: 3 |
> +------------+
>
> Family[i] = pCat;
>
> pCat Family
> +----------+ +------------+
> | o | | o----------------+
> +----|-----+ +------------+ |
> | | o------------+ |
> | +------------+ | |
> | | | | |
> | +------------+ | |
> | | | | |
> | . . | |
> | . . | |
> | . . | |
> | | | | |
> | +------------+ | |
> | | | | |
> | +------------+ | |
> | | v
> | | +------------+
> | | | Age: 1 |
> | | +------------+
> | |
> | v
> | +------------+
> +------------------------------------->| Age: 3 |
> +------------+
>
> I guess you can now see what is going on:
> New CAT objects are allocated, a pointer to them is held in pCat.
> The object gets assigned some value and finaly the pointer in the
> Family array is made to point to that object. So in the end, the
> graphics would look like this: The family rectangle has pointers
> pointing all over the place to CAT object, each with an Age number
> set.
>
> Now the final loop in the original program simply walks through
> that array,
>
> for( i = 0; i < 500; i++ )
>
> follows each pointer
>
> Family[i]->
>
> tells the object at the end of the arrow to give its age
>
> Family[i]->GetAge()
>
> and outputs the number returned by the object
>
> cout << Family[i]->GetAge() << endl;
>
> So far so good.
> But then came you and modified the program. Lets see how this
influences
> the whole program:
>
> Again 2 Pointers are created:
>
> CAT * Family[500];
> CAT * pCat;
>
> pCat Family
> +----------+ +------------+
> | | | |
> +----------+ +------------+
> | |
> +------------+
> | |
> +------------+
> | |
> . .
> . .
> . .
> | |
> +------------+
> | |
> +------------+
>
> in the loop ( i == 0 )
>
> pCat = new CAT;
> pCat->SetAge( 2*i+1);
> Family[i] = pCat;
>
> pCat Family
> +----------+ +------------+
> | o | | o----------------+
> +----|-----+ +------------+ |
> | | | |
> | +------------+ |
> | | | |
> | +------------+ |
> | | | |
> | . . |
> | . . |
> | . . |
> | | | |
> | +------------+ |
> | | | |
> | +------------+ |
> | v
> | +------------+
> +----------------------------------------->| Age: 1 |
> +------------+
>
> Nothing new so far.
> But now it comes:
>
> delete pCat;
>
> Well. delete destroyes the object pCat points to. It wipes it out of
memory.
>
> pCat Family
> +----------+ +------------+
> | o | | o----------------+
> +----|-----+ +------------+ |
> | | | |
> | +------------+ |
> | | | |
> | +------------+ |
> | | | |
> | . . |
> | . . |
> | . . |
> | | | |
> | +------------+ |
> | | | |
> | +------------+ |
> | v
> |
> +----------------------------------------->
>
>
> pCat = 0;
>
> pCat Family
> +----------+ +------------+
> | 0 | | o----------------+
> +----------+ +------------+ |
> | | |
> +------------+ |
> | | |
> +------------+ |
> | | |
> . . |
> . . |
> . . |
> | | |
> +------------+ |
> | | |
> +------------+ |
> v
>
> Next run through the loop ( i == 1 )
>
> pCat = new CAT;
> pCat->SetAge( 2*i+1);
> Family[i] = pCat;
>
> pCat Family
> +----------+ +------------+
> | o | | o----------------+
> +----|-----+ +------------+ |
> | | o-----------+ |
> | +------------+ | |
> | | | | |
> | +------------+ | |
> | | | | |
> | . . | |
> | . . | |
> | . . | |
> | | | | |
> | +------------+ | |
> | | | | |
> | +------------+ | |
> | | v
> | |
> | |
> | v
> | +----------+
> +---------------------------------->| Age: 3 |
> +----------+
>
> and again:
>
> delete pCat;
> pCat = 0;
>
> pCat Family
> +----------+ +------------+
> | O | | o----------------+
> +----------+ +------------+ |
> | o-----------+ |
> +------------+ | |
> | | | |
> +------------+ | |
> | | | |
> . . | |
> . . | |
> . . | |
> | | | |
> +------------+ | |
> | | | |
> +------------+ | |
> | v
> |
> |
> v
>
>
> I think you can imagine, what this leads to in the end. Family
> still holds a lot of pointers. But there are no objects at the
> end of all those arrows. They have all been deleted.
>
> Well. Above I said: it has been wiped out of memory. Technically
> this is impossible. Memory is memory. There is still memory at
> all the end points of all those arrows and it is impossible for
> memory to not hold some value. But logically those objects are no
> longer there. The content of that memory is random and depends on a
> lot of things outside your control. This is why the loop in your
final
> program outputs 0. It could have operated otherwise also. You program
> could have simply crashed, because in the loop you take the pointer,
> the arrow, follow it and tell the object at the end of the arrow
> to give its age. But there is no object there any more!
>
>
> --
> Karl Heinz Buchegger
> kbuchegg@gascad.at

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