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
.
