Lot of questions.

CodeMonkey · 01-03-2009, 10:24 PM

Exactly it.

*i becomes 4, but k remains 3*

Ryan0773 · 01-03-2009, 10:36 PM

Ok so when using i++, the int k will become what i was but then i will change and the k will not change because the post-increment makes it so that only the i changes. But the pre-increment makes the i change before the int k makes itself equal to i so they would both be the same.(Does that make sense?)

PS: This is completely unrelated to anything we were talking about but i was just wondering, how do you get a signature?

CodeMonkey · 01-03-2009, 10:41 PM

That's one way to think about it. Think about it more literally and it might be simpler.

++i adds one to i and returns i

i++ returns a temporary copy of i and then adds one to the real i.

*edit* "user cp" for the signature.

Ryan0773 · 01-04-2009, 10:21 AM

Thanks alot!

Ryan0773 · 01-04-2009, 10:56 AM

I still dont completely understand how to use pointers. I barely understand what their actual purpose is. Apparently they help to make sure that you have enough space for your program and that you arent wasting any space either. Please help, the whole purpose of pointers just isnt clear. How are they supposed to help your program? when should you use them? HOW do you use them? They just dont seem to make any sense.

**laserlight** · 01-04-2009, 11:23 AM

Quote:

Originally Posted by Ryan0773

I still dont completely understand how to use pointers. I barely understand what their actual purpose is.

Have you read the pointers tutorial?

Quote:

Originally Posted by Ryan0773

Apparently they help to make sure that you have enough space for your program and that you arent wasting any space either. Please help, the whole purpose of pointers just isnt clear. How are they supposed to help your program? when should you use them? HOW do you use them? They just dont seem to make any sense.

Well, one use of pointers is to avoid copying an expensive to copy object when you want to pass that object around. However, in C++ this use is largely replaced by references, unless you also want to make passing an object optional (i.e., the caller can pass a null pointer instead of a pointer to an object).

Another use would be to manage a dynamically allocated array, or to create links between the nodes of a linked list. However, this use is largely replaced by containers that abstract away the use of pointers.

Ryan0773 · 01-04-2009, 11:35 AM

Yes, i did read the tutorial. I can sortof make a basic pointer that points to another integer. but whats the purpose of codes like

Code:

int *pnt = new int;

and

Code:

delete ptr;

The first one is supposed do something like take a free pointer and make it point to something that becomes unavailable to other programs or something (dont really understand what the means)
The second one is supposed to delete a pointer that was allocated with new. Thats doesn't really help me if I dont understand the purpose of new.

**laserlight** · 01-04-2009, 11:46 AM

Quote:

Originally Posted by Ryan0773

The first one is supposed do something like take a free pointer and make it point to something that becomes unavailable to other programs or something (dont really understand what the means)

You can think of it as "creating an object of type int". The pointer named pnt points to this object.

Quote:

Originally Posted by Ryan0773

The second one is supposed to delete a pointer that was allocated with new.

You can think of it as "destroying the object of type int which ptr points to".

Ryan0773 · 01-04-2009, 11:53 AM

Quote:

Originally Posted by laserlight

You can think of it as "creating an object of type int". The pointer named pnt points to this object.

But then how can i give the object of type int a term?

Quote:

Originally Posted by laserlight

You can think of it as "destroying the object of type int which ptr points to".

So this makes the ptr a null pointer?

**laserlight** · 01-04-2009, 11:58 AM

Quote:

Originally Posted by Ryan0773

But then how can i give the object of type int a term?

What do you mean?

Quote:

Originally Posted by Ryan0773

So this makes the ptr a null pointer?

No, ptr merely points to a destroyed object and hence may not be dereferenced unless and until you make it point to an object that exists. To set ptr to be a null pointer after the object that it points to is destroyed you would write:

Code:

ptr = 0;

or if the <cstddef> header is directly or indirectly included you can also write:

Code:

ptr = NULL;

Ryan0773 · 01-04-2009, 12:08 PM

Quote:

Originally Posted by laserlight

What do you mean?

Like when you write a code that has an int with a certain interger assigned to it

Code:

int x(10);

Make it so that x has a value of ten. When i make a pointer point to something else, how can i make it so that the int that it points to has a value (srry i think i might have used the wrong word to explain it)

Quote:

Originally Posted by laserlight

No, ptr merely points to a destroyed object and hence may not be dereferenced unless and until you make it point to an object that exists. To set ptr to be a null pointer after the object that it points to is destroyed you would write:

Code:

ptr = 0;

or if the <cstddef> header is directly or indirectly included you can also write:

Code:

ptr = NULL;

So if i want to make a pointer null (or point at 0 or nothing) then i would have to do it manually by inserting the code. it wouldn't do it by itself.

**laserlight** · 01-04-2009, 12:21 PM

Quote:

Originally Posted by Ryan0773

Like when you write a code that has an int with a certain interger assigned to it

Oh, you can initialise the int with the value of 10 by writing:

Code:

int* ptr = new int(10);

After the object has been created, you can assign the value of 10 to it by writing:

Code:

*ptr = 10;

This latter syntax was covered by the tutorial.

Quote:

Originally Posted by Ryan0773

So if i want to make a pointer null (or point at 0 or nothing) then i would have to do it manually by inserting the code. it wouldn't do it by itself.

Yes, and for further reading read Stroustrup's answer to the FAQ: Why doesn't delete zero out its operand?

Ryan0773 · 01-04-2009, 12:35 PM

Quote:

Originally Posted by laserlight

This latter syntax was covered by the tutorial.

I'm not sure that it was.

Quote:

Originally Posted by laserlight

Yes, and for further reading read Stroustrup's answer to the FAQ: Why doesn't delete zero out its operand?

Alright ill check it out,thanks. But does is there a way that i can change the characteristics of a certain pointer like *p which point to int i, could i change it so that later in the script *p would go from i to x?

**laserlight** · 01-04-2009, 12:40 PM

Quote:

Originally Posted by Ryan0773

But does is there a way that i can change the characteristics of a certain pointer like *p which point to int i, could i change it so that later in the script *p would go from i to x?

Just assign the address of x to p.

Ryan0773 · 01-04-2009, 12:50 PM

Quote:

Originally Posted by laserlight

Just assign the address of x to p.

How would i write that in code would it be something like...

Code:

{
int i;
int x;
int *p

p = &i
cout<< *p <<"\n";
int *p = new int(x);
cout<< *p <<"\n";
delete p;
*p = 0;
cin.get();
}

I have no idea if this would work or not. Can you tell me if i made any small errors or if i did it completely wrong?

01-03-2009, 10:24 PM	#16
CodeMonkey Kiss the monkey. Join Date: Sep 2001 Posts: 810	Exactly it. i becomes 4, but k remains 3 __________________ "If you tell the truth, you don't have to remember anything" -Mark Twain
CodeMonkey is offline

01-03-2009, 10:36 PM	#17
Ryan0773 C++ noob Join Date: Jan 2009 Posts: 43	Ok so when using i++, the int k will become what i was but then i will change and the k will not change because the post-increment makes it so that only the i changes. But the pre-increment makes the i change before the int k makes itself equal to i so they would both be the same.(Does that make sense?) PS: This is completely unrelated to anything we were talking about but i was just wondering, how do you get a signature?
Ryan0773 is offline

01-03-2009, 10:41 PM	#18
CodeMonkey Kiss the monkey. Join Date: Sep 2001 Posts: 810	That's one way to think about it. Think about it more literally and it might be simpler. ++i adds one to i and returns i i++ returns a temporary copy of i and then adds one to the real i. edit "user cp" for the signature. __________________ "If you tell the truth, you don't have to remember anything" -Mark Twain Last edited by CodeMonkey; 01-04-2009 at 02:09 AM.
CodeMonkey is offline

01-04-2009, 10:21 AM	#19
Ryan0773 C++ noob Join Date: Jan 2009 Posts: 43	Thanks alot!
Ryan0773 is offline

01-04-2009, 10:56 AM	#20
Ryan0773 C++ noob Join Date: Jan 2009 Posts: 43	I still dont completely understand how to use pointers. I barely understand what their actual purpose is. Apparently they help to make sure that you have enough space for your program and that you arent wasting any space either. Please help, the whole purpose of pointers just isnt clear. How are they supposed to help your program? when should you use them? HOW do you use them? They just dont seem to make any sense.
Ryan0773 is offline

01-04-2009, 11:35 AM	#22
Ryan0773 C++ noob Join Date: Jan 2009 Posts: 43	Laserlight reply Yes, i did read the tutorial. I can sortof make a basic pointer that points to another integer. but whats the purpose of codes like Code: int *pnt = new int; and Code: delete ptr; The first one is supposed do something like take a free pointer and make it point to something that becomes unavailable to other programs or something (dont really understand what the means) The second one is supposed to delete a pointer that was allocated with new. Thats doesn't really help me if I dont understand the purpose of new.
Ryan0773 is offline

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Several Questions, main one is about protected memory	Tron 9000	C Programming	3	06-02-2005 07:42 AM
Trivial questions - what to do?	Aerie	A Brief History of Cprogramming.com	23	12-26-2004 09:44 AM
Questions.	anonytmouse	A Brief History of Cprogramming.com	4	05-19-2004 02:16 PM
C++ test questions	Mister C	C++ Programming	9	09-08-2002 12:05 PM