pointers *shudder*

I believe i have a fair understanding of pointers. What i don't understand or rather if i'm under estimating the powerfulness of pointers. So basically we can by-pass call by value restrictions, they can be used as part of linked lists but what else?

I have come to conclusion that pointers offer the following benefits?

- We can dynamically modify whatever the pointer is pointing to. So for example we could have a method which does something and modify the contents of the pointed value, we can then print the value in the main method by de-referencing the pointer and finding out what its pointing *TO* (i.e. *xPtr = 2) makes x =2 if *xPtr = &x (if *xPtr points to X), rather than using global variables (which are bad).

- They're the equivalent of getters and setters in java (or kind of??)

And i've also written something small to demonstrate my basic understanding. Please correct me if i'm wrong:

Code:

#include <stdio.h>
void calculate(int *xPtr);
int main()
{
	int x = 0;
	int *xPtr; // Create a pointer and a memory location is allocated.
	
	xPtr = &x;
	*xPtr = 2;
	//calculate(&x); // Pass a reference to the variable x, which will make xPtr point to x
	
	printf("%d", x); // print what x contains
	printf("%d", *xPtr); // print what xPtr is pointing to in this case x.
	
	return 0;
}

void calculate(int *xPtr)
{
	*xPtr = 2; //set whatever xPtr is pointing to (x) to 2.
}

Also what stumps me is the following:

If i pass a reference of x to calculate it makes xptr point to X and sets xptr to 2 which makes X 2 since xptr is pointing to x now. But for some reason i can't de-reference *xPtr and print it the program just crashes ?

Code:

#include <stdio.h>
void calculate(int *xPtr);
int main()
{
	int x = 0;
	int *xPtr; // Create a pointer and a memory location is allocated.
	
	//xPtr = &x;
	calculate(&x); // Pass a reference to the variable x, which will make xPtr point to x
	
	printf("%d", x); // print what x contains
	printf("%d", *xPtr); // print what xPtr is pointing to in this case x.
	
	return 0;
}

void calculate(int *xPtr)
{
	*xPtr = 2; //set whatever xPtr is pointing to (x) to 2.
}

You forgot about dynamic memory allocation, which is probably the most important function of the pointer. This is why your second example is broken.

>But for some reason i can't de-reference *xPtr and print it the program just crashes ?
You're trying to dereference an uninitialized pointer. Before you can dereference a pointer, it must point to memory that you own, either by assigning the address of an existing object or allocating dynamic memory.

I am thinking about your compilers warning level, it should have warned you about using uninitialized variable.

Originally Posted by Prelude

either by assigning the address of an existing object or allocating dynamic memory.

x is an exisiting object i'm also assigning it &x and pass it to calculate which accepts a pointer.

Isn't it the same as :

*xPtr = &x;

so:

Code:

#include <stdio.h>
void calculate(int *xPtr);
int main()
{
	int x = 0;
	int *xPtr = &x; // Make xPtr point to x.
	
	calculate(&x); // Pass a reference to the variable x, which will make xPtr point to x??? OR
	
	printf("%d", x); // print what x contains
	printf("%d", *xPtr); // print what x contains
	
	return 0;
}

void calculate(int *xPtr)
{
	*xPtr = 2; //set whatever xPtr is pointing to (x) to 2.
}

it still defeats the purpose of calculate accepting a memory reference...

Originally Posted by Axel

Isn't it the same as :

*xPtr = &x;

That would be all wrong since *xPtr while being uninitialized is pointing "nowhere" or "somewhere" And you try to assign it value which is reference of x.

Originally Posted by Rennor

That would be all wrong since *xPtr while being uninitialized is pointing "nowhere" or "somewhere" And you try to assign it value which is reference of x.

sorry i meant xPtr = &x;

ok i'm guessing:

Code:

void calculate(int *xPtr)
{
	*xPtr = 2; //set whatever xPtr is pointing to (x) to 2.
}

this only sets whatever xPtr is pointing to before this it's not pointing to no (i.e unalloacted reference) where so therefore is the reason why it crashed.

>sorry i meant xPtr = &x;
You failed to do that in the code that had the problem. To be more specific, you commented it out:

Code:

int *xPtr;
	
//xPtr = &x;
calculate(&x); /* xPtr is still uninitialized here */

what confuses me is that:

void calculate(int *xPtr)


i'm passing an &x to it. When the arguments accept a pointer (i.e. xPtr is pointing to x) Why is that?

Originally Posted by Axel

what confuses me is that:

void calculate(int *xPtr)


i'm passing an &x to it. When the arguments accept a pointer (i.e. xPtr is pointing to x) Why is that?

Huh? I don't understand what your question is.

Originally Posted by Axel

what confuses me is that:

void calculate(int *xPtr)


i'm passing an &x to it. When the arguments accept a pointer (i.e. xPtr is pointing to x) Why is that?

xPtr is a pointer which accepts the address of an integer. x is an integer, so you pass the address of x (&x) to the pointer.

&x generates a pointer, a pointer to x.

Actually it generates the address of a given variable.


Quzah.