Pointer vs. variable, Objective-C

Question

I have some difficulties grasping some concepts. Grateful for help.

Let's say you have the following piece of code:

int *intPtr;   // creates a pointer
int count = 10; // initiates an intiger variable
intptr = &count; // ??

The & operator gives the address to a variable, in this case the integer count. The address is then assigned to the intptr. My question is: Why is intptr = count; not sufficient. I know that count is a variable and intptr is a pointer, but isn´t a variable also just referring to some place in memory?

Hans

Answer 1

count refers to the VALUE of the variable. You don't want to assign the value of count to intptr, you want to assign the address of count. Thus the & operator is used.

If you do intptr = count, you'd be pointing to memory address 10 in this case, which is sure to be in system memory, not your application memory and you'd crash.

Answer 2

It is important to understand that pointers have actually a different data type.

A int variable will hold integer values.

A pointer variable will hold memory addresses.

So it is not right to assign an int variable to a pointer variable (as you suggested intptr = count;)

I believe using a typedef could help you better understand the difference.

Here's a small example:

#include <stdio.h>

typedef int* int_pointer;

int main() {

    int n; // integer
    int_pointer p; // pointer

    n = 5;

    p = &n; // p's value is now n's address

    *p = *p + 1; // add 1 to the value stored at the location that p points to
                 // and put that value back in the same location

    printf("n = %d\n", n);
    printf("*p = %d\n", *p);

    return 0;
}

This program will print

n = 6
*p = 6