Question

A pointer is a variable that points to a location in memory.

int *pointer1;
int *pointer2 = pointer1;

Let’s say that both variables are pointing to memory location

0xA

Then I perform

pointer2++;

Now, pointer2 points to

0xB

Because both addresses point to the same place, I fully expect pointer1 to point to

0xB

But it does not. pointer1 still points to 0xA.

How is this possible? Does the pointer have another address to specify which pointer it actually is? If yes, what is this second address called?

Answer 1

You are confusing the value stored in the pointer with the value to which the pointer points.

The two pointers, per se, are completely independent, they just happen to point to the same memory location. When you write

pointer2++;

you are incrementing the value stored in pointer2 (i.e. address to which it points), not the value stored at the pointed location; being pointer and pointer2 independent variables, there’s no reason why also pointer should change its value.

---

More graphically:

int var=42;
int * ptr1 = &var;
int * ptr2 = ptr2;

Supposing that var is stored at memory location 0x10, we’ll have this situation:

+----------+
|          |  0x00
+----------+
|          |  0x04
+----------+                      
|          |  0x08               +------------+
+----------+              +------| ptr1: 0x10 |
|          |  0x0C        |      +------------+
+----------+              |      +------------+
| var: 42  |  0x10   <----+------| ptr2: 0x10 |
+----------+                     +------------+
|          |  0x14      
+----------+
|          |  0x18
+----------+
|          |

Now we increment ptr2

ptr2++;

(due to pointer arithmetic the stored address gets incremented of sizeof(int), that here we assume being 4)

+----------+
|          |  0x00
+----------+
|          |  0x04
+----------+                      
|          |  0x08               +------------+
+----------+              +------| ptr1: 0x10 |
|          |  0x0C        |      +------------+
+----------+              |      +------------+
| var: 42  |  0x10   <----+   +--| ptr2: 0x14 |
+----------+                  |  +------------+
|          |  0x14   <--------+
+----------+
|          |  0x18
+----------+
|          |

Now ptr2 points to 0x14 (what’s there is not important in this example); ptr1 is left untouched pointing to 0x10.

(naturally both ptr1 and ptr2 have an address where they are stored, as any other variable; this is not shown in the diagrams for clarity)