If you add 1 to a pointer, does it really add 1, or does it add 4 or 32?

Question

Suppose you have a pointer, produced by something like

int a = 5;
int* pointer = &a;

And suppose you add 1 to the pointer, as in

pointer = pointer + 1;

Now suppose the original bitwise representation of the pointer was

00000000 00000000 00000000 00000000

Then is the new representation 00000000 00000000 00000000 00000001? Or 00000000 00000000 00000001 00000000? Or neither of those?

I am confused because I feel like adding 1 to a number should add 1 to its bitwise representation. But if you add 1 to the index of an array, the new address is 32 bits away from the old address.

Lookup "pointer arithmetic" in [your favorite search engine](http://google.com) — Eugene Sh., Jun 09 '15 at 21:46
Simple answer is: it adds a number of bytes equal to the size of the data type of the pointer. If that type is `int`, then it increases with 4 or 8, depending on the architecture. — Marius Bancila, Jun 09 '15 at 21:52
"*I feel like adding 1 to a number should add 1 to its bitwise representation*" Why? Isn't it much more sensible for adding 1 to a number should add one to its value? Adding one to a value's encoding is something you would almost never want to do. — David Schwartz, Jun 09 '15 at 21:53
@DavidSchwartz Also, pointers are not numbers. If they were, you could add two pointers. — fredoverflow, Jun 09 '15 at 22:02
@fredoverflow The OP's comment wasn't about pointers, it was about numbers. — David Schwartz, Jun 09 '15 at 23:08

score 6 · Answer 1 · edited Jun 20 '20 at 09:12

6

When you add 1 to pointer, then compiler do the following arithmetic behind the scene

pointer = pointer + sizeof(int)*1

So, it will depend on the size of data type. On a 32-bit machine size of int is 4-bytes, therefore 4*1 = 4 will be added to pointer.

For example, if pointer is pointing to memory addresss 0x00000000 then it will be pointing at 0x00000004 after adding 1 to it.

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edited Jun 20 '20 at 09:12

Community

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answered Jun 09 '15 at 21:42

haccks

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Wait, so would it be 00000000 00000000 00000000 00000100? – Jessica Jun 09 '15 at 21:46
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Yes. It would be `00000000 00000000 00000000 00000100`. – haccks Jun 09 '15 at 21:47

score 3 · Accepted Answer · edited Jun 09 '15 at 21:50

It depends on the type of pointer.

The pointer actually are all the same, as far as the actual data is concerned. An int* keeps the same kind of data as a char* (integer values representing memory locations). The reason why we care about the kind of data a pointer points to is, besides knowing how to decode it (i.e. dereference it), to know how to point to the next element of the same type. So, let p be a pointer to an object (object not as in oop, but as any variable that has some storage associated) of type T. Then, intptr_t(p + 1) = intptr_t(p) + sizeof(T).

This is particularly useful in working with arrays and, if you think a little bit, it makes sense. Consider the declaration int *p = 0x100 and assume that at address 100 is a valid integer value. What sense would it make to access the integer value from 0x101 (this is actually illegal, since ints must start on addresses divisible by 4 - considering sizeof(int) = 4)?

Thanks, that provided good intuition. – Jessica Jun 09 '15 at 21:57 — Jessica, Jun 09 '15 at 21:57

If you add 1 to a pointer, does it really add 1, or does it add 4 or 32?

2 Answers2