Strange behavior when adding values to pointers

Question

Well, I actually don't know how I should describe this:

uintptr_t * ptr = (uintptr_t *) 0x21CC9004;

std::cout << ptr + 0x10 << std::endl;
std::cout << std::hex << 0x21CC9004 + 0x10 << std::endl;

The first line yields 21CC9044 and the second line yields 21CC9014.

How is that? Can someone explain this and how I can add 0x10 to my pointer to get the result from the second line?

Pointer arithmetic is defined in terms of `sizeof(T)` units (where `T` is the referenced type), not in terms of bytes. — , Jan 26 '14 at 21:17
Also, `ptr + 0x10` is undefined behavior unless `(uintptr_t *) 0x21CC9004` is a valid pointer. — , Jan 26 '14 at 21:21
@h2so3 no, that is only superficially similar. At least find a good question and answer to mark something as a duplicate. — Yakk - Adam Nevraumont, Jan 26 '14 at 21:55

score 1 · Answer 1 · answered Jan 26 '14 at 21:20

1

In the first case the pointer arithmetic is used. It means that adding 0x10 actually adds 0x10 * sizeof( unsigned int ) to the value of the pointer that is equal to 0x40 provided that sizeof( unsigned int ) = 4.

In the second case there is the usual arithmetic of integer numbers.

answered Jan 26 '14 at 21:20

Vlad from Moscow

301,070
26
186
335

1

There's no need to post another answer. There are already many duplicates. – Jan 26 '14 at 21:20
Well that makes sense to me, is there any way to tell the compiler to treat ptr als integral number? any cast? – user3183815 Jan 26 '14 at 21:23
Them the question should be marked as duplicated. – Vlad from Moscow Jan 26 '14 at 21:23
1

@VladfromMoscow I am doing **just that.** Along with other users. – Jan 26 '14 at 21:26
@user3183815 It is better to use the pointer arithmetic instead of trying to cast pointer either to char * or to some integral type and then again to the type of the pointer. – Vlad from Moscow Jan 26 '14 at 21:26
@user3183815 Exactly, a typecast. But you really do not want to do that. – Jan 26 '14 at 21:26
And how about something like (ptr + (0x10 / sizeof(uintptr_t)))? Could you provide a example and tell me if I'm wrong with that? Thanks for your help! – user3183815 Jan 26 '14 at 21:33
Use the KISS principle. Instead of (ptr + (0x10 / sizeof(uintptr_t))) it would be simpler to write ptr + 4 – Vlad from Moscow Jan 26 '14 at 21:37

Strange behavior when adding values to pointers

1 Answers1