Adding an offset to a pointer

Question

If I have a pointer to an object and I want to get a pointer to an object that is say 16 bytes after the pointer how do I add the 16 byte offset to the pointer?

Also, memory addresses in 32bit systems look like this 0x00000000. If I change an address like 0x00000001 to 0x00000002 how many bytes are skipped?

Answer 1

Pointers count bytes, so to point at the next byte you would need to change a pointer's value by 1. Pointer arithmetic, however, counts the objects pointed to by the pointer, and incrementing a pointer increases its value by the size of its pointee type. If you want to point at bytes, use a char pointer, since char has size 1 by definition, and pointer arithmetic on char pointers is lets you point at bytes:

T * p  = get_pointer();

char * cp = reinterpret_cast<char*>(p);

cp += 16;

Casting pointers to and from char types does not constitute type punning and is explicitly allowed by the standard. However, you must not use the resulting pointer to access any objects that aren't actually at that address.

Answer 2

If I have a pointer to an object and I want to get a pointer to an object that is say 16 bytes after the pointer how do I add the 16 byte offset to the pointer?

Casting through char* will work, but this may be considered bad practice depending on the details of your scenario:

T *ptr = initialize_ptr(); // Do whatever initialization you need to.

ptr = (T*)(((char*)ptr) + 16);

Also, memory addresses in 32bit systems look like this 0x00000000. If I change an address like 0x00000001 to 0x00000002 how many bytes are skipped?

The difference between 2 and 1 is 1 -- exactly one byte would be skipped.

Answer 3

You would do this :

char *arr = (char*)((void*) ptrToSomeObject);
&arr[15]

Whats happening under the hood

1. Any ptr can be converted to 'void*'
2. 'void *' can be converted to any type of ptr.
3. arr[15] == (arr+15bytes)