Pointer Arithmetic

First, the binky video may help. It’s a nice video about pointers. For arithmetic, here is an example:

int * pa = NULL;
int * pb = NULL;
pa += 1; // pa++. behind the scenes, add sizeof(int) bytes
assert((pa - pb) == 1);

print_out(pa); // possibly outputs 0x4
print_out(pb); // possibly outputs 0x0 (if NULL is actually bit-wise 0x0)

(Note that incrementing a pointer that contains a null pointer value strictly is undefined behavior. We used NULL because we were only interested in the value of the pointer. Normally, only use increment/decrement when pointing to elements of an array).

The following shows two important concepts

  • addition/subtraction of a integer to a pointer means move the pointer forward / backward by N elements. So if an int is 4 bytes big, pa could contain 0x4 on our platform after having incremented by 1.
  • subtraction of a pointer by another pointer means getting their distance, measured by elements. So subtracting pb from pa will yield 1, since they have one element distance.

On a practical example. Suppose you write a function and people provide you with an start and end pointer (very common thing in C++):

void mutate_them(int *begin, int *end) {
    // get the amount of elements
    ptrdiff_t n = end - begin;
    // allocate space for n elements to do something...
    // then iterate. increment begin until it hits end
    while(begin != end) {
        // do something

ptrdiff_t is what is the type of (end – begin). It may be a synonym for “int” for some compiler, but may be another type for another one. One cannot know, so one chooses the generic typedef ptrdiff_t.

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