C++ Pointers Pointer Arithmetic


Increment / Decrement

A pointer can be incremented or decremented (prefix and postfix). Incrementing a pointer advances the pointer value to the element in the array one element past the currently pointed to element. Decrementing a pointer moves it to the previous element in the array.

Pointer arithmetic is not permitted if the type that the pointer points to is not complete. void is always an incomplete type.

char* str = new char[10]; // str = 0x010
++str;                    // str = 0x011  in this case sizeof(char) = 1 byte

int* arr = new int[10];   // arr = 0x00100
++arr;                    // arr = 0x00104 if sizeof(int) = 4 bytes

void* ptr = (void*)new char[10];
++ptr;    // void is incomplete.

If a pointer to the end element is incremented, then the pointer points to one element past the end of the array. Such a pointer cannot be dereferenced, but it can be decremented.

Incrementing a pointer to the one-past-the-end element in the array, or decrementing a pointer to the first element in an array yields undefined behavior.

A pointer to a non-array object can be treated, for the purposes of pointer arithmetic, as though it were an array of size 1.

Addition / Subtraction

Integer values can be added to pointers; they act as incrementing, but by a specific number rather than by 1. Integer values can be subtracted from pointers as well, acting as pointer decrementing. As with incrementing/decrementing, the pointer must point to a complete type.

char* str = new char[10];  // str = 0x010
str += 2;                  // str = 0x010 + 2 * sizeof(char) = 0x012

int* arr = new int[10];    // arr = 0x100
arr += 2;                  // arr = 0x100 + 2 * sizeof(int) = 0x108, assuming sizeof(int) == 4.

Pointer Differencing

The difference between two pointers to the same type can be computed. The two pointers must be within the same array object; otherwise undefined behavior results.

Given two pointers P and Q in the same array, if P is the ith element in the array, and Q is the jth element, then P - Q shall be i - j. The type of the result is std::ptrdiff_t, from <cstddef>.

char* start = new char[10];  // str = 0x010
char* test = &start[5];
std::ptrdiff_t diff = test - start; //Equal to 5.
std::ptrdiff_t diff = start - test; //Equal to -5; ptrdiff_t is signed.