Pointers and Arrays

It’s Only Memory

Pointers and arrays are closely related to each other

• Automatic conversion array ⟶ pointer

• Array-type function parameters are in fact pointers to the first (0-th) array element

• Index operator (a[i]) is pointer arithmetic

• True strength of C

int a[] = { 42, 1, 23 };
char str[] = { 'h', 'a', 'l', 'l', 'o', '\0' };

int *pa1 = &a[0];
int *pa2 = a;

Conversion Array ⟶ Pointer

int a[3]; int *pa1 = &a[0]; int *pa2 = a;

Pointer Arithmetic (1)

Pointer and array index Pointer + Integer = Pointer Equivalent to subscript (“index”) operator Just like subscript there is no range check being made ⟶ Errors happen But: performance!

Pointer Arithmetic (2)

Pointer increment int *pa = a; ++pa;
Pointer decrement int *pa = &a[1]; --pa;

Pointer Arithmetic (3)

Pointers don’t necessarily point to something that is valid …

pa = a + 4; // or "pa += 4" pa -= 2; i = *pa; /* ok */
pa = a - 1; pa += 2; i = *pa; /* ok */

Pointer Arithmetic: Difference

How many elements are there between two pointers?

p = &a[0]; q = &a[3]; // *one past* last valid element num = q - p; /* 2 */

Often (C++ STL) it is done like so:

• Beginning of an array (“a set of elements”) is the pointer to the first element

• End is pointer to one past the last valid element

Pointer Arithmetic: Array Algorithms

Iterating over all elements of the array

int sum(const int *begin, const int *end)
{
    int sum = 0;

    while (begin < end)
        sum += *begin++; /* precedence? what? */
    return sum;
}

Beautiful, isn’t it?

Pointer Arithmetic: Jump Width? (1)

So far: pointer to int - how are arrays of other (even compound) types handled?

⟶ just the same!

• Pointer + n: points n elements further

• Type system is not stupid (only sometimes)

• Pointer know which type is being pointed to

• Be careful with void and void*: sizeof(void) is undefined!

Pointer Arithmetic: Jump Width? (2)

struct point
{
    int x, y;
};

struct point points[3], *begin, *end;

begin = points;
end = points + sizeof(points)/sizeof(struct point);

while (begin < end) {
    ...
    ++begin;
}

Pointer Arithmetic: Arbitrary Datatypes

sizeof: size (in bytes) of a type/variable sizeof(int) sizeof(struct point) sizeof(i) sizeof(pi) sizeof(pp)