Pointer Arithmetic

Since pointers are just integers used as addresses of other objects in memory, D provides a set of features for performing arithmetic on pointers. However, pointer arithmetic is not identical to integer arithmetic. Pointer arithmetic implicitly adjusts the underlying address by multiplying or dividing the operands by the size of the type referenced by the pointer. The following D fragment illustrates this property:

int *x;

BEGIN
{
    trace(x);
    trace(x + 1);
    trace(x + 2);
}

This fragment creates an integer pointer x and then trace its value, its value incremented by one, and its value incremented by two. If you create and execute this program, DTrace reports the integer values 0, 4, and 8.

Since x is a pointer to an int (size 4 bytes), incrementing x adds 4 to the underlying pointer value. This property is useful when using pointers to refer to consecutive storage locations such as arrays. For example, if x were assigned to the address of an array a like the one shown in Figure 5-2, the expression x + 1 would be equivalent to the expression &a[1]. Similarly, the expression *(x + 1) would refer to the value a[1]. Pointer arithmetic is implemented by the D compiler whenever a pointer value is incremented using the +=, +, or ++ operators.

Pointer arithmetic is also applied when an integer is subtracted from a pointer on the left-hand side, when a pointer is subtracted from another pointer, or when the -- operator is applied to a pointer. For example, the following D program would trace the result 2:

int *x, *y;
int a[5];

BEGIN
{
    x = &a[0];
    y = &a[2];
    trace(y - x);
}