Pointer Arithmetic

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Pointer arithmetic is limited to addition, subtraction, and comparison. Arithmetical operations on object pointers of type "pointer to type" automatically take into account the size of type; that is, the number of bytes needed to store a type object.

The internal arithmetic performed on pointers depends on the memory model in force and the presence of any overriding pointer modifiers.

When performing arithmetic with pointers, it is assumed that the pointer points to an array of objects. Thus, if a pointer is declared to point to type, adding an integral value to the pointer advances the pointer by that number of objects of type. If type has size 10 bytes, then adding an integer 5 to a pointer to type advances the pointer 50 bytes in memory. The difference has as its value the number of array elements separating the two pointer values. For example, if ptr1 points to the third element of an array, and ptr2 points to the tenth element, then the result of ptr2 - ptr1 would be 7.

The difference between two pointers has meaning only if both pointers point into the same array

When an integral value is added to or subtracted from a "pointer to type," the result is also of type "pointer to type."

There is no such element as "one past the last element," of course, but a pointer is allowed to assume such a value. If P points to the last array element, P + 1 is legal, but P + 2 is undefined. If P points to one past the last array element, P - 1 is legal, giving a pointer to the last element. However, applying the indirection operator * to a "pointer to one past the last element" leads to undefined behavior.

Informally, you can think of P + n as advancing the pointer by (n * sizeof(type)) bytes, as long as the pointer remains within the legal range (first element to one beyond the last element).

Subtracting two pointers to elements of the same array object gives an integral value of type ptrdiff_t defined in stddef.h. This value represents the difference between the subscripts of the two referenced elements, provided it is in the range of ptrdiff_t. In the expression P1 - P2, where P1 and P2 are of type pointer to type (or pointer to qualified type), P1 and P2 must point to existing elements or to one past the last element. If P1 points to the i-th element, and P2 points to the j-th element, P1 - P2 has the value (i - j).