Question 6.3

So what is meant by the ``equivalence of pointers and arrays'' in C?

Much of the confusion surrounding arrays and pointers in C can be traced to a misunderstanding of this statement. Saying that arrays and pointers are ``equivalent'' means neither that they are identical nor even interchangeable.

``Equivalence'' refers to the following key definition:

An lvalue of type array-of-T which appears in an expression decays (with three exceptions) into a pointer to its first element; the type of the resultant pointer is pointer-to-T.

(The exceptions are when the array is the operand of a sizeof or & operator, or is a string literal initializer for a character array.)

As a consequence of this definition, the compiler doesn't apply the array subscripting operator [] that differently to arrays and pointers, after all. In an expression of the form a[i], the array decays into a pointer, following the rule above, and is then subscripted just as would be a pointer variable in the expression p[i] (although the eventual memory accesses will be different, as explained in question 6.2). If you were to assign the array's address to the pointer:

	p = a;
then p[3] and a[3] would access the same element.

See also question 6.8.

References: K&R1 Sec. 5.3 pp. 93-6
K&R2 Sec. 5.3 p. 99
ANSI Sec., Sec., Sec. 3.3.6
ISO Sec., Sec., Sec. 6.3.6
H&S Sec. 5.4.1 p. 124

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