Take the address of a one-past-the-end array element via subscript: legal by the C++ Standard or not?

Question

I have seen it asserted several times now that the following code is not allowed by the C++ Standard:

int array[5];
int *array_begin = &array[0];
int *array_end = &array[5];

Is &array[5] legal C++ code in this context?

I would like an answer with a reference to the Standard if possible.

It would also be interesting to know if it meets the C standard. And if it isn't standard C++, why was the decision made to treat it differently from array + 5 or &array[4] + 1?

Answer 1

Yes, it's legal. From the C99 draft standard:

§6.5.2.1, paragraph 2:

A postfix expression followed by an expression in square brackets [] is a subscripted designation of an element of an array object. The definition of the subscript operator [] is that E1[E2] is identical to (*((E1)+(E2))). Because of the conversion rules that apply to the binary + operator, if E1 is an array object (equivalently, a pointer to the initial element of an array object) and E2 is an integer, E1[E2] designates the E2-th element of E1 (counting from zero).

§6.5.3.2, paragraph 3 (emphasis mine):

The unary & operator yields the address of its operand. If the operand has type ‘‘type’’, the result has type ‘‘pointer to type’’. If the operand is the result of a unary * operator, neither that operator nor the & operator is evaluated and the result is as if both were omitted, except that the constraints on the operators still apply and the result is not an lvalue. Similarly, if the operand is the result of a [] operator, neither the & operator nor the unary * that is implied by the [] is evaluated and the result is as if the & operator were removed and the [] operator were changed to a + operator. Otherwise, the result is a pointer to the object or function designated by its operand.

§6.5.6, paragraph 8:

When an expression that has integer type is added to or subtracted from a pointer, the result has the type of the pointer operand. If the pointer operand points to an element of an array object, and the array is large enough, the result points to an element offset from the original element such that the difference of the subscripts of the resulting and original array elements equals the integer expression. In other words, if the expression P points to the i-th element of an array object, the expressions (P)+N (equivalently, N+(P)) and (P)-N (where N has the value n) point to, respectively, the i+n-th and i−n-th elements of the array object, provided they exist. Moreover, if the expression P points to the last element of an array object, the expression (P)+1 points one past the last element of the array object, and if the expression Q points one past the last element of an array object, the expression (Q)-1 points to the last element of the array object. If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined. If the result points one past the last element of the array object, it shall not be used as the operand of a unary * operator that is evaluated.

Note that the standard explicitly allows pointers to point one element past the end of the array, provided that they are not dereferenced. By 6.5.2.1 and 6.5.3.2, the expression &array[5] is equivalent to &*(array + 5), which is equivalent to (array+5), which points one past the end of the array. This does not result in a dereference (by 6.5.3.2), so it is legal.