The
usual arithmetic conversions are performed for operands of arithmetic or
enumeration type.

additive-expression:multiplicative-expressionadditive-expression+multiplicative-expressionadditive-expression-multiplicative-expression

For addition, either both operands shall have arithmetic or unscoped enumeration
type, or one operand shall be a pointer to a completely-defined object
type and the other shall have integral or unscoped enumeration type.

For subtraction, one of the following shall hold:

- both operands are pointers to cv-qualified or cv-unqualified versions of the same completely-defined object type; or
- the left operand is a pointer to a completely-defined object type and the right operand has integral or unscoped enumeration type.

The result of the binary + operator is the sum of the operands.

The result of the binary - operator is the difference resulting
from the subtraction of the second operand from the first.

When an expression J that has integral type
is added to or subtracted from an expression P of pointer type,
the result has the type of P.

- Otherwise, the behavior is undefined.

When two pointer expressions P and Q are subtracted,
the type of the result is an implementation-defined signed
integral type; this type shall be the same type that is defined as
std::ptrdiff_t in the <cstddef>
header ([support.types]).

An object that is not an array element is considered to belong to a
single-element array for this purpose; see [expr.unary.op].

A pointer past the last element of an array x of n elements
is considered to be equivalent to a pointer to a hypothetical element
x[n] for this purpose; see [basic.compound].