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

additive-expression: multiplicative-expression additive-expression + multiplicative-expression additive-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 have arithmetic or unscoped enumeration type; or
- 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, if P points to an array element i of an array object x with n elements ([dcl.array]),77 the expressions P + J and J + P (where J has the value j) point to the (possibly-hypothetical) array element of x if and the expression P - J points to the (possibly-hypothetical) array element of x if .
- 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.layout]).

As specified in [basic.compound],
an object that is not an array element
is considered to belong to a single-element array for this purpose and
a pointer past the last element of an array of n elements
is considered to be equivalent to a pointer to a hypothetical array element
n for this purpose.

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