7 Expressions [expr]

7.6 Compound expressions [expr.compound]

7.6.6 Additive operators [expr.add]

The additive operators + and - group left-to-right.
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.
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]).
For addition or subtraction, if the expressions P or Q have type “pointer to cv T”, where T and the array element type are not similar, the behavior is undefined.
Note
:
In particular, a pointer to a base class cannot be used for pointer arithmetic when the array contains objects of a derived class type.
— end note
 ]
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 array element n for this purpose; see [basic.compound].