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.
  • If P evaluates to a null pointer value and J evaluates to 0, the result is a null pointer value.
  • Otherwise, if P points to element x[i] of an array object x with n elements,85 the expressions P + J and J + P (where J has the value j) point to the (possibly-hypothetical) element if and the expression P - J points to the (possibly-hypothetical) element 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]).
  • If P and Q both evaluate to null pointer values, the result is 0.
  • Otherwise, if P and Q point to, respectively, elements x[i] and x[j] of the same array object x, the expression P - Q has the value .
  • Otherwise, the behavior is undefined.
    [Note
    :
    If the value is not in the range of representable values of type std::ptrdiff_­t, the behavior is undefined.
    end note
    ]
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 element x[n] for this purpose; see [basic.compound].