7 Expressions [expr]

7.6 Compound expressions [expr.compound]

7.6.6 Additive operators [expr.add]

1
#
The additive operators + and - group left-to-right.
The usual arithmetic conversions are performed for operands of arithmetic or enumeration type.
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.
2
#
For subtraction, one of the following shall hold:
  • (2.1)
    both operands have arithmetic or unscoped enumeration type; or
  • (2.2)
    both operands are pointers to cv-qualified or cv-unqualified versions of the same completely-defined object type; or
  • (2.3)
    the left operand is a pointer to a completely-defined object type and the right operand has integral or unscoped enumeration type.
3
#
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.
4
#
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.
  • (4.1)
    If P evaluates to a null pointer value and J evaluates to 0, the result is a null pointer value.
  • (4.2)
    Otherwise, if P points to an array element i of an array object x with n elements ([dcl.array]),68 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 .
  • (4.3)
    Otherwise, the behavior is undefined.
[Note 1: 
Adding a value other than 0 or 1 to a pointer to a base class subobject, a member subobject, or a complete object results in undefined behavior.
— end note]
5
#
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]).
  • (5.1)
    If P and Q both evaluate to null pointer values, the result is 0.
  • (5.2)
    Otherwise, if P and Q point to, respectively, array elements i and j of the same array object x, the expression P - Q has the value .
  • (5.3)
    Otherwise, the behavior is undefined.
    [Note 2: 
    If the value is not in the range of representable values of type std​::​ptrdiff_t, the behavior is undefined.
    — end note]
6
#
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.
[Example 1: int arr[5] = {1, 2, 3, 4, 5}; unsigned int *p = reinterpret_cast<unsigned int*>(arr + 1); unsigned int k = *p; // OK, value of k is 2 ([conv.lval]) unsigned int *q = p + 1; // undefined behavior: p points to an int, not an unsigned int object — end example]
68)68)
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.