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 that has integral type is added to or subtracted from a pointer, the result has the type of the pointer operand. If the expression P points to element x[i] of an array object x with n elements86, the expressions P + J and J + P (where J has the value j) point to the (possibly-hypothetical) element x[i + j] if 0 ≤ i + j ≤ n; otherwise, the behavior is undefined. Likewise, the expression P - J points to the (possibly-hypothetical) element x[i - j] if 0 ≤ i - j ≤ n; otherwise, the behavior is undefined.
When two pointers to elements of the same array object 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 the expressions 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 i - j; otherwise, the behavior is undefined. [ Note: If the value i - j 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 ([conv.qual]), 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 ]
If the value 0 is added to or subtracted from a null pointer value, the result is a null pointer value. If two null pointer values are subtracted, the result compares equal to the value 0 converted to the type std::ptrdiff_t.
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].