Subtracting pointer from an integer

Question

    int *p = new int; 
    int a = 10; 
    cout << a+p;
    cout << a-p;

Printing a+p, p+a, p-a gives an address, but a-p gives me an error. Why is this so? Aren't pointer addresses integers? And if so, shouldn't a-p give me a negative value?

Answer 1

In terms of pointer arithmetic, pointers aren't equivalent of "integer representing some address". Even addition isn't that simple:

• You can't add pointer to pointer
• Writing p+1 doesn't mean "get address value from p variable and increase it by one". Actual address increment amount depends on size of pointed variable, so incrementing int64_t *p will give different result from incrementing int8_t *p

Why do we have such "strange behavior" for addition and subtraction of pointers? Simple answer: because it's defined that way in C++ standard:

5.7 Additive operators

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.

But there's a reason behind that restriction (which is actually describe on the same page of standard). Pointer arithmetic is intended for the following usage: if p points to some element in array, p+1 points to the next element, p-1 points to the previous and p1-p2 shows number of elements between p1 and p2. In these terms p1+p2 doesn't make any sense, the same for 1-p, so they are treated as syntax errors.