Is incrementing/decrementing or adding an integer value to a pointer that is not pointing to an element in a sequence Undefined Behavior?

Question

I know that pointers (to array element) and iterators can be incremented/decremented to walk a sequence of elements and can jump back-and-for elements in the sequence.

But what will happen if I increment a pointer to a single object or add to it an integer value? is it undefined behavior or it is OK but we cannot access that memory?

int x = 551;
int* p = &x;
++p;
--p;
std::cout << *p << '\n';

Because I've already read that we should not increment/decrement a pointer that doesn't point to an element in a sequence or an array for example.

So can someone explain what will happen and whether my example is OK (de-referencing pointer p)? Thank you!

Answer 1

when pointer arithmetic applies to a pointer that points to an object, the pointer is considered to point to an array of that object type with only one element, as said in the standard.

An object that is not an array element is considered to belong to a single-element array for this purpose

In you example, the pointer p as if it point to int arr[1] = {551} So, the corresponding operation is similar to apply to a pointer that point to the arr. That means, ++p will make p point to the element arr[1] (hypothetical), and --p will make p point to the first element arr[0] again. So, in the last, de-referenceing pointer p is OK and does not result in any UB.