Pointer addition and element size

Question

At: http://www.fredosaurus.com/notes-cpp/arrayptr/26arraysaspointers.html

Under: Pointer addition and element size

There is the following code:

// Assume sizeof(int) is 4.
int b[100];  // b is an array of 100 ints.
int* p;      // p is a a pointer to an int.
p = b;       // Assigns address of first element of b. Ie, &b[0]
p = p + 1;   // Adds 4 to p (4 == 1 * sizeof(int)). Ie, &b[1]

How did "p" in the last line become "4"?

Thanks.

Answer 1

(I assume that you mean "1" in the last line, not "p")

Pointer arithmetic in both C and C++ is a logical addition, not a numeric addition. Adding one to a pointer means "produce a pointer to the object that comes in memory right after this one," which means that the compiler automatically scales up whatever you're incrementing the pointer with by the size of the object being pointed at. This prevents you from having a pointer into the middle of an object, or a misaligned pointer, or both.