what is pointer past the end of an object means?

Question

In C++ Primer, Chapter 2, \"Variables and Basic Types\", it says:

it is possible for a pointer to an object and a pointer one past the end of a different

Answer 1

Suppose you have an array, int foo[5] = {1,2,3,4,5};. It's laid out in memory like this:

-----------
|1|2|3|4|5|
-----------

It's legal to have a pointer that points to any member of the array; but it's also legal for a pointer to point one past the end of the array (usually to signal that it has reached the end of the array when doing an STL algorithm using iterators) - although it isn't legal to dereference that pointer. Like so:

-------------
|1|2|3|4|5|?|
-------------
 ^         ^
 |         |
 p         q

p is a pointer into the array; q is a pointer one-past-the-end.

Now, suppose you also have an array const char bar[3] = "Hi";. It's possible that the two arrays have been allocated next to each other in memory, like so:

<--foo---> <-bar->
-----------------
|1|2|3|4|5|H|i|0|
-----------------
 ^         ^
 |         |
 p         q

Then q is both a one-past-the-end for foo, and also pointing to the physical location for bar[0].