Pre/Post Increment Pointers in C++

Question

*(p1++)

int array[10] = {1,2};
int *p1 = array;
*p1=24;
*p1= *(p1++);
for (int i : array)
    cout << i << " ";

Output is 24 24

*(++p1)

int array[10] = {1,2};
int *p1 = array;
*p1=24;
*p1= *(++p1);
for (int i : array)
    cout << i << " ";

Output is 24 2

It seems like this is the exact opposite of doing increment with values. Can someone explain what is going on here? Thanks!

Answer 1

There is an undefined behavior in

*p1 = *(p1++);

because, quoting §1.9/15:

If a side effect on a scalar object is unsequenced relative to either another side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined.

Side effect here is an increment of p1 and value computation is a computation of address using p1.

So you shouldn't rely on the exact outcome of your examples.

Answer 2

*p1= *(p1++);

This just doesn't make sense. The semantic meaning of this operation is different depending on which side of the = is evaluated first. So there's no way you can make any sense out of it.