increment of pointer to array

Question

I'm trying to understand why the following C++ code does not compile

int main () {
  int a[10];
  int (*p)[10] = &a;
  int *q = static_cast<int *>(++p);
}

If it's not obvious, what I was trying to do is find a pointer to the end of the array by using pointer arithmetic.

My understanding so far is that p has type pointer to array of ten ints and so does the expression ++p. Normally, I can assign an array of ints to a variable of type pointer to int but this fails on the incremented pointer ++p.

I first tried without the static_cast but that didn't work either.

@GregKikola Yes, I could do that, but that's not what I want to do. I think the pointer arithmetic should be different in the two cases. — Sidious Lord
– Sidious Lord, Commented Jan 28, 2017 at 18:47
Oh, I misinterpreted, I didn't see the note about wanting q to point to the end of the array. In that case you could do int *q = a + 10;. Alternatively, with C++11, int* q = std::end(a); (just include <iterator>). — Greg Kikola
– Greg Kikola, Commented Jan 28, 2017 at 19:02
Even if you get this to work, the next person to look at this code will likely not understand it. I think (a+10) is much clearer. — brian beuning
– brian beuning, Commented Jan 28, 2017 at 19:15
@brianbeuning I'm not planning to use this in production code. I was just playing with pointers... — Sidious Lord
– Sidious Lord, Commented Jan 28, 2017 at 19:20

Sam Varshavchik · Accepted Answer · 2017-01-28 18:42:26Z

7

p has type pointer to array of ten ints

This is correct.

Normally, I can assign an array of ints to a variable of type pointer to int

This is also correct. Except, if you compare your two statements, you should see the obvious difference: p is not an array of ints, it is a pointer to an array of ints. Hence, the difference.

Now, if you dereference that pointer to an array, your result will be an expression whose type is an array (loosely speaking), and that can be decayed to a pointer:

int *q = static_cast<int *>(*++p);

answered Jan 28, 2017 at 18:42

Sam Varshavchik

119k6 gold badges109 silver badges164 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Sidious Lord Over a year ago

Thanks, that makes sense.

Sidious Lord Over a year ago

The weird thing is that if I print p and q I get the same thing. So there seems to be something weird about pointer arithmetic on pointers to arrays.

Greg Kikola Over a year ago

@SidiousLord p and q are different types, but they hold the same address, both pointing to the element just after the end of the array. p points to an array of 10 ints starting at that address, q points to an int at that same address.

Sidious Lord Over a year ago

@GregKikola I thing I found my error. To get what I wanted I need to use *p++ not *++p. Then p will point at the beginning of the array while q points to the end of the array.

Collectives™ on Stack Overflow

increment of pointer to array

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related