7.5 Postincrement and Postdecrement

Using ‘++’ or ‘--’ after an lvalue does something peculiar: it gets the value directly out of the lvalue and then increments or decrement it. Thus, the value of i++ is the same as the value of i, but i++ also increments i “a little later.” This is called postincrement or postdecrement.

For example,

#include <stdio.h>   /* Declares printf. */

int
main (void)
{
  int i = 5;
  printf ("%d\n", i);
  printf ("%d\n", i++);
  printf ("%d\n", i);
  return 0;
}

prints lines containing 5, again 5, and 6. The expression i++ has the value 5, which is the value of i at the time, but it increments i from 5 to 6 just a little later.

How much later is “just a little later”? That is flexible. The increment has to happen by the next sequence point. In simple cases, that means by the end of the statement. See Sequence Points.

If a unary operator precedes a postincrement or postincrement expression, the increment nests inside:

-a++   is equivalent to   -(a++)

That’s the only order that makes sense; -a is not an lvalue, so it can’t be incremented.