Is this example causing an undefined behaviour?

Question

Possible Duplicate:
Undefined Behavior and Sequence Points

The variable i is changed twice, but is the next example going to cause an undefined behaviour?

#include <iostream>
int main()
{
    int i = 5;
    std::cout << "before i=" << i << std::endl;
    ++ i %= 4;
    std::cout << "after i=" << i << std::endl;
}

The output I get is :

before i=5
after i=2

Answer 1

Yes, it's undefined. There is no sequence point on assignment, % or ++ And you cannot change a variable more than once within a sequence point.

A compiler could evaluate this as:

++i;
i = i % 4;

or

i = i % 4;
++i;

(or something else)