Why is i = v[i++] undefined?

Question

From the C++ (C++11) standard, §1.9.15 which discusses ordering of evaluation, is the following code example:

void g(int i, int* v) {
    i = v[i++]; // the beha         


        

Answer 1

The reason is not just historical. Example:

int f(int& i0, int& i1) {
    return i0 + i1++;
}

Now, what happens with this call:

int i = 3;
int j = f(i, i);

It's certainly possible to put requirements on the code in f so that the result of this call is well defined (Java does this), but C and C++ don't impose constraints; this gives more freedom to optimizers.