What the difference?

Question

Hi everyone! Tell someone please why cout<<x++++; give an error and cout<<(x-++--x); give output?

Answer 1

cout << x-++--x;

is actually interpreted as:

cout << x - (++(--x));

and

cout << x++++;

is simply not legit.

Answer 2

why not working (x++)++ as x+=2 after code string executed? simply not legit is not an answer i looking for here