int y;　　　// define y as an integer variable
y = 4;　　　// r-value 4 evaluates to 4, which is then assigned to l-value y
y = 2 + 5;　// r-value 2 + r-value 5 evaluates to r-value 7, which is then assigned to l-value y

int x;　　　// define x as an integer variable
x = y;　　　// l-value y evaluates to 7 (from before), which is then assigned to l-value x.
x = x;　　　// l-value x evaluates to 7, which is then assigned to l-value x (useless!)
x = x + 1;　// l-value x + r-value 1 evaluate to r-value 8, which is then assigned to l-value x.