foo.cc: In function 'int main()':
foo.cc:10:7: warning: statement has no effect [-Wunused-value]
   cout;
       ^
foo.cc:8:13: warning: unused variable 'r' [-Wunused-variable]
  int x0, y, r;
             ^
foo.cc:9:6: warning: 'x0' is used uninitialized in this function [-Wuninitialized]
  if(g(x0,y)==0)
     ~^~~~~~
foo.cc:9:6: warning: 'y' is used uninitialized in this function [-Wuninitialized]