foo.cc: In function ‘bool cmp(cr, cr)’:
foo.cc:10:40: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
   10 |         return a.num<b.xb||a.num==b.num&&a.xb<b.xb;
      |                            ~~~~~~~~~~~~^~~~~~~~~~~
foo.cc: In function ‘int main()’:
foo.cc:21:15: error: lvalue required as decrement operand
   21 |         while(a--)
      |               ^
foo.cc:30:50: error: ‘v’ was not declared in this scope
   30 |                                         a[i].num=v;
      |                                                  ^