/in/foo.cc: In function 'int main()':
/in/foo.cc:11:13: warning: 'num1' is used uninitialized in this function [-Wuninitialized]
  cout << 20 - num1 - num2 - num3 << endl;
          ~~~^~~~~~
/in/foo.cc:11:20: warning: 'num2' is used uninitialized in this function [-Wuninitialized]
  cout << 20 - num1 - num2 - num3 << endl;
          ~~~~~~~~~~^~~~~~
/in/foo.cc:11:29: warning: 'num3' is used uninitialized in this function [-Wuninitialized]
  cout << 20 - num1 - num2 - num3 << endl;
                             ^~~~