Is there any difference between List x; and List x();?

A big difference!

Suppose that List is the name of some class. Then function f() declares a localList object called x:

 

 void f()
 {
   List x;     // Local object named x (of class List)
   ...
 }

But function g() declares a function called x() that returns aList:

 

 void g()
 {
   List x();   // Function named x (that returns a List)
   ...
 }