View Compilation Error

0_0_21382521_13478.cpp: In function 'int main()':
0_0_21382521_13478.cpp:14:13: error: 'S1' cannot be used as a function
         S1(0);
             ^
0_0_21382521_13478.cpp:15:13: error: 'S2' cannot be used as a function
         S2(0);
             ^