/in/foo.cc: In function 'int main()':
/in/foo.cc:6:14: error: 'strlen' was not declared in this scope
    6 |   int lena = strlen(a);
      |              ^~~~~~
/in/foo.cc:2:1: note: 'strlen' is defined in header '<cstring>'; did you forget to '#include <cstring>'?
    1 | #include<iostream>
  +++ |+#include <cstring>
    2 | using namespace std;