You have now successfully created your new UDF shared extension. To activate it you have to load the extension into a mysql server. This is done using CREATE FUNCTION ...

When you open the generated code file x3.cpp you'll see a comment block at the top showing the CREATE statements needed to install all functions provided by this extension. 

/*
register the functions provided by this UDF module using
CREATE FUNCTION ex_sqr RETURNS REAL SONAME "example.so";

register the functions provided by this UDF module using
DROP FUNCTION ex_sqr;
*/

mysql> CREATE FUNCTION ex_sqr RETURNS REAL SONAME "example.so";
Query OK, 0 rows affected (0.00 sec)

mysql> select ex_sqr(3);
+-----------+
| ex_sqr(3) |
+-----------+
|         9 |
+-----------+
1 row in set (0.01 sec)

mysql> DROP FUNCTION ex_sqr;  
Query OK, 0 rows affected (0.01 sec)