This example shows how to use persistent class inheritance.

The example consists of the following files:

employee.hxx
  Header file defining the 'person' and 'employee' abstract persistent
  classes as well as the 'permanent_employee', 'temporary_employee', and
  'contractor' concrete persistent classes.

employee-odb.hxx
employee-odb.ixx
employee-odb.cxx
employee.sql
  The first three files contain the database support code and the last file
  contains the database schema for the employee.hxx header.

  These files are generated by the ODB compiler from employee.hxx using the
  following command line:

  odb -d <database> --generate-schema --generate-query employee.hxx

  Where <database> stands for the database system we are using, for example,
  'mysql'.

database.hxx
  Contains the create_database() function which instantiates the concrete
  database class corresponding to the database system we are using.

driver.cxx
  Driver for the example. It includes the employee.hxx and employee-odb.hxx
  headers to gain access to the persistent classes and their database support
  code. It also includes database.hxx for the create_database() function
  declaration.

  In main() the driver first calls create_database() to obtain the database
  instance. It then persists a number of permanent and temporary employee
  objects as well as a number of contractor objects. The next transaction
  looks up a contractor based on the email address. Finally, the driver
  performs a database query which uses a data member from the base class
  in its criterion.

To run the example we may first need to create the database schema (for some
database systems, such as SQLite, the schema is embedded into the generated
code which makes this step unnecessary). Using MySQL as an example, this
can be achieved with the following command:

mysql --user=odb_test --database=odb_test < employee.sql

Here we use 'odb_test' as the database login and also 'odb_test' as the
database name.

Once the database schema is ready, we can run the example (using MySQL as
the database):

./driver --user odb_test --database odb_test