This example shows how to use polymorphism inheritance with ODB. This
inheritance style is normally used to provide polymorphic behavior through
a common interface. The base class defines a number of virtual functions and,
normally, a virtual destructor while the derived classes provide specific
implementations of these virtual functions.

The other commonly used inheritance style is reuse inheritance. Refer to the
inheritance/reuse example for more information on this style.

The example consists of the following files:

employee.hxx
employee.cxx
  Header and source files defining the 'person' abstract polymorphic
  persistent class as well as the '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 employee and contractor objects
  via their common base (person). The next transaction loads these objects,
  also via their common base. Once loaded, the driver calls the print()
  virtual function on each of them. Next, the driver changes an employee
  from temporary to permanent and updates its state in the database, again
  using the base class interface. The driver then queries the database for
  all the person objects that have Doe as the last name. The result set of
  this query contains a mix of employee and contractor objects. The driver
  iterates over this result set and calls the print() virtual function for
  each object. Finally, the driver erases the state of the persistent
  objects from the database, again using the base class interface.

To compile and link the example manually from the command line we can use
the following commands (using  MySQL as an example; replace 'c++' with your
C++ compiler name):

c++ -c employee.cxx
c++ -c employee-odb.cxx
c++ -DDATABASE_MYSQL -c driver.cxx
c++ -o driver driver.o employee.o employee-odb.o -lodb-mysql -lodb

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