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This example shows how to use virtual data members to implement a persistent
class that employs the pimpl C++ idiom.

The example consists of the following files:

person.hxx
person.cxx
  Header and source file implementing the 'person' persistent class that
  uses the pimpl idiom.

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

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

  odb -d <database> --generate-query --generate-schema person.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 person.hxx and person-odb.hxx
  headers to gain access to the persistent class and its 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. Then it executes a number of database transactions on the 'person'
  objects.

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 < person.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