This example shows how to use object sections to implement lazy-loading
and change-updating of a subset of data members in a persistent class.

The example consists of the following files:

person.hxx
  Header file defining the 'person' persistent class. It contains a number of
  data members some of which are made to belong to a section.

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-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 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 'person' object, loads it back, including
  the section, and prints the contents of its members. Finally, the driver
  shows how to update the state of the section data members in various ways,
  then re-loads and prints the object to verify that the changes have been
  made persistent.

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 person-odb.cxx
c++ -DDATABASE_MYSQL -c driver.cxx
c++ -o driver driver.o person-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 < 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