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This example shows how to map persistent C++ classes to a custom database
schema. In particular, it shows how to map all the commonly-used constructs,
including containers, object relationships, and composite value types.

The example uses the shared_ptr smart pointer from TR1 and requires a C++ 
compiler with TR1 support or an external TR1 implementation, such as the
one provided by Boost.

The example consists of the following files:

employee.hxx
  Header file defining the 'employee' and 'employer' persistent classes
  as well as the 'name' composite value type. ODB pragmas are used to
  assign custom database tables to persistent classes as well as custom
  database types and columns to data members.

employee-odb.hxx
employee-odb.ixx
employee-odb.cxx 
  These files contain the database support code for the employee.hxx header
  and are generated by the ODB compiler from employee.hxx using the following
  command line:

  odb -d <database> --generate-query --default-pointer std::tr1::shared_ptr \
  employee.hxx

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

  The --default-pointer option is used to make TR1 shared_ptr the default
  object pointer.

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 programmatically creates the database schema by executing
  a series of SQL statements. After that the driver creates a number of
  'employee' and 'employer' objects, sets the relationships between them,
  and persists them in the database. Finally, the driver performs a database
  query and prints the information about the returned objects.

To run the driver, using MySQL as an example, we can execute the following
command:

./driver --user odb_test --database odb_test

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