1. Mapping

The mapping of C++ classes to database tables is accomplished through a set of custom #pragma directives that are placed in the C++ header file. They allow the application developer to control various aspects of the mapping such as table and column names, C++ types to SQL types mapping, etc. The following example illustrates a typical persistent class declaration:

#pragma db object table("people") class person { ... private: friend class odb::access; person (); #pragma db id auto unsigned long id_; string first_; string last_; #pragma type("INT UNSIGNED") unsigned short age_; };

The ODB-specific pragmas can also be placed outside of the class declaration.

2. Persistence

ODB provides an object-oriented database API that allows the application developer to perform various operations on persistent objects. The following code fragment illustrates the use of the most common operations:

unsigned long joe_id = 1; person john ("John", "Doe", 31); person jane ("Jane", "Doe", 29); transaction t (db.begin ()); // Persist in the database. // db.persist (john); db.persist (jane); // Load from the database. // auto_ptr<person> joe (db.load<person> (joe_id)); // Update in the database. // jane.age (jane.age () + 1); db.update (jane); // Delete from the database. // db.erase (jane); t.commit ();

3. Query Language

ODB provides an object-oriented query language, called ODB Query Language, which is integrated into C++ allowing the application developer to write expressive and type-safe queries that look and feel like ordinary C++. For example:

typedef odb::query<person> query; typedef odb::result<person> result; transaction t (db.begin ()); result r (db.query<person> (query::last == "Doe" && query::age < 30)); for (result::iterator i (r.begin ()); i != r.end (); ++i) { cout << "Hello, " << i->first () << endl; } t.commit ();

Query parameters can be bound to C++ variables either by value or by reference. If desired, native SQL queries can be executed in which case ODB still provides support for parameter binding. ODB also supports prepared queries which are a thin wrapper around the underlying database system's prepared statements functionality. Prepared queries provide a way to perform potentially expensive query preparation tasks only once and then execute the query multiple times.

4. Views

Besides persistent objects, ODB also has the notion of views. An ODB view is a C++ class that embodies a light-weight, read-only projection of one or more persistent objects or database tables or the result of a native SQL query execution.

Some of the common applications of views include loading a subset of data members from objects or columns from database tables, executing and handling results of arbitrary SQL queries, including aggregate queries, as well as joining multiple objects and/or database tables using object relationships or custom join conditions.

Many relational databases also define the concept of views. However, ODB views are not mapped to database views. Rather, by default, an ODB view is mapped to an SQL SELECT query. But, if desired, it is possible to create an ODB view that is based on a relational database view.

As an example, consider a view that returns the number of people stored in the database:

#pragma db view object(person) struct person_count { #pragma db column("count(" + person::id_ + ")") std::size_t count; };

The following code fragment shows how we can use this view to find out how many people are younger than 30:

typedef odb::query<person_count> query; typedef odb::result<person_count> result; transaction t (db.begin ()); result r (db.query<person_count> (query::age < 30)); const person_count& c (*r.begin ()); cout << c.count << endl; t.commit ();

ODB views can be defined in terms of one or more persistent objects, database tables, a combination of the two, or as a native SQL query.