Library Version 18.1.40

Db
Prev | Chapter 2. The Db Handle | Next

## Db

    #include <db_cxx.h>

    class Db {
    public:
            Db(DbEnv *dbenv, u_int32_t flags);
            ~Db();

             DB *Db::get_DB();
             const DB *Db::get_const_DB() const;
             static Db *Db::get_Db(DB *db);
             static const Db *Db::get_const_Db(const DB *db);
             ...
    };

The Db handle is the handle for a Berkeley DB database, which may or may not be part of a database environment.

Db handles are free-threaded if the DB_THREAD flag is specified to the Db::open() method when the database is opened or if the database environment in which the database is opened is free-threaded. The handle should not be closed while any other handle that refers to the database is in use; for example, database handles must not be closed while cursor handles into the database remain open, or transactions that include operations on the database have not yet been committed or aborted. Once the Db::close() , Db::remove() , Db::rename() , or Db::verify() methods are called, the handle may not be accessed again, regardless of the method's return.

The constructor creates a Db object that is the handle for a Berkeley DB database. The constructor allocates memory internally; calling the Db::close() , Db::remove() , or Db::rename() methods will free that memory.

Note that destroying the Db object is synonomous with calling `Db::close(0)`.

Each `Db` object has an associated `DB` struct, which is used by the underlying implementation of Berkeley DB and its C-language API. The `Db::get_DB()` method returns a pointer to this struct. Given a `const Db` object, `Db::get_const_DB()` returns a const pointer to the same struct.

Given a `DB` struct, the `Db::get_Db()` method returns the corresponding `Db` object, if there is one. If the `DB` object was not associated with a `Db` (that is, it was not returned from a call to the `Db::get_DB()` method), then the result of `Db::get_Db()` is undefined. Given a `const DB struct`, `Db::get_const_Db()` returns the associated `const Db`object, if there is one.

These methods may be useful for Berkeley DB applications including both C and C++ language software. It should not be necessary to use these calls in a purely C++ application.

### Parameters

#### dbenv

If no dbenv value is specified, the database is standalone; that is, it is not part of any Berkeley DB environment. Since it is not part of any environment, any DB_CONFIG file in the application’s current working directory is ignored.

If a dbenv value is specified, the database is created within the specified Berkeley DB environment. The database access methods automatically make calls to the other subsystems in Berkeley DB, based on the enclosing environment. For example, if the environment has been configured to use locking, the access methods will automatically acquire the correct locks when reading and writing pages of the database.

#### flags

The flags parameter must be set to 0 or the following value:

- `DB_CXX_NO_EXCEPTION`

  The Berkeley DB C++ API supports two different error behaviors. By default, whenever an error occurs, an exception is thrown that encapsulates the error information. This generally allows for cleaner logic for transaction processing because a try block can surround a single transaction. However, if this flag is specified, exceptions are not thrown; instead, each individual function returns an error code.

  If a dbenv value is specified, this flag is ignored, and the error behavior of the specified environment is used instead.

### Class

Db

### See Also

Database and Related Methods

Prev | Up | Next
db_copy | Home | Db::del()