CodeIgniter底层数据库类继承关系

1、CI_DB_mysql_driver 继承 CI_DB,  CI_DB这个类是不存在的，每次调用文件中~/CodeIgniter_2.1.3/system/database/DB.php函数&DB来初始化database的时候，

根据用户是否开启active_record，生成继承CI_DB_active_record或CI_DB_driver类的CI_DB类，

2、CI_DB_active_record类继承CI_DB_driver类

    require_once(BASEPATH.'database/DB_driver.php');

    if ( ! isset($active_record) OR $active_record == TRUE)
    {
        require_once(BASEPATH.'database/DB_active_rec.php');

        if ( ! class_exists('CI_DB'))
        {
            eval('class CI_DB extends CI_DB_active_record { }');
        }
    }
    else
    {
        if ( ! class_exists('CI_DB'))
        {
            eval('class CI_DB extends CI_DB_driver { }');
        }
    }

3、最终会调用顶级父类CI_DB_driver的构造方法，将数据库配置的各字段赋值到CI_DB_driver类的各个字段上，

$active_group = 'default';
$active_record = TRUE;

$db['default']['hostname'] = 'localhost';
$db['default']['username'] = '';
$db['default']['password'] = '';
$db['default']['database'] = '';
$db['default']['dbdriver'] = 'mysql';
$db['default']['dbprefix'] = '';
$db['default']['pconnect'] = TRUE;
$db['default']['db_debug'] = TRUE;
$db['default']['cache_on'] = FALSE;
$db['default']['cachedir'] = '';
$db['default']['char_set'] = 'utf8';
$db['default']['dbcollat'] = 'utf8_general_ci';
$db['default']['swap_pre'] = '';
$db['default']['autoinit'] = TRUE;

        var $username;
    var $password;
    var $hostname;
    var $database;
    var $dbdriver        = 'mysql';
    var $dbprefix        = '';
    var $char_set        = 'utf8';
    var $dbcollat        = 'utf8_general_ci';
    var $autoinit        = TRUE;
    var $port            = '';
    var $pconnect        = FALSE;
    var $db_debug        = FALSE;    

如果autoinit自动初始化为TRUE，则将DB初始化，这个DB对象是CI_DB_mysql_driver类

    require_once(BASEPATH.'database/DB_driver.php');

    if ( ! isset($active_record) OR $active_record == TRUE)
    {
        require_once(BASEPATH.'database/DB_active_rec.php');

        if ( ! class_exists('CI_DB'))
        {
            eval('class CI_DB extends CI_DB_active_record { }');
        }
    }
    else
    {
        if ( ! class_exists('CI_DB'))
        {
            eval('class CI_DB extends CI_DB_driver { }');
        }
    }

    require_once(BASEPATH.'database/drivers/'.$params['dbdriver'].'/'.$params['dbdriver'].'_driver.php');

    // Instantiate the DB adapter
    $driver = 'CI_DB_'.$params['dbdriver'].'_driver';
    $DB = new $driver($params);
    if ($DB->autoinit == TRUE)
    {
        $DB->initialize();
    }

    return $DB;