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C. Object Aggregation/Composition Functions| Warning | This extension is
EXPERIMENTAL. The behaviour of this extension --
including the names of its functions and anything else documented
about this extension -- may change without notice in a future release of PHP.
Use this extension at your own risk. |
In Object Oriented Programming, it is common to see the composition of
simple classes (and/or instances) into a more complex one. This is a
flexible strategy for building complicated objects and object
hierarchies and can function as a dynamic alternative to multiple
inheritance. There are two ways to perform class (and/or object)
composition depending on the relationship between the composed
elements: Association and
Aggregation.
An Association is a composition of independently constructed and
externally visible parts. When we associate classes or objects, each
one keeps a reference to the ones it is associated with. When we
associate classes statically, one class will contain a reference to an
instance of the other class. For example:
Example 1. Class association <?php
class DateTime {
function DateTime()
{
// empty constructor
}
function now()
{
return date("Y-m-d H:i:s");
}
}
class Report {
var $_dt;
// more properties ...
function Report()
{
$this->_dt = new DateTime();
// initialization code ...
}
function generateReport()
{
$dateTime = $this->_dt->now();
// more code ...
}
// more methods ...
}
$rep = new Report();
?> |
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We can also associate instances at runtime by passing a reference in a
constructor (or any other method), which allow us to dynamically change
the association relationship between objects. We will modify the example
above to illustrate this point:
Example 2. Object association <?php
class DateTime {
// same as previous example
}
class DateTimePlus {
var $_format;
function DateTimePlus($format="Y-m-d H:i:s")
{
$this->_format = $format;
}
function now()
{
return date($this->_format);
}
}
class Report {
var $_dt; // we'll keep the reference to DateTime here
// more properties ...
function Report()
{
// do some initialization
}
function setDateTime(&$dt)
{
$this->_dt =& $dt;
}
function generateReport()
{
$dateTime = $this->_dt->now();
// more code ...
}
// more methods ...
}
$rep = new Report();
$dt = new DateTime();
$dtp = new DateTimePlus("l, F j, Y (h:i:s a, T)");
// generate report with simple date for web display
$rep->setDateTime(&$dt);
echo $rep->generateReport();
// later on in the code ...
// generate report with fancy date
$rep->setDateTime(&$dtp);
$output = $rep->generateReport();
// save $output in database
// ... etc ...
?> |
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Aggregation, on the other hand, implies
encapsulation (hidding) of the
parts of the composition. We can aggregate classes by using a (static)
inner class (PHP does not yet support inner classes), in this case the
aggregated class definition is not accessible, except through the class
that contains it. The aggregation of instances (object aggregation)
involves the dynamic creation of subobjects inside an object, in the
process, expanding the properties and methods of that object.
Object aggregation is a natural way of representing a whole-part relationship,
(for example, molecules are aggregates of atoms), or can be used to
obtain an effect equivalent to multiple inheritance, without having to
permanently bind a subclass to two or more parent classes and their
interfaces. In fact object aggregation can be more flexible, in which we
can select what methods or properties to "inherit" in the aggregated
object.
We define 3 classes, each implementing a different storage method:
Example 3. storage_classes.inc <?php
class FileStorage {
var $data;
function FileStorage($data)
{
$this->data = $data;
}
function write($name)
{
$fp = fopen(name, "w");
fwrite($fp, $this->data);
fclose($data);
}
}
class WDDXStorage {
var $data;
var $version = "1.0";
var $_id; // "private" variable
function WDDXStorage($data)
{
$this->data = $data;
$this->_id = $this->_genID();
}
function store()
{
if ($this->_id) {
$pid = wddx_packet_start($this->_id);
wddx_add_vars($pid, "this->data");
$packet = wddx_packet_end($pid);
} else {
$packet = wddx_serialize_value($this->data);
}
$dbh = dba_open("varstore", "w", "gdbm");
dba_insert(md5(uniqid("", true)), $packet, $dbh);
dba_close($dbh);
}
// a private method
function _genID()
{
return md5(uniqid(rand(), true));
}
}
class DBStorage {
var $data;
var $dbtype = "mysql";
function DBStorage($data)
{
$this->data = $data;
}
function save()
{
$dbh = mysql_connect();
mysql_select_db("storage", $dbh);
$serdata = serialize($this->data);
mysql_query("insert into vars ('$serdata',now())", $dbh);
mysql_close($dbh);
}
}
?> |
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We then instantiate a couple of objects from the defined classes, and
perform some aggregations and deaggregations, printing some object information
along the way:
Example 4. test_aggregation.php <?php
include "storageclasses.inc";
// some utilty functions
function p_arr($arr)
{
foreach ($arr as $k => $v)
$out[] = "\t$k => $v";
return implode("\n", $out);
}
function object_info($obj)
{
$out[] = "Class: " . get_class($obj);
foreach (get_object_vars($obj) as $var=>$val) {
if (is_array($val)) {
$out[] = "property: $var (array)\n" . p_arr($val);
} else {
$out[] = "property: $var = $val";
}
}
foreach (get_class_methods($obj) as $method) {
$out[] = "method: $method";
}
return implode("\n", $out);
}
$data = array(M_PI, "kludge != cruft");
// we create some basic objects
$fs = new FileStorage($data);
$ws = new WDDXStorage($data);
// print information on the objects
echo "\$fs object\n";
echo object_info($fs) . "\n";
echo "\n\$ws object\n";
echo object_info($ws) . "\n";
// do some aggregation
echo "\nLet's aggregate \$fs to the WDDXStorage class\n";
aggregate($fs, "WDDXStorage");
echo "\$fs object\n";
echo object_info($fs) . "\n";
echo "\nNow let us aggregate it to the DBStorage class\n";
aggregate($fs, "DBStorage");
echo "\$fs object\n";
echo object_info($fs) . "\n";
echo "\nAnd finally deaggregate WDDXStorage\n";
deaggregate($fs, "WDDXStorage");
echo "\$fs object\n";
echo object_info($fs) . "\n";
?> |
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We will now consider the output to understand some of the side-effects
and limitation of object aggregation in PHP.
First, the newly created $fs and $ws
objects give the expected output (according to their respective class
declaration). Note that for the purposes of object aggregation,
private elements of a class/object begin with an underscore
character ("_"), even though there is not real distinction between
public and private class/object elements in PHP.
We then aggregate $fs with the
WDDXStorage class, and print out the object
information. We can see now that even though nominally the
$fs object is still of
FileStorage, it now has the property
$version, and the method store(),
both defined in WDDXStorage. One important thing
to note is that it has not aggregated the private elements defined in
the class, which are present in the $ws object. Also
absent is the constructor from WDDXStorage, which
will not be logical to aggegate.
The process of aggregation is cumulative, so when we aggregate
$fs with the class DBStorage,
generating an object that can use the storage methods of all the
defined classes.
Finally, the same way we aggregated properties and methods dynamically,
we can also deaggregate them from the object. So, if we deaggregate the
class WDDXStorage from $fs, we
will obtain:
One point that we have not mentioned above, is that the process of
aggregation will not override existing properties or methods in the
objects. For example, the class FileStorage defines a
$data property, and the class
WDDXStorage also defines a similar property
which will not override the one in the object acquired during
instantiation from the class FileStorage.
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nsapi_virtual
