Creating references within the constructor can lead to confusing
results. This tutorial-like section helps you to avoid problems.
<?php
class Foo {
function Foo($name) {
// create a reference inside the global array $globalref
global $globalref;
$globalref[] = &$this;
// set name to passed value
$this->setName($name);
// and put it out
$this->echoName();
}
function echoName() {
echo "<br />", $this->name;
}
function setName($name) {
$this->name = $name;
}
}
?>
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Let us check out if there is a difference between
$bar1 which has been created using
the copy = operator and
$bar2 which has been created using
the reference =& operator...
<?php
$bar1 = new Foo('set in constructor');
$bar1->echoName();
$globalref[0]->echoName();
/* output:
set in constructor
set in constructor
set in constructor */
$bar2 =& new Foo('set in constructor');
$bar2->echoName();
$globalref[1]->echoName();
/* output:
set in constructor
set in constructor
set in constructor */
?>
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Apparently there is no difference, but in fact there is a
very significant one: $bar1 and
$globalref[0] are _NOT_ referenced, they
are NOT the same variable. This is because "new" does not
return a reference by default, instead it returns a copy.
Замечание:
There is no performance loss (since PHP 4 and up use reference
counting) returning copies instead of references. On the
contrary it is most often better to simply work with copies
instead of references, because creating references takes some
time where creating copies virtually takes no time (unless none
of them is a large array or object and one of them gets changed
and the other(s) one(s) subsequently, then it would be wise to
use references to change them all concurrently).
To prove what is written above let us watch the code below.
<?php
// now we will change the name. what do you expect?
// you could expect that both $bar1 and $globalref[0] change their names...
$bar1->setName('set from outside');
// as mentioned before this is not the case.
$bar1->echoName();
$globalref[0]->echoName();
/* output:
set from outside
set in constructor */
// let us see what is different with $bar2 and $globalref[1]
$bar2->setName('set from outside');
// luckily they are not only equal, they are the same variable
// thus $bar2->name and $globalref[1]->name are the same too
$bar2->echoName();
$globalref[1]->echoName();
/* output:
set from outside
set from outside */
?>
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Another final example, try to understand it.
<?php
class A {
function A($i) {
$this->value = $i;
// try to figure out why we do not need a reference here
$this->b = new B($this);
}
function createRef() {
$this->c = new B($this);
}
function echoValue() {
echo "<br />","class ",get_class($this),': ',$this->value;
}
}
class B {
function B(&$a) {
$this->a = &$a;
}
function echoValue() {
echo "<br />","class ",get_class($this),': ',$this->a->value;
}
}
// try to understand why using a simple copy here would yield
// in an undesired result in the *-marked line
$a =& new A(10);
$a->createRef();
$a->echoValue();
$a->b->echoValue();
$a->c->echoValue();
$a->value = 11;
$a->echoValue();
$a->b->echoValue(); // *
$a->c->echoValue();
?>
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Результат выполнения данного примера:
class A: 10
class B: 10
class B: 10
class A: 11
class B: 11
class B: 11 |