PHP编程中的__clone()方法使用详解

可以在对象类中定义一个__clone()方法来调整对象的克隆行为。此方法的代码将在克隆操作期间执行。除了将所有现有对象成员复制到目标对象之外，还会执行__clone()方法指定的操作。下面修改corporate_drone类，增加以下方法：

function __clone() { 
 $this->tiecolor = "blue";
}

之后，创建一个新的corporate_drone对象，增加employeeid成员的值，克隆这个对象，然后输出一些数据，从而显示克隆对象的tiecolor确实是通过__clone()方法设置的。示例代码：

<?php
 // create new corporatedrone object
  $drone1 = new corporatedrone();
 
 // set the $drone1 employeeid member
  $drone1->setemployeeid("12345");
 
 // clone the $drone1 object
  $drone2 = clone $drone1;
 
 // set the $drone2 employeeid member
  $drone2->setemployeeid("67890");
 
 // output the $drone1 and $drone2 employeeid members
  echo "drone1 employeeid: ".$drone1->getemployeeid()."<br />";
  echo "drone2 employeeid: ".$drone2->getemployeeid()."<br />";
  echo "drone2 tiecolor: ".$drone2->gettiecolor()."<br />";
?>

程序运行结果

drone1 employeeid: 12345
drone2 employeeid: 67890
drone2 tiecolor:

再来一个小例子：

<?php
class fruit {
 private $name = "水果";
 private $color = "颜色";
 
 public function setname($name){
 $this->name = $name;
 }
 
 public function setcolor($color){
 $this->color = $color;
 }
 
 function showcolor(){
 return $this->color.'的'.$this->name."<br />";
 }
 
 function __destruct(){
 echo "被吃掉了（对象被回收） <br />"; 
 }
}

$apple = new fruit();
$apple->setname("大苹果");
$apple->setcolor("红色");
echo $apple->showcolor();

$clone_apple = $apple;
$clone_apple->setname("小苹果");
$clone_apple->setcolor("青色");

echo $clone_apple->showcolor();
?>

上面只是将一个类赋值给另一个类，所以此时内存中仍是一个对象。

<?php
class fruit {
 private $name = "水果";
 private $color = "颜色";
 
 public function setname($name){
 $this->name = $name;
 }
 
 public function setcolor($color){
 $this->color = $color;
 }
 
 function showcolor(){
 return $this->color.'的'.$this->name."<br />";
 }
 
 function __destruct(){
 echo "被吃掉了（对象被回收） <br />"; 
 }
 function __clone(){
 $this->name = "克隆水果"; 
 }
}

$apple = new fruit();
$apple->setname("大苹果");
$apple->setcolor("红色");
echo $apple->showcolor();

$clone_apple = clone $apple;
$clone_apple->setcolor("青色");

echo $clone_apple->showcolor();
?>

clone方法克隆出了一个新的类，所以此时内存中有两个对象。

php的__clone()方法对一个对象实例进行的浅复制,对象内的基本数值类型进行的是传值复制，而对象内的对象型成员变量,如果不重写__clone方法,显式的clone这个对象成员变量的话,这个成员变量就是传引用复制,而不是生成一个新的对象.如下面一个例子的第28行注释所说

<?php
  class account {
    public $balance;
    
    public function __construct($balance) {
      $this->balance = $balance;
    }
  }
 
  class person {
    private $id;
    private $name;
    private $age;
    public $account;
    
    public function __construct($name, $age, account $account) {
      $this->name = $name;
      $this->age = $age;
      $this->account = $account;
    }
    
    public function setid($id) {
      $this->id = $id;
    }
    
    public function __clone() {  #复制方法,可在里面定义再clone是进行的操作
      $this->id = 0;
      $this->account = clone $this->account;  #不加这一句,account在clone是会只被复制引用,其中一个account的balance被修改另一个也同样会被修改
    }
  }
  
  $person = new person("peter", 15, new account(1000));
  $person->setid(1);
  $person2 = clone $person;
  
  $person2->account->balance = 250;
  
  var_dump($person, $person2);
  
 ?>

输出:

object(person)#1 (4) { ["id":"person":private]=> int(1) ["name":"person":private]=> string(5) "peter" ["age":"person":private]=> int(15) ["account"]=> object(account)#2 (1) { ["balance"]=> int(1000) } } object(person)#3 (4) { ["id":"person":private]=> int(0) ["name":"person":private]=> string(5) "peter" ["age":"person":private]=> int(15) ["account"]=> object(account)#4 (1) { ["balance"]=> int(250) } }