如何限制用户只能创建有限数量的对象

<?php 
class Book 
{ 

var $name; 
    function setName($name){ 
      $this->name = $name; 
    } 
    function getName(){ 
     return $this->name ; 
    } 
} 
$objectfirst = new Book; 
$objectfirst->setName('English'); 
echo $objectfirst->getName(); 
$objectsecond = new Book; 
$objectsecond->setName('Science'); 
echo $objectsecond->getName(); 
?>

如何限制用户只能创建有限数量的对象。对于上面的例子，如果我将创建一个更多的对象，那么我会抛出一个错误。如何限制用户只能创建有限数量的对象

来源

2016-04-25 Rajiv

另一种方法是创建一个单独的类，这样可以确保始终有类的1个实例从不2 。 – Daan

你的情况应该是什么限制？ 2个对象？ – RomanPerekhrest

在我的情况下，它不应该超过2。 – Rajiv

添加静态计数器变量到您的类，添加构造函数和析构函数来增加和减少它。检查值在构造函数中：

<?php 
class Book 
{ 

    var $name; 
    private static $counter=0; 

    function __construct() 
    { 
    self::$counter++; 
    if(self::$counter > 2) 
     throw new Exception('Limit exceeded'); 
    } 

    function __destruct() 
    { 
    self::$counter--; 
    } 

    function setName($name){ 
      $this->name = $name; 
    } 

    function getName(){ 
     return $this->name ; 
    } 
} 

$objectfirst = new Book; 
$objectfirst->setName('English'); 
echo $objectfirst->getName(); 
$objectsecond = new Book; 
$objectsecond->setName('Science'); 
echo $objectsecond->getName(); 

$objectthird = new Book; 
$objectthird->setName('Test'); 
echo $objectthird->getName();

脚本输出：

EnglishScience 
Fatal error: Uncaught exception 'Exception' with message 'Limit exceeded' in sandbox/scriptname.php:12 
Stack trace: 
#0 sandbox/scriptname.php(36): Book->__construct() 
#1 {main} 
    thrown in sandbox/scriptname.php on line 12

来源

2016-04-25 07:25:31 kay27

将静态属性'$ counter'标记为私有，以避免外部修改 – RomanPerekhrest

@RomanPerekhrest thx，done – kay27

我会更改异常消息文本，但是，无论如何...你有我的upvote（+10） – RomanPerekhrest

解决的办法是：

保持private的Book类的构造方法。
声明名为$number_of_objects和$threshold_number_of_objects的两个类成员。
使用单独的类方法，说getInstance()方法来创建Book类的新实例。如果已创建的对象数量小于阈值，则此getInstance()方法将创建类Book的新实例，否则它将返回错误。

所以，你的代码应该是这样的：

class Book{ 

    private $name; 
    private static $number_of_objects = 0; 
    private static $threshold_number_of_objects = 2; 

    // Since it's constructor method is private 
    // it prevents objects from being created from 
    // outside of the class 
    private function __construct(){} 

    // It creates an object if the number of objects 
    // already created is less than the threshold value 
    public static function getInstance() { 
     if(self::$number_of_objects < self::$threshold_number_of_objects){ 
      ++self::$number_of_objects; 
      return new Book(); 
     }else{ 
      echo "Error: Number of objects crossed the threshold value"; 
      return false; 
     } 
    } 

    public function setName($name){ 
      $this->name = $name; 
    } 

    public function getName(){ 
     return $this->name ; 
    } 
} 

//$objectfirst = new Book; invalid 
$objectfirst = Book::getInstance(); 
if($objectfirst){ 
    $objectfirst->setName('English'); 
    echo $objectfirst->getName() . "<br />";  
} 

//$objectsecond = new Book; invalid 
$objectsecond = Book::getInstance(); 
if($objectsecond){ 
    $objectsecond->setName('Science'); 
    echo $objectsecond->getName() . "<br />"; 
} 

//$objectthird = new Book; invalid 
$objectthird = Book::getInstance(); 
if($objectthird){ 
    $objectthird->setName('Engineering'); 
    echo $objectthird->getName() . "<br />";  
}

输出：

English 
Science 
Error: Number of objects crossed the threshold value

来源

2016-04-25 07:37:51

如何限制用户只能创建有限数量的对象

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