Question

<?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();
?>

如何将用户限制为仅创建有限数量的对象。对于上面的例子，如果我将再创建一个对象，那么我将抛出一个错误。

Answer 1

将静态计数器变量添加到您的类中，添加构造函数和析构函数以增加和减少它。检查构造函数中的值：

<?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

Answer 2

解决方案是：

保留Book类private。
声明两个名为$number_of_objects和$threshold_number_of_objects的成员。
使用单独的类方法，比如说getInstance()方法来创建Book类的新实例。如果已创建的对象数小于阈值，则此getInstance()方法将创建类Book的新实例，否则将返回错误。

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

如何将用户限制为仅创建有限数量的对象

2 个答案: