PHP Singleton设计模式继承错误 [英] PHP Singleton design pattern inheritance error

问题描述

从下面的php单身课程

 <？php 
 class Singleton 
 {
 / ** 
 * @var Singleton这个类的* Singleton *实例的引用
 * / 
 private static $ instance; 
 
 / ** 
 *返回此类的* Singleton *实例。 
 * 
 * @return Singleton * Singleton *实例。 
 * / 
 public static function getInstance（）
 {
 if（null === static :: $ instance）{
 static :: $ instance = new static （）; 
} 
 
 return static :: $ instance; 
} 
 
 / ** 
 *受保护的构造函数，以防止通过此类外部的new运算符创建
 * * Singleton *的新实例。 
 * / 
 protected function __construct（）
 {
} 
} 
  

我试图继承新的小孩类

  class SingletonChild extends Singleton {
 
  

但是当我做测试

  $ obj = Singleton :: getInstance（）; 
 $ obj_two = SingletonChild :: getInstance（）; 
 var_dump（$ obj === Singleton :: getInstance（））; // bool（true）
 var_dump（$ obj === $ obj_two）; // false 
  

我正在收到php致命错误。

PHP致命错误：未捕获错误：无法访问属性
SingletonChild :: $ instance

解决方案

在PHP 7.0中继承单元格 c> class 在PHP 7.0中很难，但是您可以通过在

首先使你的 Singleton class 到抽象

 抽象类Singleton {
 
} 
  

将您的 $ instance 变量更改为数组 $ instance（s）

  private $ instances = []; 
  

现在更改 getInstance（）

  public static function getInstance（）{
 if（！isset（self :: $ instances [static :: class ]）{
 self :: $ instances [static :: class] = new static（）; 
} 
 
 return self :: $ instances [static :: class]; 
} 
  

更改测试

记住现在你不能调用 Singleton :: getInstance（）由于抽象

  class SingletonChild extends Singleton {
} 
 
 class SingletonChildTwo extends SingletonChild {
} 
 
 $ obj = SingletonChild :: getInstance（）; 
 $ obj_two = SingletonChildTwo :: getInstance（）; 
 var_dump（$ obj === SingletonChild :: getInstance（））; // true 
 var_dump（$ obj === $ obj_two）; //将 - > false 
  

From php singleton class below

<?php
class Singleton
{
    /**
     * @var Singleton The reference to *Singleton* instance of this class
     */
    private static $instance;

    /**
     * Returns the *Singleton* instance of this class.
     *
     * @return Singleton The *Singleton* instance.
     */
    public static function getInstance()
    {
        if (null === static::$instance) {
            static::$instance = new static();
        }

        return static::$instance;
    }

    /**
     * Protected constructor to prevent creating a new instance of the
     * *Singleton* via the `new` operator from outside of this class.
     */
    protected function __construct()
    {
    }
}

I am trying to inherit new child class

class SingletonChild extends Singleton {
}

but when I do testing

$obj = Singleton::getInstance();
$obj_two = SingletonChild::getInstance();
var_dump($obj === Singleton::getInstance());             // bool(true)
var_dump($obj === $obj_two);   // false

I'm getting php fatal error.

PHP Fatal error: Uncaught Error: Cannot access property SingletonChild::$instance

解决方案

Inheriting Singleton class in PHP is difficult, event in PHP 7.0, but you can do this with some changes on your class to work.

first make your Singleton class to abstract

abstract class Singleton {

}

change your $instance variable to array $instance(s)

private $instances = [];

Now change getInstance() method like below

public static function getInstance() {
  if (!isset(self::$instances[static::class]) {
    self::$instances[static::class] = new static();
  }

  return self::$instances[static::class];
}

And change your test

remember now you can't call Singleton:: getInstance() due to abstract

class SingletonChild extends Singleton {
}

class SingletonChildTwo extends SingletonChild {
}

$obj = SingletonChild::getInstance();
$obj_two = SingletonChildTwo::getInstance();
var_dump($obj === SingletonChild::getInstance()); // true
var_dump($obj === $obj_two); // will -> false

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