Question

<?php
class LoveBase
{
    protected static $_instance = NULL;
    protected function __construct() {}
    public static function app()
    {
        if(self::$_instance == NULL) {
            self::$_instance = new self();
        }
        return self::$_instance;
    }

    public function get()
    {
        return 'LoveBase';
    }

}

class Love extends LoveBase
{
    public static function app()
    {
        if(self::$_instance == NULL) {
            self::$_instance = new self();
        }
        return self::$_instance;
    }
    public function get()
    {
        return 'Love';
    }
}

// Print "LoveLove" in this case(first case)
echo Love::app()->get(); 
echo LoveBase::app()->get();

// Print "LoveBaseLoveBase" in this case(second case)
// echo LoveBase::app()->get();
// echo Love::app()->get();

为什么两种不同的方法会产生相同的结果？
比较这两种情况，该方法在首次实例化时会起作用。

（对不起，我不擅长英语，希望你能做到这一点）

Answer 1

定义两个静态函数，它们都使用相同的静态变量（$ _instance） - 基类的静态成员也可以通过子类访问（只要它不是私有的）。请记住静态内容（方法和变量）是继承的，但不是克隆的。

解决方案：将成员变量设为私有，并为每个类创建一个。

class LoveBase
{
    private static $_instance = NULL;
    // ...

class Love extends LoveBase
{
    private static $_instance = NULL;
    // ...

Answer 2

// Print "LoveLove" in this case(first case)

//Set self::$_instance to Love object id
echo Love::app()->get(); 

//Static property $_instance is now already set, so LoveBase::app() won't create new self(), it will just return created and saved Love object
echo LoveBase::app()->get();

// Print "LoveBaseLoveBase" in this case(second case)

// Here is the same case, but static property $_instance filled with new self() in LoveBase class
// echo LoveBase::app()->get();
// echo Love::app()->get();

在PHP中扩展单例类

2 个答案: