瞭解爲什麼父級屬性可以從子類訪問，但父對象不能

Question

我很難理解爲什麼這不起作用。我可以訪問父類的屬性，但不能訪問對象。

我讀過其他線程，表明我應該從下面的Session類中調用parent::__construct()，但這只是創建一個無限循環並失敗。

爲什麼$TestVariable在Session類中可用，但$Cookie對象不是？

主應用程序文件：

class Application 
{ 
    //Class Objects 
    public $Cookie = NULL; 
    public $Session = NULL; 
    public $TestVariable = "Hello World"; 

    //Object initialization. Singleton design. 
    protected function __construct() 
    { 
     //Initialize cookie object. 
     $this->Cookie = Cookie::getCookie(); 

     //Initialize session object. 
     $this->Session = Session::getSession(); 
    } 

    //Returns the singleton instance of The Curator class. Singleton design. 
    public static function Initialize() 
    { 
     static $instance = NULL; 

     if($instance === NULL) 
     { 
      $instance = new static(); 
     } 

     return $instance; 
    } 

    //Singleton design. 
    private function __clone() {} 

    //Singleton design. 
    private function __wakeup() {} 
} 

Cookie文件：

class Cookie 
{ 
    //Object initalization. Singleton design. 
    protected function __construct() 
    { 
     //CODE HERE 
    } 

    //Singleton design. 
    private function __clone() {} 

    //Singleton design. 
    private function __wakeup() {} 

    //Returns the singleton instance of the cookie class. Singleton design. 
    public static function getCookie() 
    { 
     static $cookieInstance = NULL; 

     if($cookieInstance === NULL) 
     { 
      $cookieInstance = new static(); 
     } 

     return $cookieInstance; 
    } 

    //Removes all cookies. 
    public function destroyCookies() 
    { 
     //CODE HERE 
    } 
} 

會議文件：

class Session extends Application 
{ 
    //Object initialization. Singleton design. 
    protected function __construct() 
    { 
     //This FAILS! 
     $this->Cookie->destroyCookies(); 

     //This WORKS! 
     echo $this->TestVariable; 
    } 

    //Singleton design. 
    private function __clone() {} 

    //Singleton design. 
    private function __wakeup() {} 

    //Returns the singleton instance of the session class. Singleton design. 
    public static function getSession() 
    { 
     static $sessionInstance = NULL; 

     if($sessionInstance === NULL) 
     { 
      $sessionInstance = new static(); 
     } 

     return $sessionInstance; 
    } 
} 

Answer 1

你需要調用從子類的父類的構造。 而遞歸是因爲你有這樣的：

• 會話構造函數調用父:: __結構（）
• 應用構造函數調用會話::的getSession（）
• 的會話中::的getSession（），我們有new Session聲明
• 會話構造函數被調用
• 會話構造函數調用父:: __結構（）
• ...等

因此，使它工作，你需要打破鏈和移動會話初始化了應用程序構造：

class Application 
{ 
    //Class Objects 
    public $Cookie = NULL; 
    public $Session = NULL; 
    public $TestVariable = "Hello World"; 

    //Object initialization. Singleton design. 
    protected function __construct() 
    { 
     //Initialize cookie object. 
     $this->Cookie = Cookie::getCookie(); 
    } 

    protected function init() { 
     //Initialize session object. 
     $this->Session = Session::getSession(); 
    } 

    //Returns the singleton instance of The Curator class. Singleton design. 
    public static function Initialize() 
    { 
     static $instance = NULL; 

     if($instance === NULL) 
     { 
      $instance = new static(); 
      // Call init after construction 
      $instance->init(); 
     } 

     return $instance; 
    } 

... 

這裏是full code。

但的確，設計看起來不對，可能Session根本不應該繼承Application。