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函数参数NULL有关问题

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Release： 2016-06-13 11:59:03
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函数参数NULL问题
一个数据连接初始化的代码，本人之前学C#的，就是构造函数里的$dbo=NULL这个看不懂，请指教。
我可不可以不要=NULL呢，直接用$dbo,我个人理解是$dbo=NULL这条语句就表示$dbo不是object类型了，那它下面为什么还要去判断？ $dbo=NULL这个NULL值不会带到函数里面去吗？
protected function __construct($dbo)
{
      //...
}

<br /><br />	class DB_Connect{<br />		<br />		protected $db;<br />		<br />		protected function __construct($dbo=NULL)<br />		{<br />			if(is_object($dbo))<br />			{<br />				$this->db=$dbo;				<br />			}<br />			else<br />			{<br />				$dsn="mysql:host=".DB_HOST."; dbname=".DB_NAME;<br />				try <br />				{<br />					$this->db=new PDO($dsn,DB_USER,DB_PASS);<br />				}<br />				catch(Exception $e)<br />				{<br />					die($e->getMessage());<br />				}<br />			}<br />		}		<br />	<br />	}	<br />
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------解决方案--------------------
function __construct($dbo=NULL)
表示 $dbo 这个参数是可缺省的，因为他有初值 NULL
如果仅是
function __construct($dbo)
那么 $dbo 这个参数就一定要传入的

由于是可缺省参数，所以
new DB_Connect();
new DB_Connect($db);
都不会出错


C# 支持重载，所以对于这种情况你可能是这样写
DB_Connect::__construct($dbo) {}
DB_Connect::__construct() {}
------解决方案--------------------
但是面向对象编程，重载是一个很重要的概念（方法）

Related labels：
connect nbsp null
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