php oop 受保护的属性、受保护的方法、受保护的构造 [英] php oop protected property, protected method, protected construct

问题描述

这些是我写的一些示例代码

these are some example code I have written

<?php
/**************** code block 1 *****************/
class Database1
{
    public $rows;

    public function __construct()
    {

    }

    public function assingrow()
    {
        $this->rows=5;
    }   
}


$database1 = new Database1();
$database1->assingrow();
echo $database1->rows;

//no error




/**************** code block 2 *****************/
class Database2
{
    protected $rows;//

    public function __construct()
    {

    }

    public function assingrow()
    {
        $this->rows=5;
    }   
}


$database2 = new Database2();
$database2->assingrow();
echo $database2->rows;
//error message
//Fatal error: Cannot access protected property Database2::$rows in E:\xampp\htdocs\pdo\4.php on line 46





/**************** code block 3 *****************/
class Database3
{
    public $rows;

    public function __construct()
    {

    }

    protected function assingrow()////
    {
        $this->rows=5;
    }   
}


$database3 = new Database3();
$database3->assingrow();
echo $database3->rows;
//error message
//Fatal error: Call to protected method Database3::assingrow() from context '' in E:\xampp\htdocs\pdo\4.php on line 68





/**************** code block 4 *****************/
class Database4
{
    public $rows;

    protected function __construct()
    {

    }

    public function assingrow()
    {
        $this->rows=5;
    }   
}


$database4 = new Database4();
$database4->assingrow();
echo $database4->rows;
//error message
//Fatal error: Call to protected Database4::__construct() from invalid context in E:\xampp\htdocs\pdo\4.php on line 91

谁能解释为什么这些

1. 为什么不能在代码块 2 中为受保护的属性赋值
2. 为什么不能在代码块 3 中使用受保护的方法为公共属性赋值
3. 为什么不能在代码块 4 中保护构造

推荐答案

这是可见性.

在第 2 块中，您的财产受到保护.这意味着它只能被类本身(Database2)和继承的类访问.当您尝试从外部 echo 变量时发生错误.

In Block 2 your property is protected. This means it can only be accessed the class itself (Database2) and by inherited classes. The error occurs when you try to echo the variable from the outside.

同样适用于 Block 3 中的方法.

The same goes for the method in Block 3.

构造函数可以被protected 甚至private.但是你不能再从外面调用它了.但这样的事情是可能的:

The Constuctor can be protected or even private. But you cannot call it from the outside anymore. But something like this is possible:

class Foo
{
    private function __construct()
    {
    }

    public static function create()
    {
        return new self();
    }
}

$foo = Foo::create();

这篇关于php oop 受保护的属性、受保护的方法、受保护的构造的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！