PHP 致命错误：无法访问空属性

PHP Fatal error: Cannot access empty property

提问by Bishan

I'm new to phpand I have executed below code.

我是新手php，我已经执行了下面的代码。

<?php
class my_class{

    var $my_value = array();
    function my_class ($value){
        $this->my_value[] = $value;
    }
    function set_value ($value){
    // Error occurred from here as Undefined variable: my_value
        $this->$my_value = $value;

    }

}

$a = new my_class ('a');
$a->my_value[] = 'b';
$a->set_value ('c');
$a->my_class('d');

foreach ($a->my_value as &$value) {
    echo $value;
}

?>

I got below errors. What could be the error?

我得到以下错误。可能是什么错误？

Notice: Undefined variable: my_value in C:\xampp\htdocs\MyTestPages\f.php on line 15

Fatal error: Cannot access empty property in C:\xampp\htdocs\MyTestPages\f.php on line 15

回答by Philipp

You access the property in the wrong way. With the $this->$my_value = ..syntax, you set the property with the name of the value in $my_value. What you want is $this->my_value = ..

您以错误的方式访问该属性。使用$this->$my_value = ..语法，您可以使用 $my_value 中的值的名称设置属性。你想要的是$this->my_value = ..

$var = "my_value";
$this->$var = "test";

is the same as

是相同的

$this->my_value = "test";

To fix a few things from your example, the code below is a better aproach

要从您的示例中修复一些问题，下面的代码是一个更好的方法

class my_class {

    public  $my_value = array();

    function __construct ($value) {
        $this->my_value[] = $value;
    }

    function set_value ($value) {
        if (!is_array($value)) {
            throw new Exception("Illegal argument");
        }

        $this->my_value = $value;
    }

    function add_value($value) {
        $this->my_value = $value;
    }
}

$a = new my_class ('a');
$a->my_value[] = 'b';
$a->add_value('c');
$a->set_value(array('d'));

This ensures, that my_value won't change it's type to string or something else when you call set_value. But you can still set the value of my_value direct, because it's public. The final step is, to make my_value private and only access my_value over getter/setter methods

这可确保当您调用 set_value 时，my_value 不会将其类型更改为字符串或其他类型。但是你仍然可以直接设置 my_value 的值，因为它是公开的。最后一步是将 my_value 设为私有，并且只能通过 getter/setter 方法访问 my_value

回答by Marko D

First, don't declare variables using var, but

首先，不要使用 var 声明变量，但是

public $my_value;

Then you can access it using

然后你可以使用它访问它

$this->my_value;

and not

并不是

$this->$my_value;

回答by Val

To access a variable in a class, you must use $this->myVarinstead of $this->$myvar.

要访问类中的变量，您必须使用$this->myVar代替$this->$myvar。

And, you should use access identifier to declare a variable instead of var.

并且，您应该使用访问标识符来声明一个变量而不是var.

Please read the doc here.

请在此处阅读文档。

回答by Abu Roma?ssae

As I see in your code, it seems you are following an old documentation/tutorial about OOP in PHP based on PHP4 (OOP wasn't supported but adapted somehow to be used in a simple ways), since PHP5 an official support was added and the notation has been changed from what it was.

正如我在您的代码中看到的那样，您似乎正在遵循有关基于 PHP4 的 PHP 中 OOP 的旧文档/教程（不支持 OOP，但以某种方式进行了调整，以便以简单的方式使用），因为添加了 PHP5 官方支持并且符号已经从原来的样子改变了。

Please see this code review here:

请在此处查看此代码：

<?php
class my_class{

    public $my_value = array();

    function __construct( $value ) { // the constructor name is __construct instead of the class name
        $this->my_value[] = $value;
    }
    function set_value ($value){
    // Error occurred from here as Undefined variable: my_value
        $this->my_value = $value; // remove the $ sign
    }

}

$a = new my_class ('a');
$a->my_value[] = 'b';
$a->set_value ('c'); // your array variable here will be replaced by a simple string 
// $a->my_class('d'); // you can call this if you mean calling the contructor 


// at this stage you can't loop on the variable since it have been replaced by a simple string ('c')
foreach ($a->my_value as &$value) { // look for foreach samples to know how to use it well
    echo $value;
}

?>

I hope it helps

我希望它有帮助

回答by gazdagergo

This way you can create a new object with a custom property name.

通过这种方式，您可以创建一个具有自定义属性名称的新对象。

$my_property = 'foo';
$value = 'bar';
$a = (object) array($my_property => $value);

Now you can reach it like:

现在你可以像这样到达它：

echo $a->foo;  //returns bar

回答by vikingmaster

Interesting:

有趣的：

1. You declared an array var $my_value = array();
2. Pushed value into it $a->my_value[] = 'b';
3. Assigned a string to variable. (so it is no more array) $a->set_value ('c');
4. Tried to push a value into array, that does not exist anymore. (it's string) $a->my_class('d');

1. 你声明了一个数组 var $my_value = array();
2. 将价值推入其中 $a->my_value[] = 'b';
3. 将字符串分配给变量。（所以它不再是数组）$a->set_value ('c');
4. 试图将一个值推送到数组中，该数组不再存在。（这是字符串）$a->my_class('d');

And your foreachwont work anymore.

你foreach不会再工作了。

回答by John Rix

I realise this answer is not a direct response to the problem described by the OP, but I found this question as a result of searching for the same error message. I thought it worth posting my experience here just in case anybody is muddling over the same thing...

我意识到这个答案不是对 OP 描述的问题的直接回应，但我在搜索相同的错误消息时发现了这个问题。我认为值得在这里发布我的经验，以防万一有人对同一件事感到困惑......

You can encounter the error in question as a result of a poorly formatted forloop over an associative array. In a fit of bone-headedness, I was using -> instead of => in my forstatement:

由于for关联数组上的错误格式循环，您可能会遇到有问题的错误。一头雾水，我在for声明中使用了 -> 而不是 => ：

        foreach ($object->someArray as $key->$val) {
            // do something
        }

Of course, I should have had:

当然，我应该有：

        foreach ($object->someArray as $key=>$val) {
            // do something
        }

I confused myself at first, thinking the reported error was referring to the someArray property!

一开始我很困惑，以为报告的错误是指 someArray 属性！