为什么（对象）NULL或新的stdClass IN一个php函数？ [英] why (object) NULL or new stdClass IN a php function?

问题描述

谁可以解释为什么会返回错误：

Who can explain why this returns an error:

$test = new myclass();

class myclass {
    private $object = (object) NULL;

    public function addmember() {
        $this->object->member1 = 'member 1';
    }
}
$test -> addmember();

...这是正常的：

$test = new myclass();

class myclass {
    private $object = '';

    public function addmember() {
        $this->object = (object) NULL;// new stdClass();
        $this->object->member1 = 'member 1';
    }
}
$test -> addmember();

但是为什么？谁能解释为什么第一个例子是wroing？
为什么我必须在函数中加上（object）NULL的行？

But why? Who can explain why the first example is wroing? Why I have to put the line with "(object)NULL" IN the function?

推荐答案

从 php.net ：

From php.net:

这个声明可以包括一个初始化，但是这个
初始化必须是一个常量值 - 也就是说，它必须能够在编译时评估
，并且不能依赖于运行时

This declaration may include an initialization, but this initialization must be a constant value--that is, it must be able to be evaluated at compile time and must not depend on run-time information in order to be evaluated.

例如，您无法执行此操作：

For example, you can not do this:

<?php
class A {
    public $x = 1 + 2; // < expression
}
?>

但可以这样做：

<?php
class A {
    public $x;

    public function __construct(){
        $this->x = 1 + 2;
    }
}
?>

此外，您可以通过常量值在类主体中初始化属性，不需要对分析过程：

Also, you can initialize property within a class body by constant value, that does not need to be evaluated on parse process:

<?php
class A {
    public $x = 123; // < constant value
}
?>

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