无法查看从父类继承的属性 [重复]答案

【问题标题】：Unable to view property inherited from parent class [duplicate]无法查看从父类继承的属性 [重复]
【发布时间】：2019-08-11 14:02:47
【问题描述】：

我创建了一个带有以下变量的银行账户类，$Balance 变量是私有的，所以我使用 setter 和 getter 函数来访问 balance 的私有属性。

class BankAccount
{
    public $HoldersName;
    public $AccountNumber;
    public $SortCode;
    private $Balance = 1;
    public $APR = 0;
    public $Transactions = []; //array defined.

    public function __construct($HoldersName, $AccountNumber, $SortCode, $Balance = 1) //constructor where i.e. HoldersName is the same as the variable $HoldersName. 
    {                                                                                  //constructor automatically called when object is created.
        echo "Holders Name: " . $this->HoldersName  = $HoldersName . "</br>";
        echo "Account Number: " . $this->AccountNumber = $AccountNumber . "</br>";
        echo "Sort Code: " . $this->SortCode = $SortCode . "</br>";
        $this->Balance = $Balance >= 1 ? $Balance : 1;
    }


    public function set_Balance($Balance) //SET balance.
    {
        return $this->Balance=$Balance;
    }

    public function get_Balance() //GET balance.
    {
        return $this->Balance; //Allows us to access the prviate property of $Balance.
    }

}

SavingsAccount 类扩展了 BankAccount，因此它继承了 BankAccount 类的所有内容。我创建了一个函数来计算利息，即余额 6800 * (0.8+0.25) * 1。

class SavingsAccount extends BankAccount
{
    public $APR = 0.8;
    public $APRPaymentPreference;
    public $ExtraBonus = 0.25;

    public function CalculateInterest()
    {
        $this->Balance = $this->Balance * ($this->APR + $this->ExtraBonus) * 1; //this is where i get the error now

    }

    public function FinalSavingsReturn()
    {

    }
}

这里我创建了一个SavingsAccount类的实例，余额为6800，我尝试调用函数SavingsAccount::CalculateInterest()却出现了这个错误：

注意未定义的属性：第 42 行的 SavingsAccount::$Balance

//define objects
$person2 = new SavingsAccount ('Peter Bond', 987654321, '11-11-11', 6800); //create instance of class SavingsAccount
echo "<b>Interest:</b>";
print_r ($person2->CalculateInterest());

【问题讨论】：

标签： php oop

【解决方案1】：

正如您所说，$Balance 属性是私有的。私有意味着它不能从定义类外部访问，甚至不能从继承该属性的子类访问。

<?php
class Foo {
    private $bar="baz";
    public function __construct(){echo $this->bar;}
    public function getBar(){return $this->bar;}
}
class Bar extends Foo {
    public function __construct(){echo $this->bar;}
}
new Foo;
new Bar;

如果我们运行这段代码，我们会得到错误：

baz
PHP Notice:  Undefined property: Bar::$bar in php shell code on line 2

所以，你有两个选择。您可以改用您的吸气剂：

<?php
class Foo {
    private $bar="baz";
    public function __construct(){echo $this->bar;}
    public function getBar(){return $this->bar;}
}
class Bar extends Foo {
    public function __construct(){echo $this->getBar();}
}
new Foo;
new Bar;

或者，您可以将属性声明为protected 而不是private：

<?php
class Foo {
    protected $bar="baz";
    public function __construct(){echo $this->bar;}
    public function getBar(){return $this->bar;}
}
class Bar extends Foo {
    public function __construct(){echo $this->bar;}
}
new Foo;
new Bar;

其中任何一个都给出了预期的输出：

baz
baz

有关属性和方法可见性的更多详细信息，请参阅documentation here。

【讨论】：