PHP 面向对象基础：参数类型约束 1 个改进

PHP 5 可以使用类型约束。函数的参数可以指定必须为对象（在函数原型里面指定类的名字）。如果一个类或接口指定了类型约束，则其所有的子类或实现也都如此。

class Parents {}

实现 Parents 类

class Son extends Parents
{
    public function read()
    {
        echo 'Son is reading.' . '<br />';
    }

    public function write()
    {
        echo 'Son is writing.' . '<br />';
    }
}

class Daugthter extends Parents
{
    public function read()
    {
        echo 'Daugthter is reading.' . '<br />';
    }

    public function write()
    {
        echo 'Daugthter is writing.' . '<br />';
    }
}

类型约束示例

class Doing
{
    /**
     * Son 对象约束
     * @param Son $son
     */
    public function sonRead(Son $son)
    {
        $son->read();
    }

    /**
     * Daughter 对象约束
     * @param Daugthter $daugthter
     */
    public function daugtherRead(Daugthter $daugthter)
    {
        $daugthter->read();
    }

    /**
     * Parents 对象约束
     * @param Parents $parents
     */
    public function parentsWrite(Parents $parents)
    {
        $parents->write();
    }
}

$doing = new Doing();
$doing->sonRead(new Son()); // Son is reading.
$doing->daugtherRead(new Daugthter()); // Daugthter is reading.
$doing->parentsWrite(new Son()); // Son is writing.
$doing->parentsWrite(new Daugthter()); // Daugthter is writing.

类型约束也能使用在函数里

function doing (Son $son)
{
    $son->read();
}
doing(new Son());  // Son is reading.

类型约束允许 null 值

function test(Son $obj = null) {
    echo 'Nothing here.' . '<br />';
}

test(null);  // Nothing here.

函数调用的参数与定义的参数类型不一致时会抛出致命错误

// Fatal error: Uncaught TypeError: Argument 1 passed to Doing::sonRead() must be an instance of Son, instance of Daugthter given
$doing->sonRead(new Daugthter());

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