php_异常解决_sessioin

写购物车时遇到问题，

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Fatal error : DangdangService::addCartItem() [dangdangservice.addcartitem]: The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "Cart" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition

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查询发现;

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$car = null;
if(empty($_SESSION['car'])){
?? $uid = $_SESSION['uid'];???
?? $car = DangdangService::getInstance()->createCart($uid, $bookId);
?? //php 特性，对象，往session存，序列化
?? $_SESSION['car'] = serialize($car);
?? //$_SESSION['car'] = $car;
}else{
??? $car = unserialize($_SESSION['car']);
??? //$car = $_SESSION['car'];
??? $_SESSION['car'] = serialize(DangdangService::getInstance()->addCartItem($car, $bookId));
??? // $_SESSION['car'] = DangdangService::getInstance()->addCartItem($car, $bookId);
}
$oSmarty->assign("items",$car->getItems());
$oSmarty->display("car.html");

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php 在session 中存取对象时要序列化? :)