while-switch循环中的istream崩溃

我在while循环中有一个开关 . 在我调用选项4三次之后，程序在下次输入int时崩溃，这决定了在交换机中进入哪种情况 . 我不知道为什么会这样 . 这是while循环的代码：

void Menu::start()
{
    Store st;
    int op=1,num,quantity;
    string name;
    while(op!=0)
    {
        cin>>op;
        try
        {
            switch(op)
            {
                    case 1:
            {
                cin>>num>>name;
                st.addProduct(num,name);
                break;
            }
            case 4:
                {
                    cin>>num>>quantity;
                    st.sellProduct(num,quantity);
                    break;
                }
            case 0:
                break;
            default:
                throw(exception("Unknown option, try again.\n"));
            } //end of switch
        } //end of try
//catches
    } //end of while
}

/*****************************************************************************
* function name: addProduct
* The Input: This Store, const& int num, const& string name
* The output: If product with given num doesn't exist in store, adds it to
* store.
* The Function operation: uses the products map.
*****************************************************************************/
void Store::addProduct( const int& num,const string& name )
{
    //if product doesn't exist in map, add it
    if(prods.find(num)==prods.end())
        prods.insert(pair<int,Product>(num,Product(num,name)));
    //otherwise issue an error
    else
        throw(AddProdException(num));
}

/*****************************************************************************
* function name: sellProduct
* The Input: This Store, const int& prodNum, const unsigned int& quantityBought
* The output: If product doesn't exist or quantityBought is more than 10 units
* more than quantity in stock, issues an error. Otherwise, sells the product
* and if needed, issues a shipment such that after the purchase the store will
* be left with 20 units.
* The Function operation: uses the products and orders map.
*****************************************************************************/
void Store::sellProduct( const int& prodNum, const unsigned int& quantityBought )
{
    if(prods.find(prodNum)!=prods.end())
    {
        Product& pr = prods.find(prodNum)->second;
        const int& signedQB=quantityBought, signedPQ=pr.getQuantity();
        if( signedPQ<signedQB-10 )
            //store can't supply product
            throw(BuyQuanException(prodNum,quantityBought));
        //make purchase
        else
        {
            //purchase only what left in stock
            if(signedPQ<signedQB )
            {
                //issue shipment
                Order order=Order(prodNum,20+quantityBought-pr.getQuantity());
                orders.insert(pair<int,Order>(order.getID(),order));
                //document order
                purchaseDocs.add(new Documentation(pr,quantityBought,
                    orders.find(order.getID())->second));
                //buy product
                pr.decreaseQuantity( pr.getQuantity() );
            }
            //purchase requested amount
            else
            {
                //buy product
                pr.decreaseQuantity( quantityBought );
                //document order
                purchaseDocs.add(new Documentation(pr,quantityBought));
            }
        } //else regarding making the purchase

    } //if regarding found the product
    //otherwise issue an error
    else
        throw(BuyProdException(prodNum));
}

在三个进入案例4之后（并且仅在案例4之后，仅在3次之后），它在下一次到达cin >> op时在istream文件内崩溃 . 崩溃，我的意思是弹出以下错误消息：“Ex6.exe中0x4a34870c处的未处理异常：0xC0000005：访问冲突 . ”欢迎帮助！