特定功能的訪問沖突

Question

我的CustomerList.cpp文件中有很多功能，下面僅顯示其中一個無效的功能（斷點帶有注釋）。 注意：Store類是正確的，並且m_pHead是CustomerList私有變量（但這無關緊要）。

bool CustomerList::removeStore(int ID)
{

    Store *back, *temp;

    if(m_pHead = NULL)
        {
            cout << "\nError! Store " << ID << " not found in the List!\n";
            system("pause");
            return false; // nothing to delete
        }

    // Search for the item to delete
    back = NULL;
    temp = m_pHead;

    while((temp != NULL) && (temp->getStoreID() != ID))
    {
        back = temp;
        temp = temp->m_pNext;
    }

    if(back == NULL)    // Delete the first item in the list
    {
        m_pHead = temp->m_pNext; // THE FUNCTION BREAKS HERE
        delete temp;
        cout << "\nSuccess! Store " << ID << " added to List!\n";
        system("pause");
        return true;
    }
    else if(temp != NULL)  // Delete from middle or end of list
    {
        back->m_pNext = temp->m_pNext;
        delete temp;
        cout << "\nSuccess! Store " << ID << " added to List!\n";
        system("pause");
        return true;
    }
    else
    {
        cout << "\nError! Store " << ID << " not found in the List!\n";
        system("pause");
        return false;    // Didn't find the item to delete
    }

}

每次我調用此函數時，即使Store的ID不在列表中，它也會中斷（它不應在函數中走得那么遠）。 這是一個呼叫示例：

// Creating a new Customer List
CustomerList *newList = new CustomerList();
newList->removeStore(3);

我到底在做什么錯？

Answer 1

您的代碼中存在一些邏輯錯誤。 最值得注意的是，這條線；

if(m_pHead = NULL)

在比較之前將NULL分配給m_pHead 。 因此， temp為NULL，而back仍為NULL，這就是為什么代碼到達注釋位置並崩潰的原因。

您需要使用==比較運算符，而不是=賦值運算符（您的編譯器應已對此發出警告）：

if(m_pHead == NULL)

或更安全：

if(!m_pHead)

現在，您可以將剩下的代碼簡化為以下代碼：

bool CustomerList::removeStore(int ID)
{
    Store *temp, *previous;

    // Search for the item to delete

    previous = NULL;
    temp = m_pHead;

    while (temp != NULL)
    {
        if (temp->getStoreID() == ID)
        {
            if (m_pHead == temp)
            {
                // Deleting the first item in the list
                m_pHead = temp->m_pNext;
            }

            if (previous != NULL)
            {
                // Deleting from middle or end of list
                previous->m_pNext = temp->m_pNext;
            }

            delete temp;
            cout << "\nSuccess! Store " << ID << " removed from List!\n";
            system("pause");
            return true;
        }

        previous = temp;
        temp = temp->m_pNext;
    }

    cout << "\nError! Store " << ID << " not found in the List!\n";
    system("pause");
    return false; // nothing to delete
}

或者，如果您使用標准的C ++容器，例如std::list ，而不是創建自己的手動鏈表，則可以這樣做：

struct isStoreID
{
    int m_id;
    isStoreID(int id) : m_id(id) {}
    bool operator()(const Store &store) { return (store.getStoreID() == m_id); }
};

bool CustomerList::removeStore(int ID)
{
    // Search for the item to delete

    // where m_list is a std::list<Store>...
    std::list<Store>::iterator iter = std::find_if(m_list.begin(), m_list.end(), isStoreID(ID));

    bool bWasFound = (iter != m_list.end());
    if (bWasFound)
    {
        m_list.erase(iter);
        cout << "\nSuccess! Store " << ID << " removed from List!\n";
    }
    else
        cout << "\nError! Store " << ID << " not found in the List!\n";

    system("pause");
    return bWasFound;
}