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c++ - 尝试创建链表时出现错误“写访问冲突。this->tail was nullptr”

问题描述

我正在尝试将 csv 文件中的数据输入到链接列表中。

程序编译但没有输出。我正在使用调试器逐行浏览代码，一切似乎都很顺利，直到我被带到该行：tail->next = temp 然后出现一个错误，指出“抛出未处理的异常：写访问冲突。这- >tail 是 nullptr”。每次我尝试执行另一行代码时，我都会被带回到那个点。

struct Customer {
std::string raw_User_ID;
std::string Product_ID;
std::string raw_Gender;
std::string age;
std::string raw_Occupation;
std::string raw_City_Category;
std::string Stay_In_Current_City_Years;
std::string raw_Marital_Status;
std::string raw_Product_Category_1;
std::string raw_Product_Category_2;
std::string raw_Product_Category_3;
std::string raw_Purchase;
std::string allData;
double User_ID;
char Gender;
int Occupation;
char City_Category;
bool Marital_Status;
int Product_Category_1;
int Product_Category_2;
int Product_Category_3;
double Purchase;
Customer *next;
   };

   class List
     {
    private:
Customer *head, *tail;
      public:
List()
{
    head = NULL;
    tail = NULL;
}

void inputCustomers(std::ifstream& infile) {
    infile.open("BlackFriday.csv");
    if (!infile) {
        std::cout << "The file could not be found\n";
    }
    else {
        std::cout << "The file was found\n";
    }


    while (!infile.eof()) {
        Customer *temp = new Customer;
        std::getline(infile, temp->raw_User_ID, ',');
        std::getline(infile, temp->Product_ID, ',');
        std::getline(infile, temp->raw_Gender, ',');
        std::getline(infile, temp->age, ',');
        std::getline(infile, temp->raw_Occupation, ',');
        std::getline(infile, temp->raw_City_Category, ',');
        std::getline(infile, temp->Stay_In_Current_City_Years, 
        ',');
        std::getline(infile, temp->raw_Marital_Status, ',');
        std::getline(infile, temp->raw_Product_Category_1, 
         ',');
        std::getline(infile, temp->raw_Product_Category_2, 
        ',');
        std::getline(infile, temp->raw_Product_Category_3, 
         ',');
        std::getline(infile, temp->raw_Purchase, '\n');
        if (temp->raw_Product_Category_1 == "") {
            temp->raw_Product_Category_1 = '0';
        }
        if (temp->raw_Product_Category_2 == "") {
            temp->raw_Product_Category_2 = '0';
        }
        if (temp->raw_Product_Category_3 == "") {
            temp->raw_Product_Category_3 = '0';
        }
        std::stringstream(temp->raw_User_ID) >> temp->User_ID;
        std::stringstream(temp->raw_Gender) >> temp->Gender;
        std::stringstream(temp->raw_Occupation) >> temp->Occupation;
        std::stringstream(temp->raw_City_Category) >> temp->City_Category;
        std::stringstream(temp->raw_Marital_Status) >> temp->Marital_Status;
        std::stringstream(temp->raw_Product_Category_1) >> temp->Product_Category_1;
        std::stringstream(temp->raw_Product_Category_2) >> temp->Product_Category_2;
        std::stringstream(temp->raw_Product_Category_3) >> temp->Product_Category_3;
        std::stringstream(temp->raw_Purchase) >> temp->Purchase;
        temp->next = NULL;

        if (head == NULL)
        {
            head = temp;
            tail = temp;
        }
        else
        {
            tail->next = temp;
            tail = temp->next;
        }
    }
}

void displayhead() {
    Customer *temp = new Customer;
    temp = head;
    std::cout << temp->age << std::endl;
    std::cout << temp->Gender << std::endl;
    std::cout << temp->allData << std::endl;
}
     };

    int main() {

List AllCustomers;

std::ifstream input;
AllCustomers.inputCustomers(input);

AllCustomers.displayhead();


getchar();
return 0;
   }

标签： c++

解决方案

我怀疑这对于前两行输入运行良好。您设置temp->next = NULL;, 并在 else 子句中执行,在第三次运行中tail = temp->next;有效地设置tail为。NULL由于tail是指向最后一个元素，只需更改

tail = temp->next;

至

tail = temp;

在 else 子句中。

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