文件处理问题，以读取模式打开文件时

Question

我有一个任务是从用户那里获取输入并将其写入文件并成功写入文件，如下面给出的函数，它运行良好

void add_category(string NAME, int ID_category)
{
    category* newnode;
    newnode = new category;
    newnode->name_of_category = NAME;
    newnode->id = ID_category;
    if (headd == NULL) {
        headd = newnode;
        newnode->next = NULL;
        newnode->pre = NULL;
        lastd = newnode;
    }
    else {
        lastd->next = newnode;
        newnode->pre = lastd;
        newnode->next = NULL;
        lastd = newnode;
    }
    dlength++;

    ofstream file("Category.txt", ios::app);
    category* temp = headd;
    category* temp2;
    while (temp != NULL) {
        file << temp->name_of_category << endl;
        file << temp->id << endl;
        temp2 = temp;
        temp = temp->next;
        delete temp2;
    }
    file.close();
}

但问题是，当通过代码打开文件时会崩溃或 TBH，我不知道如何打开文件。我正在使用双链表。我的任务是以读取模式打开文件并删除用户输入的类别。这是我删除类别的功能：

void delete_category(string remove_name)
{
    category* currentptr = headd;
    category* temp;
    category* temp2;
    category* temp3 = headd;

    ifstream file("Category.txt");

    if (!file) {
        cout << "file can not open" << endl;
    }
    else {
        while (temp3 != NULL && !file.eof()) {
            file >> temp3->name_of_category;
            file >> temp3->id;
            temp3 = temp3->next;
        }
    }

    int save = search_category(remove_name);

    cout << save << endl;

    if (dlength <= 0) {
        cout << " NO category found" << endl;
    }
    else {
        if (save == 0) {
            temp = headd;
            headd = headd->next;
            headd->pre = NULL;
        }
        else {
            for (int i = 1; i < save; i++) {
                currentptr = currentptr->next;
            }

            temp = currentptr->next;
            temp2 = temp->next;

            currentptr->next = temp2;
            temp2->pre = currentptr;
        }

        dlength--;

        delete temp;

        cout << " Successfully Deleted the Category" << endl;
    }
}

由于我在删除类别中调用了搜索功能，因此我在搜索功能中使用相同的方法打开文件：

int search_category(string search)
{
    category* currentptr = headd;
    int flag = 0;
    category* temp3 = headd;

    ifstream file("Category.txt");

    if (!file) {
        cout << "file can not open" << endl;
    }
    else {
        while (temp3 != NULL && !file.eof()) {

            file >> temp3->name_of_category;
            file >> temp3->id;

            temp3 = temp3->next;
        }
    }

    for (int i = 0; i < dlength; i++) {
        if (currentptr->name_of_category == search) {
            flag = 1;
            return i;
        }

        currentptr = currentptr->next;
    }

    if (flag == 0) {
        cout << " Sorry not found" << endl;
        return -1;
    }
}

请纠正我在打开文件时出错的地方。

Answer 1

无需从同一个文件中多次读取数据，只需将其一次读入内存并处理即可。

using Category = std::pair<int, std::string>;
std::vector<Category> categories;

{
  Category cat;
  std:ifstream file("Category.txt");

  while (file >> cat.first >> cat.second)
    categories.push_back(cat);
}

auto novels = std::find_if(std::begin(categories), std::end(categories),
    [](const Category &category) { return category.second == "novels"; });

Answer 2

在我看来，问题在于ios::app 模式的使用。请参阅link。 app 模式用于追加不写入，所以发生的情况是文件指示符位于文件末尾。 我希望这可以帮助您解决问题。