Question

我创建了一个班级Student 学生班：

class Student
{
    friend ostream& operator<<(ostream& os, const Student& s)
    {
        return os <<
            "Roll no.: " << s.roll_no << '\n' <<
            "Name: " << s.name << '\n' <<
            "Phone no.: " << s.phone_no << '\n' <<
            "Address: " << s.address << '\n';
    }
public:
    Student() = default;
    Student(int r, const char* n, int p_no, const char* a):
        roll_no(r), phone_no(p_no)
    {
        strcpy(name, n);
        strcpy(address, a);
    }
    int get_roll() const
    {
        return roll_no;
    }
private:
    int roll_no;
    char name[40 + 1];
    int phone_no;
    char address[100 + 1];
};

在StudentList类中，我将一些Student对象存储在二进制文件中。

class StudentList
{
public:
    StudentList(const string& fname):
        filename(fname)
    {
        make_index();
    }
    Student get_student(int roll)
    {
        fstream ifs(filename, ios::in | ios::binary);
        int pos = index[roll];
        ifs.seekg(pos * sizeof(Student));
        Student s;
        ifs.read(reinterpret_cast<char*>(&s), sizeof(s));
        return s;
    }
    void change_student(Student s)
    {
        fstream ofs(filename, ios::out | ios::binary);
        int pos = index[s.get_roll()];
        ofs.seekp(pos * sizeof(s));
        ofs.write(reinterpret_cast<const char*>(&s), sizeof(s));
    }
    void add_student(Student s)
    {
        fstream ofs(filename, ios::out | ios::binary | ios::app);
        ofs.write(reinterpret_cast<const char*>(&s), sizeof(s));
        int total_no = index.size() + 1;
        index[s.get_roll()] = total_no - 1;
    }
private:
    string filename;
    map<int, int> index;
    void make_index()
    {
        fstream ifs(filename, ios::in | ios::binary);
        int pos = 0;
        if (ifs.is_open()) {
            while (!ifs.eof()) {
                Student s;
                ifs.read(reinterpret_cast<char*>(&s), sizeof(s));
                index[s.get_roll()] = pos;
                pos++;
            }
        }
    }
};

但是，此代码输出错误。运行程序时，二进制文件不存在。所以StudentList::make_index在构造函数中调用它时会创建一个新文件。

int main()
{
    StudentList sl("student.dat");
    sl.add_student(Student(14, "V", 12, "14/14"));
    sl.add_student(Student(1, "A", 12, "13/13 Tollygunge"));
    cout << sl.get_student(14) << '\n';
    cout << sl.get_student(1) << '\n';
    sl.change_student(Student(1, "B", 12, "14/14, Bosepukur Road"));
    cout << sl.get_student(14) << '\n';
    cout << sl.get_student(1) << '\n';
    return 0;
}

输出：

Roll no.: 14
Name: V
Phone no.: 12
Address: 14/14

Roll no.: 1
Name: A
Phone no.: 12
Address: 13/13 Tollygunge

Roll no.: 0
Name: 
Phone no.: 0
Address: 

Roll no.: 1
Name: B
Phone no.: 12
Address: 14/14, Bosepukur Road

尝试使用gdb进行调试时，我发现函数StudentList::change_student(Student)无法正常工作。在函数中，尽管使用seekp在文件中的第一个Student对象之后设置写入位置（作为pos = 1），但第一个Student对象也以某种方式被修改。登记/> 修改
我想我发现了错误。在StudentList::change_student(Student s)中更改此行：

fstream ofs(filename, ios::out | ios::binary);

为：

fstream ofs(filename, ios::out | ios::binary | ios::in);

给出正确的输出输出：

Roll no.: 14
Name: V
Phone no.: 12
Address: 14/14

Roll no.: 1
Name: A
Phone no.: 12
Address: 13/13 Tollygunge

Roll no.: 14
Name: V
Phone no.: 12
Address: 14/14

Roll no.: 1
Name: B
Phone no.: 12
Address: 14/14, Bosepukur Road

最有可能是Aumnayan建议的，之前的开放模式是在change_student函数中删除文件的内容。

Answer 1

尝试在change_student方法中添加ios :: app标志。我缝合记住，ios :: out是破坏性的，这将留下一个文件，学生（1）传播，学生14设置为（大概）0。自从我玩这种类型的文件以来，已经有一段时间了，所以YMMV。

使用seekg和seekp获取错误的输出

1 个答案: