从二进制文件读取结构到链表导致无限循环

Question

This is how my structure is constructed

struct dNascimento{

    int day, month, year;

};

struct morada{

    string street;
    int doorNum;
    string postCode;

};

typedef struct student{

    unsigned int id; //Número mecanográfico
    string name;
    string password;
    morada addressStudent;
    string course;
    dNascimento birth;
    int money;

    student* next;

}* studentPointer;

I'm adding student records to the linked list through this function

void addStudent(studentPointer studentToAdd) {

    studentToAdd->next = NULL;

    if (head != NULL) {

        current = head;

        while (current->next != NULL)
            current = current->next;

        current->next = studentToAdd;


    }
    else
        head = studentToAdd;

}

This is the function I use to save the students in the linked list to the binary file

void saveStudentsToFile() {

    FILE *file = fopen("records.bin", "wb");

    if (file != NULL) {

        if (head != NULL) {

            current = head;
            int numberOfStudents = 0;

            while (current != NULL){

                if (fwrite(current, sizeof(student), 1, file) != 1)
                    cout << "\n\t> Erro ao guardar o aluno " << current->name << " no ficheiro!\n" << endl;

                current = current->next;
                numberOfStudents++;

            };

            cout << "\n\t> " << numberOfStudents << " aluno(s) guardados com sucesso!\n" << endl;
            //^ Number of students saved successfully
            fclose(file);

        }else
            cout << "\n\t> Não há registos de alunos para guardar!\n" << endl;
            //^ There are no students to save to the file
    }else
        cout << "\n\t> Erro ao abrir o ficheiro para guardar registo de alunos!\n" << endl;
        //^ Error opening file to save students
}

This is the method I'm using to load the students saved from the file to the linked list

void loadFile() {

    FILE *file = fopen("records.bin", "rb");

    if (file != NULL) {

        studentPointer studentReader = new student;
        studentReader = (studentPointer) malloc(sizeof(student));
        int numberOfStudentsLoaded = 0;

        while (!(fread(studentReader, sizeof(student), 1, file) != 1)){
                addStudent(studentReader);
                numberOfStudentsLoaded++;
        };

        fclose(file);

        cout << "\n\t> " << numberOfStudentsLoaded << " aluno(s) carregado(s) com sucesso!\n" << endl;
        //^ Number of students loaded successfully
    }
    else
        cout << "\n\t> Erro ao abrir o ficheiro dos registos!\n" << endl;
        //^ Error opening file to load records
}

This is the method I'm using to list the students in the linked list

void listStudents() {

    if (head != NULL) {

        current = head;
        while (current != NULL) {
            cout << "\n\t> ID #" << current->id << endl;
            cout << "\t> " << current->name << endl;
            cout << "\t> " << current->password << endl;
            current = current->next;
        };

    }
    else
        cout << "\n\t> Não tem registos guardados!\n" << endl;
        //^ No records saved to display
}

When I add records manually to the linked list and use listStudents() to display them, they show up perfectly, but when I save the student records to the binary file and load them, the listStudents() method goes on an infinite loop to only display the last record saved in the binary file.

Keep in mind that every variable in the structure is correctly initialized before saved.

How students show up through listStudents() when I insert them manually http://image.prntscr.com/image/73459c40d81d462ca043ee414d1556d0.png

How students show up after I saveStudentsToFile() and loadFile() http://image.prntscr.com/image/c3dfb61235de46b7adb757d0762883a9.png

Answer 1

One of the problems with your code is that you cannot use fread to read student data from file (the same applies to fwrite ). Your student class is not POD (Plain Old Data), it contains string variables which cannot be initialized by reading from binary file with fread. You should use fstream to write field by field to output file, and then read field by field. Or convert your student struct to POD, ie: replace all string name; with char name[64]; .

Also, if you store: student* next; then you should not use its value, address will not be valid.

Your code looks more like C than C++, for example here:

Why do you:

    studentPointer studentReader = new student;
    studentReader = (studentPointer) malloc(sizeof(student));

you dont need (studentPointer) malloc(sizeof(student)); . Whats more you should not use malloc as it will not call constructors for string variables.

In the end never ending loop is probably caused by Undefined Behaviours in your code.

[edit]

The reason for infinite loop is because you dont allocate each time you read a student from file, so you end up with all the same students. Below is corrected code.:

    int numberOfStudentsLoaded = 0;
    while (true){
            studentReader = (studentPointer) malloc(sizeof(student));

            if ((fread(studentReader, sizeof(student), 1, file) != 1)
               break;

            addStudent(studentReader);
            numberOfStudentsLoaded++;
    };