从c++二进制文件中读取结构

所以我试图从我写的二进制文件中读取结构，但有些东西似乎是错误的。下面是我的main:

//Fraction structure
struct Fraction
{
int num, den;
};

int main()
{
int menu_selection;
char slash;
fstream datafile; //creates file object
Fraction* f; //creates pointer for dam
//menu to the user
menu_selection = main_menu();
bool condition = true;
while (condition)
{
    switch(menu_selection)
    {
    case 1: //adds a new Fraction structure to binary file
        f = new Fraction; //dynamically allocating memory
        cout << "Enter fraction to add: "; //adds fraction to file
        cin >> f->num >> slash >> f->den;//stores input
        datafile.open("fractions.dat",ios:: out | ios:: app | ios::binary); //creates file
        datafile.write(reinterpret_cast<char *>(f),sizeof(Fraction));//stores fraction to file
        datafile.close();
        menu_selection = main_menu();
        break;
    case 2:
        menu_selection = main_menu();
        break;
    case 3: //displays the contents of the file to user
        datafile.open("fractions.dat", ios::in | ios::binary);
        while(!datafile.eof())
        {
            datafile.read(reinterpret_cast<char *>(f),sizeof(f));
            cout << f->num << "/" << f->den << " ";
        }
        datafile.close();
        menu_selection = main_menu();
        break;
    case 4:
        menu_selection = main_menu();
        break;
    case 5:
        menu_selection = main_menu();
        break;
    case 6: condition = false;
        return 0;
        break;
    default: cout << endl << "That is an invalid option. Please try again." << endl; //default option
        break;
    }
}
delete f;
}

当用户输入1时，我动态地为结构Fraction分配内存并将其写入二进制文件。我使用ios::app是因为我想在再次调用时添加到文件中。写或读功能有什么问题吗?当我按原样运行程序时，如果我输入一个分数，比如1/5，它显示1/5 5/5 5/5，但是如果我输入两个分数，比如1/5和2/6，它显示2/6 6/6 6/6。