c++ - 附加随机访问文件

Question

我在附加到随机访问文件时遇到问题。我似乎无法覆盖文件中的字段。相反，我只是附加到文件，并保持旧名称不变。

fstream Airline("AirLine.text", ios::out | ios::in | ios::app | ios::binary); 
if(!Airline)
{
    cout << "File Could not be opened." << endl;  
    system("PAUSE");
    exit (1);
}

cout << "Enter Seat Number to Update" << "(1 to 100, 0 to end)?";

NewPassenger passenger;
cin >> passenger.seat_number;

while (0 < passenger.seat_number && passenger.seat_number <= 100)
{
    Airline.seekp((passenger.seat_number - 1 ) * sizeof(NewPassenger));
    Airline.read(reinterpret_cast<char *>(&passenger), sizeof(NewPassenger));

    cout << " Found Record. Now Enter New Name:";
    cin >> passenger.name;

    Airline.seekg((passenger.seat_number - 1 ) * sizeof(NewPassenger),ios::beg);
    Airline.write(reinterpret_cast<const char *>(&passenger), sizeof(NewPassenger));

    cout << "\nEnter Seat Number: ?";
    cin >> passenger.seat_number;
}

Airline.close();
cout << endl;

NewPassenger 定义如下:

struct NewPassenger
{ 
    int seat_number;
    char name[20];
    char onward[3];
};

Answer 1

fstream Airline("AirLine.text", ios::out | ios::in | ios::app | ios::binary);

查看cplusplus.com on fstream::open我看到附加标志描述为

app (append) Set the stream's position indicator to the end of the stream before each output operation.

(强调)。看起来你正在得到你要求的行为。也许你想要

ate (at end) Set the stream's position indicator to the end of the stream on opening.