1

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,我只是附加到文件中,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];
};
4

2 回答 2

1
fstream Airline("AirLine.text", ios::out | ios::in | ios::app | ios::binary);

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

app (append)在每次输出操作之前将流的位置指示器设置为流的末尾。

(强调补充)。看起来你正在得到你要求的行为。也许你想要

ate (at end) 将流的位置指示器设置为打开时流的末尾。

于 2011-11-27T19:01:34.437 回答
0

seekg与正确的位置参数一起使用:

seekg(abs_pos, std::ios::beg);  // seek relative to the beginning

默认值为std::ios::cur,即相对于当前位置。

seekp.


具体来说,这样说:

Airline.seekp((passenger.seat_number - 1 ) * sizeof(Newpassenger), std::ios::beg);
//                                                               ^^^^^^^^^^^^^^^

于 2011-11-27T12:17:08.433 回答