Ofstream 文件在打开时被覆盖

Question

所以我必须为一项作业制作一个银行程序，而我的教授想要每个“操作”的日志文件（他对此真的很模糊，我决定记录输入和输出）。我遇到的问题是，每次我调用一个新函数时，我都必须重新打开文件，它会覆盖以前存在的内容。据我所知，重复使用 .open 会导致我所在的任何函数忽略之前的输出。
我尝试声明一个全局 ofstream 希望它会改变，但问题仍然存在。冲洗和/或关闭似乎没有帮助，但我完全有可能滥用它们或语法错误。下面的代码是main函数、read_accts函数、menu函数。如果我在调用 read_accts 函数之前终止程序，日志文件将显示“有多少帐户？”但是如果我允许程序调用其他两个函数，那么日志文件只有菜单文件的输出。我为这篇长篇文章道歉，但我不知道出了什么问题。

int main()
{
    ofstream log;
    log.open("Log.txt");
    int MAX_NUM = 100;
    BankAccount account[MAX_NUM];
    int num_accts = 0;
    char selection = 'Z';
    cout << "How many accounts are there?" << endl;
    log << "How many accounts are there?" << endl;
    cin >> MAX_NUM;
    read_accts(account, MAX_NUM, num_accts);
    cout << endl;
    cout << "There are " << num_accts << " accounts" << endl;
    log << "There are " << num_accts << " accounts" << endl;
    cout << endl;
    print_accts(account, MAX_NUM);
    cout << endl;
    `

    while (selection != 'Q' && selection != 'q')
    {

        menu();
        cin >> selection;
        log << selection;
        switch (selection)
        {
            case 'W':
                case 'w':
                withdrawal(account, num_accts);
                break;
            case 'D':
                case 'd':
                deposit(account, num_accts);
                break;
            case 'N':
                case 'n':
                new_acct(account, num_accts);
                break;
            case 'B':
                case 'b':
                balance(account, num_accts);
                break;
            case 'I':
                case 'i':
                account_info(account, num_accts);
                break;
            case 'C':
                case 'c':
                close_acct(account, num_accts);
                break;
            case 'Q':
                case 'q':
                print_accts(account, num_accts);
                break;
            default:
                cout << "Invalid selection" << endl;
                log << "Invalid selection" << endl;

        }
        cout << endl;
    }
    print_accts(account, MAX_NUM);
    cout << "Goodbye" << endl;
    log.close();
    return 0;
}

int read_accts(BankAccount account[], int MAX_NUM, int &num_accts)
{
    ofstream log;
    log.open("log.txt", std::ofstream::app);
    string f;
    string l;
    int social;
    int acct;
    string type;
    double bal;
    int i = 0;
    ifstream readfile;
    readfile.open("bankdatain.txt");
    if (!readfile)
    {
        cout << "Can't open input file." << endl;
        log << "Can't open input file." << endl;
        exit(1);
    }
    while (readfile >> f >> l >> social >> acct >> type >> bal)
    {
        account[i].setfname(f);
        account[i].setlname(l);
        account[i].setssnum(social);
        account[i].setacctnum(acct);
        account[i].settype(type);
        account[i].setbalance(bal);
        i++;
        num_accts++;
    }

    return num_accts;
}

void menu()
{
    ofstream log;
    log.open("Log.txt");
    cout << "W - Withdrawal" << endl;
    log << "W - Withdrawal" << endl;
    cout << "D - Deposit" << endl;
    log << "D - Deposit" << endl;
    cout << "N - New account" << endl;
    log << "N - New account" << endl;
    cout << "B - Balance" << endl;
    log << "B - Balance" << endl;
    cout << "I - Account Info" << endl;
    log << "I - Account Info" << endl;
    cout << "C - Close Account" << endl;
    log << "C - Close Account" << endl;
    cout << "Q - Quit" << endl;
    log << "Q - Quit" << endl;
    cout << "Please make your selection: " << endl;
    log << "Please make your  selection: " << endl;
}

Answer 1

据我所知，默认情况下，ofstream 将以 std::fstream::out 模式打开文件，如果将删除现有文件或创建新文件。

将其更改为 std::fstream::app http://www.cplusplus.com/reference/fstream/fstream/open/

编辑： 好像您只对一个文件使用了两个 ostream。不能拒绝访问Using std:fstream how to deny access (read and write) to the file

将ostream传递给被调用函数

int read_accts(BankAccount account[],int MAX_NUM,int &num_accts, ostream& log)
{
log.open("log.txt",std::ofstream::app); // remove this line since you already open it in main()

在 main 中调用它

read_accts(account,MAX_NUM,num_accts, log);

Answer 2

我认为你唯一的问题是你必须在写入文件之前添加这个。

log.seekp(0, std::ios_base::end);

这会将文件指针设置到文件末尾并继续从那里写入。

您的文件已经在int main () 中打开，如果您将变量log 传递给函数int read_accts(BankAccount account[], int MAX_NUM, int &num_accts) 会更好 并使其成为int read_accts(BankAccount account[], int MAX_NUM, int &num_accts, ofstream log );

并做出定义

int read_accts(BankAccount account[], int MAX_NUM, int &num_accts, ofstream log )
{
    string f;
    string l;
    int social;
    int acct;
    string type;
    double bal;
    int i = 0;
    ifstream readfile;
    readfile.open("bankdatain.txt");
    if (!readfile)
    {
        cout << "Can't open input file." << endl;
        log << "Can't open input file." << endl;
        exit(1);
    }
    while (readfile >> f >> l >> social >> acct >> type >> bal)
    {
        account[i].setfname(f);
        account[i].setlname(l);
        account[i].setssnum(social);
        account[i].setacctnum(acct);
        account[i].settype(type);
        account[i].setbalance(bal);
        i++;
        num_accts++;
    }

    return num_accts;
} 

PS seekp() 的语法是

  ostream& seekp (streampos pos);
  ostream& seekp (streamoff off, ios_base::seekdir way);

参考here

PSifstream 的等效项是 seekg ( 0, std::ios_base::end )