C++ 以前的记录在写入新对象时被删除

Posted 2023-02-22

【中文标题】C++ 以前的记录在写入新对象时被删除

【英文标题】：C++ Previous Record Gets deleted when writing a new object

【发布时间】：2017-05-29 06:26:57

【问题描述】：

我正在尝试从一个类中保存一个对象。当我添加新记录（添加正在保存的新对象）时，以前的记录将被删除。我究竟做错了什么？似乎一旦我添加了一个新对象，之前创建的对象就会因为 idk.已经尝试了几个小时。好像没看懂。

void Department2File(HANDLE screen, Department &Departments, int deptcount)

    Department Testing;
    fstream file;
    file.open("Departmentsdata.dat", ios::out | ios::binary);
    file.close();
    file.open("Departmentsdata.dat", ios::in | ios::out | ios::binary);

    if (!file)
    
        cout << "Error opening file.";
    

    // writting data
    long size = (deptcount * sizeof(Departments));

    if (deptcount == 0)
    
        size = (deptcount * sizeof(Departments));
    
    else
    
        size = (deptcount * sizeof(Departments))+1;
    

    cout << "\nsaved!" << endl;;
    file.clear();
    file.seekp(size, ios::beg);
    file.write(reinterpret_cast <char *>(&Departments), sizeof(Departments));
    file.clear();
    file.seekg(size, ios::beg);
    file.read(reinterpret_cast<char *> (&Testing), sizeof(Departments));


    cout << Testing.DepartmentID;////////////<--------------delete

    ///////////////////////////////
    ///////////////DELETE//////////
            file.clear();
            file.seekg(0, ios::beg);
            file.get(reinterpret_cast<char *> (&Testing), sizeof(Departments));
            cout <<"\n RECORD 0: "<<Testing.DepartmentID;

                                    ////////////////////////////
    file.close();

【参考方案1】：

在open函数中添加标志ios::app以在文件末尾添加输出：

file.open("Departmentsdata.dat", ios::out | ios::app | ios::binary);

查看此链接：http://www.cplusplus.com/doc/tutorial/files/

更新：尝试在您的代码中只使用一个 open：

fstream file;
file.open("Departmentsdata.dat", ios::out | ios::binary);
file.close();
file.open("Departmentsdata.dat", ios::in | ios::out | ios::binary);

像这样：

fstream file;
file.open("Departmentsdata.dat", ios::in | ios::app | ios::out | ios::binary);
//Write Something
file.close();

但这取决于你想要做什么。