MRE /概念证明

Question

我用c ++为学生数据库创建了一个程序。该数据库有7个选项可供选择。例如输入学生详细信息，编辑记录等。通过case语句可以选择不同的选项。我的问题与情况6有关。在这种情况下，我必须将输入的文本显示到文本文件中，并能够对其进行读写。我可以写但不能读。我只张贴了案例6部分。注意：我创建了一个班级来存储学生的信息，例如姓名，姓氏等，“ stud”是班级学生的对象。

    class students
    {
        public:

        char name[50];
        char sname[45];

        int idno;
        char cours[40];
        int age;
        int modules;

        void read_data();
        void display();

    }s;

   void students::read_data()
   {
        cout<<"Enter name :";
        cin>>name;
        cout<<"Enter Surname : ";
        cin>>sname;
        cout<<"Enter course : ";
        cin>>cours;
        cout<<"Enter age :";
        cin>>age;
        cout<<"Enter modules:";
        cin>>modules;
      }
    void students::display()
    {
        cout<<"-----------------------------------------------"<<endl;
        cout<<"Student ID no : "<<idno<<endl;
        cout<<"Student name is :"<<name<<endl;
        cout<<"Student Surname is : "<<sname<<endl;
        cout<<"Student course is :"<<cours<<endl;
        cout<<"Student age is  :"<<age<<endl;
        cout<<"Student modules are:"<<modules<<endl;
        cout<<"----------------------------------------------"<<endl;
    }

     int main()
     {

       char dow;
       int arr=0;
      do{

        students stud[100];

        cout<<"Press 1 Enter record \n";
        cout<<"Press 2 Display record \n";
        cout<<"\n\t Select option::";

        int idcheck=0;

        int sw;
        cin>>sw;

        switch(sw)
        {

        case 1:

        cout<<"\n Enter the data of the student no "<<arr+1<<" is :\n";

                cout<<"\t Enter the Roll No = ";
                int id2;
                int id;

                cin>>id;

                for(int j=0; j<arr; j++)
                {

                    id2=id;
                    if(id2==stud[j].idno)
                    {
                        idcheck=1;
                     }
                 }
                 if(idcheck!=1){
                    stud[arr].idno=id;

                 stud[arr].read_data();

                    arr=arr+1;
                 }
                 else
                 {
                    cout<<"This Record is Already Entered \n"; 
                 }

                 break;
    case:2   
   {
    ifstream sfile;
    sfile.open("StudentFile.txt", ios::in);

    cout << "\n-----------------------------------------------" << endl;
    for(int i = 0; i < 1; i++)
    {
    cout << setw(12) << "Rollno ||";
    cout << setw(12) << " Name   ||";
    cout << setw(12) << " Surname    || ";
    cout << setw(12) << "Age  ||";
    cout << setw(12) << " Course  ||";
    cout << setw(12) << " Module   ||";

    for(int k = 0; k < arr; k++)
    {
        if(stud[k].idno != 'd')
    {
            cout << "\n";
            cout << " ";
            cout << setw(5) << stud[k].idno;                            
            cout << setw(15) << stud[k].name;                           
            cout << setw(14) << stud[k].sname;                       
            cout << setw(14) << stud[k].age;                         
            cout << setw(12) << stud[k].cours;                       
            cout << setw(14) << stud[k].modules;                        
    }
    }
    }
     cout << "\n-------------------------------------------"   << endl;

     sfile.read((char*)&s, sizeof(s));
     sfile.close();
}

Answer 1

一个流包含两个“指针”，它们指向它正在读取和正在写入的位置。如果您正在写，则读指针也将前进。所以您首先要向后移动读取指针，使您将使用seekg()读取的字节数：

cfile.seekg(-static_cast<long>(sizeof stud), cfile.cur);

MRE /概念证明

#include <iostream>
#include <fstream>

struct Foo {
    int i = 2;
    double d = 3.13421;
    float f = 3241.41f;
};

std::ostream& operator<<(std::ostream& s, const Foo& foo) {
    return s << "i=" << foo.i << "; d=" << foo.d << "; f=" << foo.f;
}

int main() {
    std::fstream file;
    Foo foo;

    file.open("foo.txt", file.in | file.out | file.trunc);

    file << "some other text";

    file.write(reinterpret_cast<char*>(&foo), sizeof foo);

    std::cout << foo << std::endl;

    foo.i = foo.d = foo.f = 0;

    file.seekg(-static_cast<long>(sizeof foo), file.cur);

    std::cout << foo << std::endl;

    file.read(reinterpret_cast<char*>(&foo), sizeof foo);

    std::cout << foo << std::endl;
}

产生以下输出：

i=2; d=3.13421; f=3241.41
i=0; d=0; f=0
i=2; d=3.13421; f=3241.41

mport如何读写文本文件

1 个答案:

MRE /概念证明