[英]Problem while reading objects from file in c++
我正在做一個小型大學項目,我必須使用 OOP 概念在文件中添加、編輯和搜索記錄。 添加到文件中工作正常,但每當我嘗試從文件中讀取時,它都會以不可讀的文本打印。 這是完整的代碼和 output。 主文件
#include <iostream>
#include <cstdlib>
#include <fstream>
#define MIN 20
#define MAX 100
#include "student.h"
using namespace std;
void add_student();
void edit_student();
void search_student();
void addToFile(const Student&);
Student* fetchFromFile();
int getFileSize();
void updateFile(Student*);
// Student stud[MAX];
int main()
{
int choice;
system("cls");
system("Color B0");
while(1)
{
cout<<"\n\t\tWhat do you want to do?"<<endl;
cout<<"\t\t----------------------"<<endl;
cout<<"\t\t1-Add student"<<endl;
cout<<"\t\t2-Edit student"<<endl;
cout<<"\t\t3-Search student"<<endl;
cout<<"\t\t4-Quit Program"<<endl;
cout<<"\t\t----------------------"<<endl;
cout<<"Enter your choice: ";
cin>>choice;
switch(choice)
{
case 1:
add_student(); //calling add_student function to add records.
break;
case 2:
edit_student();
break;
case 3:
search_student();
break;
case 4:
return 0;
break;
default:
cout<<"Invalid choice";
break;
}
}
return 0;
}
int Student::id = getFileSize() - 1; //Initialize id equals to size of file
// setData function of class Student definition
void Student :: setData()
{
// taking input from user
cout<<"Enter student roll no in format(1XXX): ";
cin>>roll;
cout<<"Enter student name: ";
cin>>name;
cout<<"Enter stduent date of birth(dd/mm/yy): ";
cin>>dob;
cout<<"Enter stduent phone no: ";
cin>>phone;
cout<<"Enter student address: ";
cin>>address;
stdId = Student::id;
}
void Student :: showData()
{
cout<<stdId<<" ";
cout<<roll<<" ";
cout<<name<<" ";
cout<<dob<<"\t";
cout<<phone<<" ";
cout<<address<<"\n\n";
}
const int Student :: getRoll()
{
return roll;
}
Student& Student::operator = (const Student& newObj)
{
stdId = newObj.stdId;
roll = newObj.roll;
name = newObj.name;
dob = newObj.dob;
phone = newObj.phone;
address = newObj.address;
return *this;
}
void add_student()
{
Student stud;
Student::incrementId();
stud.setData();
addToFile(stud); //adding records to file
system("CLS");
cout<<endl;
cout<<"--------------------------------------------------------------------------------"<<endl;
cout<<"---------------------------Student updated record Table---------------------------------"<<endl;
cout<<"--------------------------------------------------------------------------------"<<endl;
cout<<"ID "<<"Roll "<<"Name "<<"DOB "<<"Phone no "<<"Address\n\n";
cout<<"--------------------------------------------------------------------------------"<<endl;
Student* student = fetchFromFile(); //getting records from file in array of objects
int length = getFileSize(); //getting length of array of objects
for(int i=0; i<(length-1); i++)
{
student[i].showData(); //showing all the data
}
cout<<"--------------------------------------------------------------------------------"<<endl;
cout<<"---------------------------------FINISH-----------------------------------------"<<endl;
cout<<"--------------------------------------------------------------------------------"<<endl;
cout<<"You want to add more?(Y/n): ";
char c;
cin>>c;
if(c=='y' || c=='Y')
{
add_student();
}
else{
system("pause");
}
}
void edit_student(){
//Showing existing record first before editing
cout<<endl;
cout<<"--------------------------------------------------------------------------------"<<endl;
cout<<"---------------------------Student Existing record Table---------------------------------"<<endl;
cout<<"--------------------------------------------------------------------------------"<<endl;
cout<<"ID "<<"Roll "<<"Name "<<"DOB "<<"Phone no "<<"Address\n\n";
cout<<"--------------------------------------------------------------------------------"<<endl;
Student* student = fetchFromFile(); //fetching all records from file
int length = getFileSize();
for(int i=0; i<(length-1); i++)
{
student[i].showData();
}
int idnumber;
cout<<"--------------------------------------------------------------------------------"<<endl;
cout<<"Which ID number your want to edit: ";
cin>>idnumber; //Asking the user at which ID he wants to make a change.
//checking for valid id number
if(idnumber>length || idnumber<0)
{
cout<<"\nInvalid ID Number."<<endl;
}
//showing existing information about that specific record
cout<<"\nExisted information about this record.\n\n";
cout<<"--------------------------------------------------------------------------------"<<endl;
cout<<"ID "<<"Roll "<<"Name "<<"Father\tCell no. "<<"DOB "<<"Address\n\n";
cout<<"--------------------------------------------------------------------------------"<<endl;
student[idnumber].showData();
cout<<"\n\nEnter new data for above shown record.\n\n";
student[idnumber].setData(); //Inputting data for that specific record.
updateFile(student);
cout<<"\n\nRecord updated successfully."<<endl;
cout<<endl;
cout<<"--------------------------------------------------------------------------------"<<endl;
cout<<"---------------------------Updated record Table---------------------------------"<<endl;
cout<<"--------------------------------------------------------------------------------"<<endl;
cout<<"ID "<<"Roll "<<"Name "<<"DOB "<<"Phone no "<<"Address\n\n";
cout<<"--------------------------------------------------------------------------------"<<endl;
for(int i=0; i<(length-1); i++) //Showing updated record Table
{
student[i].showData();
}
}
void search_student(){
Student* student = fetchFromFile();
int fileLenth = getFileSize() - 1;
int searchkey;
cout<<"Enter roll_no of student you want to search: ";
cin>>searchkey; //roll_no as the search key can be entered by user.
for(int i=1; i<fileLenth; i++)
{
if(searchkey==student[i].getRoll()) //checking for roll no
{
student[i].showData();
}
}
cout<<"--------------------------------------------------------------------------------"<<endl;
cout<<"---------------------------------FINISH-----------------------------------------"<<endl;
cout<<"--------------------------------------------------------------------------------"<<endl;
system("pause");
}
//FILE HANDLING
void addToFile(const Student& obj)
{
ofstream fout;
fout.open("records.txt", std::ofstream::app | std::ofstream::binary);
fout.write((char*)&obj, sizeof(obj));
cout<<"Added to file successfully!"<<endl;
fout.close();
}
Student* fetchFromFile()
{
int i=0;
Student obj;
Student* returnObj = new Student[MAX];
ifstream fin;
fin.open("records.txt", std::ifstream::binary);
while(!fin.eof())
{
fin.read((char*)&obj, sizeof(obj));
returnObj[i] = obj;
i++;
}
fin.close();
delete[] returnObj;
return returnObj;
}
int getFileSize()
{
int i=0;
Student obj;
ifstream fin;
fin.open("records.txt", std::ifstream::binary);
while(!fin.eof())
{
fin.read((char*)&obj, sizeof(obj));
i++;
}
fin.close();
return i;
}
void updateFile(Student* student)
{
ofstream fout;
fout.open("records.txt", std::ofstream::binary);
fout.write((char*)&student, sizeof(student));
fout.close();
}
student.h header 文件
// A student class that hold students attributes like id, name, address and class
// Object of this class will be craeted to store student details
class Student
{
int stdId;
int roll;
std::string name;
std::string dob;
std::string phone;
std::string address;
public:
static int id; //we will increase 'id' whenever student is added to the record
//Member functions declaration
void setData(); //this function will take input from user and set the data to attributes of class
void showData(); //This function will give student data to user when called
static void incrementId()
{
id++;
}
const int getRoll();
Student& operator = (const Student&);
};
樣品 output 1
樣品 output 2
問題 1:顯示 id 的垃圾值。
樣品 output 3
樣品 output 4
樣品 output 5
問題 2:查看記錄如何以不可讀的形式打印。
為什么會這樣。 現在,如果我關閉程序並再次運行它,它仍然以不可讀的文本顯示。 希望你能得到我的問題。 我想對此有詳細的解釋。 另外,如果我犯了其他雜項錯誤,請告訴我。 謝謝!
Student
太復雜,無法像read
和write
那樣使用未格式化的 IO 來閱讀。 用技術術語來說,它不是Trivially Copyable ,因為它包含std::string
,並且string
不是 Trivially Copyable 。
最簡單的write
read
您的大部分數據都是字符串數據,因此請為Student
編寫<<
和>>
重載並將所有內容存儲為文本。
friend std::ostream & operator<<(std::ostream & out,
const Student & stu)
{
out << stu.stdId << ',' <<stu.roll << ','<< stu.name... <<'\n';
return out;
}
讀回數據有點棘手
friend std::istream & operator>>(std::istream & in,
Student & stu)
{
std::string line;
if (std::getline(in, line))
{
std::stringstream strm(line);
if (!(strm >> stu.stdId >> comma >> stu.roll >> comma) ||
!std::getline(strm, stu.name, ',') ||
!std::getline(strm, stu.dob, ',') ||
...))
{ // If any read failed, mark the stream as failed so the caller knows.
out.setstate(std::ios_base::failbit);
}
}
return out;
}
首先,一個小技巧,將 class 的空構造函數添加到初始所有成員、整數{0} 和字符串{},這將消除一些不需要的垃圾。
Student::Student() :stdId(0), roll(0), name{}, dob{}, phone{}, address{} {;}
您的主要問題來自 function fetchfomrfile() ,您在其中刪除了獲取的學生數組,因此調用者收到了一個未定義的數據數組:
Student* fetchFromFile()
{
int i=0;
Student obj;
Student* returnObj = new Student[MAX];
//reading records from file
fin.close();
delete[] returnObj;// <<<< youe deleted
return returnObj; // and return it as undefined
}
由於您能夠計算文件的大小,我建議在調用者 function 中:
void search_student(){
// Student* student = fetchFromFile();
Student *student = new Stduent [getFileSize()];
fecchFromFile(student); // use this array in fetch_file
// other things
}
將 fetchFromFile 重寫為:
viod fetchFromFile(Stduent *ss)
{
// read file data to array ss[i];
}
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.