以 QIODevice::Append 模式打开的 QFile 意外工作。这是一个Qt错误吗？

Question

在我的 class StudentTable 中，我创建了一个函数 appendRecord，如下所示-

qint64 StudentTable::appendRecord(Student student){
QFile file(tableName);
QDataStream stream(&file);
qDebug()<<file.open(QIODevice::Append);
qint64 dumpPosition=file.pos();
qDebug()<<file.openMode();

qDebug()<<dumpPosition;
student.next=-1;
student.print();
stream<<student;
file.close();
return dumpPosition;

}

问题是，当我直接从 main() 函数调用函数时，它工作正常。但是当我从同一个 class 中的另一个函数调用同一个函数时，它什么也不做；表示不向文件附加任何内容；

也尝试了 QIODevice::Append、QIODevice::Append|QIODevice::ReadWrite，但对我没有任何作用。我试过是因为 this question : Open QFile for appending 说要尝试另一种方法；已接受答案的评论。

我正在尝试为学生制作一个文件来记录学生。制作基于记录的平面文件数据库；使用链表。

代码：AddRecord() 正在调用函数 appendRecord() 在这种情况下没有任何反应。但是当从 main() 函数调用时，它会在最后附加记录。

增加StudentTable的记录功能class...

 bool StudentTable::addRecord(Student student)
    {
        if(isEmpty()&&!isHeapAvailable())
        {
            qDebug()<<"case 1";
            student=Student("rajesh","ramesh",4,5,6);
            qint64 dumpPosition= appendRecord(student);
            Header head=header();
            head.used=dumpPosition;
            head.usedLast=dumpPosition;
            setHeader(head); 
                /*If I comment this setHeader() call here then  
                  appending works else not */

            return true;
        }

        if(!isEmpty()&&!isHeapAvailable())
        {
             qDebug()<<"case 2";
             qint64 dumpPosition= appendRecord(student);
             Header head=header();

             head.usedLast=dumpPosition;
             setHeader(head); /*If I comment this setHeader() call here then  
                  appending works else not */
             return true;
        }

        return false;

    }

我正在使用 Windows 8、64 位。

项目的所有文件：-

Header.h

#ifndef HEADER_H
#define HEADER_H
#include<QtCore>

class Header
{
public:
    qint32 magicNumber;
    qint32 streamVersion;
    qint64 used;
    qint64 usedLast;
    qint64 freeLast;
    qint64 free;
    Header();
    void debug();

};

QDataStream &operator>>(QDataStream &in, Header &painting);
QDataStream &operator<<(QDataStream &out, Header &painting);

#endif // HEADER_H

Header.cpp

#include "header.h"
#include<QtGui>
#include<QtCore>

Header::Header()
{
}

void Header::debug()
{
    qDebug()<<magicNumber<<streamVersion<<used<<free<<usedLast<<freeLast;
}

QDataStream & operator<<(QDataStream &out, Header &header)
{
    out<<header.magicNumber<<header.streamVersion<<header.used<<header.free<<header.usedLast<<header.freeLast;
    return out;
}

QDataStream & operator>>(QDataStream &in, Header &header)
{
    in>>header.magicNumber>>header.streamVersion>>header.used>>header.free>>header.usedLast>>header.freeLast;
    return in;
}

Student.h

#ifndef STUDENT_H
#define STUDENT_H
#include<QtCore>
#include "record.h"

class Student
{
public:

    bool valid;
    static QString fileName;
    quint64 recordLength;
    QString name,fname;
    quint8 age,weight,clss;
    qint64 next;

    Student();
    Student(QString name,QString fname,quint8 age,quint8 weight, quint8 clss);
    void encrypt();
    void print();

};

QDataStream &operator>>(QDataStream &in, Student &painting);
QDataStream &operator<<(QDataStream &out, Student &painting);


#endif // Student_H

Student.cpp

#include "student.h"

Student::Student(QString name, QString fname, quint8 age, quint8 weight, quint8 clss)
{

    Student::fileName="Student";
    recordLength=291;
    this->name=QString(70,' ');
    this->name.replace(0,name.length(),name);

    this->fname=QString(70,' ');
    this->fname.replace(0,fname.length(),fname);

    this->age=age;
    this->weight=weight;
    this->clss=clss;
    this->next=-1;
}

void Student::encrypt()
{

}

Student::Student()
{
    Student::fileName="Student";
    recordLength=291;
    this->name=QString(70,'0');

    this->fname=QString(70,'0');

    this->age=-1;
    this->weight=-1;
    this->clss=-1;
    valid=true;
    this->next=-1;
}

QDataStream & operator<<(QDataStream &out, Student &f)
{

    out<<f.name<<f.fname<<f.age<<f.weight<<f.clss<<f.next;

    return out;


}
QDataStream & operator>>(QDataStream &in, Student &f)
{

    in>>f.name>>f.fname>>f.age>>f.weight>>f.clss>>f.next;

    return in;
}

void Student::print()
{
    qDebug()<<name<<fname<<age<<weight<<clss<<next;
}

QString Student::fileName;

StudentTable.h

#ifndef STUDENTTABLE_H
#define STUDENTTABLE_H
#include "student.h"
#include "header.h"
#include <QtGui>
#include<QtCore>

class StudentTable
{
public:
    StudentTable();
    QString tableName;


    quint64 next;

    void create();
    Header header();
    void setHeader(Header header);
    bool addRecord(Student student);
    QStandardItemModel * getModel();
    qint64 appendRecord(Student student);
    bool isEmpty();
    bool isHeapAvailable();
    void print();
private:


};

StudentTable.cpp

#include "studenttable.h"
#include<QtGui>

#include "header.h"

StudentTable::StudentTable()
{
    tableName="Student";
}

void StudentTable::create()
{
    QFile file(tableName);
    if(file.exists())
        return;
    else
        file.open(QIODevice::Append);
    QDataStream stream(&file);

    Header header;
    header.magicNumber=2015;
    header.streamVersion=stream.version();
    header.used=-1;
    header.free=-1;
    header.usedLast=-1;
    header.freeLast=-1;

    stream<<header;
    file.close();

}

void StudentTable::setHeader(Header header)
{
    QFile file(tableName);
    if(!file.exists())
    {
        qDebug()<<"File "<<tableName<<" doesn't exists";
        return;
    }
    file.open(QIODevice::WriteOnly);
    QDataStream stream(&file);

    stream<<header;

    file.close();
}

Header StudentTable::header()
{
    QFile file(tableName);


    file.open(QIODevice::ReadOnly);
    QDataStream stream(&file);
    Header header;
    stream>>header;

    file.close();
    return header;
}

bool StudentTable::isEmpty()
{
    Header head=header();
    if(head.used==-1)
        return true;
    else return false;
}

bool StudentTable::isHeapAvailable()
{
    Header head=header();
    if(head.free==-1)
        return false;
    else return true;
}

 qint64 StudentTable::appendRecord(Student student)
{
    QFile file(tableName);
    QDataStream stream(&file);
    qDebug()<<file.open(QIODevice::Append);
    qint64 dumpPosition=file.pos();
    qDebug()<<file.openMode();

    qDebug()<<dumpPosition;
    student.next=-1;
    student.print();
    stream<<student;
    file.close();
    return dumpPosition;
}

 void StudentTable::print()
 {
     if(isEmpty())
         qDebug()<<" Table is empty";
     else
     {
         Header head=header();
     QFile file(tableName);
     file.open(QIODevice::ReadOnly);
     QDataStream stream(&file);
     file.seek(head.used);

     Student current;
     stream>>current;
     current.print();
     while(current.next!=-1)
     {
         file.seek(current.next);
         stream>>current;
         current.print();

     }
     file.close();
     }

 }
bool StudentTable::addRecord(Student student)
{
    if(isEmpty()&&!isHeapAvailable())
    {
        qDebug()<<"case 1";
        student=Student("rajesh","ramesh",4,5,6);
        qint64 dumpPosition= appendRecord(student);
        Header head=header();
        head.used=dumpPosition;
        head.usedLast=dumpPosition;
        setHeader(head); 
            /*If I comment this setHeader() call here then  
              appending works else not */

        return true;
    }

    if(!isEmpty()&&!isHeapAvailable())
    {
         qDebug()<<"case 2";
         qint64 dumpPosition= appendRecord(student);
         Header head=header();

         head.usedLast=dumpPosition;
         setHeader(head); /*If I comment this setHeader() call here then  
              appending works else not */
         return true;
    }

    return false;

}

QStandardItemModel * StudentTable::getModel()
{
    QStandardItemModel *model;
    return model;

}

main.cpp

#include "widget.h"
#include <QApplication>
#include "Student.h"
#include "studenttable.h"
#include<QDebug>
#include<QtGui>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    StudentTable studentTable;
    studentTable.create();
    Header header;
    header=studentTable.header();
    header.debug();

    for(int i=1;i<2;i++)
    {
        Student student = Student("ram","bhairu",i,i,i);
        studentTable.addRecord(student);

    }

header=studentTable.header();
header.debug();

//studentTable.print();

    Widget widget;
   // widget.table->setModel(model);
    widget.show();

    return a.exec();
}

Answer 1

只是文件打开模式的命名造成了混淆。问题出在 setHeader() 函数中。

void StudentTable::setHeader(Header header)
{
    QFile file(tableName);
    if(!file.exists())
    {
        qDebug()<<"File "<<tableName<<" doesn't exists";
        return;
    }
    file.open(QIODevice::WriteOnly);
    QDataStream stream(&file);

    stream<<header;

    file.close();
}

每次我必须写文件头时，我都会以 QIODevice::WriteOnly 模式打开文件；这听起来很正确，因为我既不阅读也不寻找文件位置。但问题是 QIODevice::WriteOnly 模式；清除文件中的所有数据，然后将数据写入文件；我期望覆盖的内容；

所以结论是：

If you have to open a file and overwrite some data; use ReadWrite mode instead of QIODevice::WriteOnly mode. QIODevice::WriteOnly will clear all previous data before writing

我不知道他们为什么这样命名。但是模式命名在 qt 和 C++ 中确实令人困惑。同样的事情也适用于 C++。

以 QIODevice::Append 模式打开的 QFile 意外工作。这是一个Qt错误吗？

QFile opened in QIODevice::Append mode working unexpectedly. Is it a Qt bug?

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