我将在二进制文件中保存一个结构并稍后加载它.我发现一种方法是使用QVariant.这是我创建的简化的Qt Widget应用程序示例.但是当我运行它时,二进制文件仍然是空的.你能帮我解决吗？另外,有没有更好的方法来做这样的事情？

mainwindow.h：

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QFile>
#include <QFileDialog>
#include <QDataStream>
#include <QString>

#include "mystruct.h"

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
};
#endif

mainwindow.cpp：

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    /* create an object of the struct */
    myStruct * aStruct = new myStruct;
    aStruct->myStringVar = QString("aaaaa");

    /* put the object in QVariant */
    QVariant aVariant;
    aVariant.setValue(aStruct);

    /* save the QVariant to binary file */
    QFile myFile("myFile");
    QDataStream save(&myFile);
    save.setVersion(QDataStream::Qt_4_6);
    save << aVariant;
    myFile.close();

    /* load the QVariant from binary file */
    QDataStream load(&myFile);
    load.setVersion(QDataStream::Qt_4_6);
    QVariant theVariant;
    load >> theVariant;
    QString theVar = theVariant.value<myStruct*>()->myStringVar;
    myFile.close();
}

MainWindow::~MainWindow()
{
    delete ui;
}

mystruct.h：

#ifndef MYSTRUCT_H
#define MYSTRUCT_H

#include <QMetaType>
#include <QString>

struct myStruct
{
    QString myStringVar;
};

Q_DECLARE_METATYPE(myStruct*)

#endif // MYSTRUCT_H

评论：作为参考,this link显示了类似的问题,但我无法正确理解/实施建议.例如,我试图定义一个新的运算符,如here所述,但它不起作用.

解决方法:

首先你不应该保存指针,因为这只是变量的内存地址,应该保存的是值.另一方面,如果要使用QDataStream,则必须覆盖流操作符,最后如果要保存为QVariant,则必须使用qRegisterMetaTypeStreamOperators.除上述内容外,您的代码还有一些错误,例如无法打开QFile.

mystruct.h

#ifndef MYSTRUCT_H
#define MYSTRUCT_H

#include <QString>
#include <QMetaType>

struct myStruct
{
    QString myStringVar;
    bool operator==(const myStruct & o){
        return o.myStringVar == this->myStringVar;
    }
    friend QDataStream &operator<<(QDataStream &out, const myStruct &rhs){
        out << rhs.myStringVar;
        return out;
    }
    friend QDataStream &operator>>(QDataStream &in, myStruct &rhs){
        in >> rhs.myStringVar;
        return in;
    }
};
Q_DECLARE_METATYPE(myStruct)

#endif // MYSTRUCT_H

main.cpp中

#include "mystruct.h"

#include <QDataStream>
#include <QFile>
#include <QVariant>

int main(int argc, char *argv[])
{
    qRegisterMetaTypeStreamOperators<myStruct>("myStruct");

    // create struct
    myStruct aStructIn{"aaa"};

    // myStruct to QVariant
    QVariant aVariant;
    aVariant.setValue(aStructIn);

    //open file
    QFile myFile("myFile");
    if(!myFile.open(QIODevice::WriteOnly))
        return -1;

    // save QVariant
    QDataStream save(&myFile);
    save.setVersion(QDataStream::Qt_4_6);
    save << aVariant;
    myFile.close();

    //open file
    if(!myFile.open(QIODevice::ReadOnly))
        return -1;

    // read QVariant
    QDataStream load(&myFile);
    load.setVersion(QDataStream::Qt_4_6);
    QVariant theVariant;
    load >> theVariant;
    myFile.close();

    // QVariant to myStruct
    myStruct aStructOut = theVariant.value<myStruct>();

    Q_ASSERT(aStructOut == aStructIn);

    return 0;
}