c++ - 将自定义 QMetaType 与 QDataStream 结合使用

Question

我正在尝试使用 QDataStream 对象加载和存储自定义 QMetaType。这是一个例子:

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QString test1("/test1/");
    const QString path1(QDir::homePath()+test1);
    qDebug() << "path1 variable is " << path1;
    QDir apphome(path1);
    qDebug() << "apphome path is " << apphome.absolutePath();
    if (!apphome.mkdir(path1)) { qDebug() << "mkdir returned false. directory already exists?"; }
    if(!apphome.setCurrent(path1)) { qDebug() << "did not set current directory"; }
    qDebug() << "apphome path is " << apphome.absolutePath();
    Basic basic1;
    Basic basic2;
    basic1.value = 14;
    QFile file1("file1name");
    if (!file1.open(QIODevice::WriteOnly)) { qDebug() << "file1 not open."; }
    QDataStream dataStream1(&file1);
    QVariant qvar1;
    qvar1.setValue(basic1);
    dataStream1 << (quint32)12345;
    dataStream1 << qvar1;
    file1.close();
    file1.open(QIODevice::ReadOnly);
    QDataStream dataStream2(&file1);
    quint32 magic;
    QVariant qvar2;
    dataStream1 >> magic;
    qDebug() << "magic number is " << magic;
    dataStream2 >> qvar2;
    file1.close();
    basic2 = qvar2.value<Basic>();
    qDebug() << "14 = " << basic1.value << " = " << basic2.value << ".";

    //MainWindow w;
    //w.show();

    return a.exec();
}

魔数(Magic Number)回来了，但是有一条消息 QVariant::save: unable to save type 'Basic' (type id: 1026). 然后当然是 QVariant::加载:无法加载类型 1026.，然后是 14 = 14 = 0。 Basic 类仅来自 QMetaType 文档:

struct Basic
{
    Basic();
    Basic(const Basic &basic);
    ~Basic();
    int value;
};
Q_DECLARE_METATYPE(Basic)

// basic.cpp

#include "basic.h"

Basic::Basic() {}

Basic::Basic(const Basic &basic)
{
    value = basic.value;
}

Basic::~Basic(){}

我的想法已经用完了，有人知道是什么导致了这个问题吗？ Qt的版本是5.10.1。

Answer 1

因为编译器无法读懂你的想法，你需要描述序列化是如何可能的，例如

struct Basic
{
    Basic();
    Basic(const Basic &basic);
    ~Basic();
    int value;

    friend QDataStream & operator << (QDataStream &arch, const Basic& object)
    {
        arch << object.value;
        return arch;
    }

    friend QDataStream & operator >> (QDataStream &arch, Basic& object)
    {
        arch >> object.value;
        return arch;
    }
};
Q_DECLARE_METATYPE(Basic);

在主函数中

qRegisterMetaType<Basic>("Basic");
qRegisterMetaTypeStreamOperators<Basic>("Basic");

在保存\加载操作发生之前。 Q_DECLARE_METATYPE 需要使用 QVariant 存储类型，这两个需要注册对象的“三巨头”以将其作为资源及其序列化方法进行管理。输出:

path1 variable is  "C:/Users/Yaroslav/test1/"
apphome path is  "C:/Users/Yaroslav/test1"
mkdir returned false. directory already exists?
apphome path is  "C:/Users/Yaroslav/test1"
magic number is  12345
14 =  14  =  14 .

附言请注意，如果您在没有窗口的情况下离开 return a.exec();，程序将永远保留在内存中，直到您停止它。