using custom QMetaType with QDataStream - c++

Question

I'm trying to load and store a custom QMetaType using QDataStream object. Here is an example:
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();
}
The magic number comes back, but there is a message QVariant::save: unable to save type 'Basic' (type id: 1026). and then of course QVariant::load: unable to load type 1026. and then 14 = 14 = 0. The Basic class is just from the QMetaType documentation:
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(){}
I'm running out of ideas, does anyone know what is causing the problem? The version of Qt is 5.10.1.

Answer 1

Because compiler can't read your mind, you need describe how serialization is possible, e.g.
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);
in main()
qRegisterMetaType<Basic>("Basic");
qRegisterMetaTypeStreamOperators<Basic>("Basic");
before save\load operation would happen. Q_DECLARE_METATYPE is required to store type with QVariant, those two are required to register object's "big three" to manage it as resource and its serialization methods. Output:
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 .
P.S. Note, if you left return a.exec(); with no window present, you get program staying in memory forever until you stop it.