Simple custom QVariant serialization not working

[Per Ivar Bruheim]

I have implemented a custom QVariant type by registering the class with Q_DECLARE_METATYPE(Vector) and defining the stream operators.

Vector.h:

class Vector
{
public:
   Vector();
   QString mFoo;
};

Q_DECLARE_METATYPE(Vector)
QDataStream& operator<<(QDataStream& stream, const Vector& v);
QDataStream& operator>>(QDataStream& stream, Vector& v);

Vector.cpp:

#include "Vector.h"

Vector::Vector()
{ }

QDataStream& operator<<(QDataStream& stream, const GVector& v)
{
   stream << v.mFoo;
   return stream;
}

QDataStream& operator>>(QDataStream& stream, Vector& v)
{
   stream >> v.mFoo;
   return stream;
}

Test code:

QMetaType metaType = QMetaType::fromName("Vector");
Q_ASSERT(metaType.hasRegisteredDataStreamOperators()); // PASSES

Vector vector;
vector.mFoo = "bar";
QVariant variant = QVariant::fromValue<Vector>(vector);

QByteArray bytes;
QDataStream stream(&bytes, QIODevice::WriteOnly);
stream << variant;

QDataStream stream2(&bytes, QIODevice::ReadOnly);
QVariant variant2;
stream >> variant2;
Vector vector2 = variant2.value<Vector>();
Q_ASSERT(vector2.mFoo == "bar"); // FAILS

The vector is not restored correctly--in fact the variant is null when inspecting it. The stream read function is not even called.

Is there something I'm missing? I assumed this is how serialization with custom types were supposed to work.

[Christian Ehrlicher]

You forgot qRegisterMetaTypeStreamOperators<>() : https://forum.qt.io/topic/1690/qvariant-save-load-unable-to-save-type-x/4

[Per Ivar Bruheim]

@Christian-Ehrlicher I'm using Qt 6, where that function has been removed. According to the docs the stream operators are detected automatically.

[Christian Ehrlicher]

The you should at least call qRegisterMetaType<>().

[Per Ivar Bruheim]

@Christian-Ehrlicher I've tried that too, but it still doesn't work.

[Christian Ehrlicher]

I tried your code here (no copy and paste) with Qt6. 6 and it worked correct, even without qRegisterMetaType<>(). Will have to find out the difference.

/edit: please output the bytearray with qDebug to see if it was properly serialized.

[Christian Ehrlicher]

@Per-Ivar-Bruheim said in Simple custom QVariant serialization not working:

QVariant variant2;
stream >> variant2;

OK, you should correct this.

[Per Ivar Bruheim]

@Christian-Ehrlicher @Christian-Ehrlicher Thank you. Such an embarrasingly simple mistake.

[Per Ivar Bruheim]

This post is deleted!