[solved] Serialize heap objects pointed to by QList

[SGaist]

Did you consider Qt's implicit sharing helper classes ?

[Chris Kawa]

As @SGaist said in the first response you just need to provide the operator>> for the pointer. The syntax is not obvious but it's quite logical when you see it. Here's an example:

struct Foo {
    Foo(const QString& foo = QString()) : data(foo) {}
    QString data;
};

QDataStream& operator<<(QDataStream& out, const Foo* foo) { out << foo->data; return out;  }
QDataStream& operator>>(QDataStream& in, Foo*& foo) { foo = new Foo(); in >> foo->data; return in;  }
//yes, that's a reference to a pointer ;)

and then...

QByteArray storage;

QList<Foo*> orig { new Foo("bar"), new Foo("bazz") };
QDataStream storeStream(&storage, QIODevice::WriteOnly);
storeStream << orig;
qDeleteAll(orig);

QList<Foo*> retr;
QDataStream retrStream(&storage, QIODevice::ReadOnly);
retrStream >> retr;
qDeleteAll(retr);

[A Former User]

Hi guys! Thanks for your replies,
@SGaist : No, I wasn't aware of that feature. Will check it out!
@Chris-Kawa : Sounds aweseommely simple, but doesn't this contradict Samuel's former statment: "You have to [...] handle the creation of the objects yourself" ?
I'm talking about the part:
QList<Foo*> retr;
QDataStream retrStream(&storage, QIODevice::ReadOnly);
retrStream >> retr; -> does Qt call my constructor for every object in the list here?
qDeleteAll(retr);
Cheers,
Kalsan

[Chris Kawa]

@kalsan said:

Sounds aweseommely simple, but doesn't this contradict Samuel's former statment: "You have to [...] handle the creation of the objects yourself" ?

No. I am handling it myself by calling new Foo() in the operator>>.

There's no magic here. When Qt deserializes a list it resizes it to the size value stored in the data and initializes each of its element by calling operator>> on it. In this case the element type happens to be a pointer, but it's exactly the same as if it were an int or a QString. We need to initialize the variable in any case and in case of a pointer we do it by assigning a new Foo() to it and filling its data from the stream.

[Chris Kawa]

To answer your question specifically - no, it does not call a constructor for your objects. It calls a (default) constructor for a pointer to your object.
In pseudocode something like this:

stream >> typeid;
if(typeid == a_type_id_of_a_list) {
    stream >> sizeOfTheList
    QList<Type> list;
    for(int i=0; i<sizeOfTheList; ++i ) {
        Type variable; //creates a default constructed value of Type
        stream >> variable;
        list << variable;
    }
}

The important thing to notice is that the Type in this case is Foo*, a pointer. If it were Foo then it would default construct Foo object, but here all it does is default construct a pointer, which simply creates a null pointer. Then an operator>> is called and the pointer variable is passed to it by reference and we initialize it.

[A Former User]

Hey Chris,
So I spend hours trying to understand why the hell the code I generated from what I learned from you and the Qt documentation wouldn't even compile. The answer is simple: Deserializing QString works fine, but if I try to do that with a qint32... BAM!
So the Qt documentation for QDataStream says, tralala (yes, I AM slowly getting nuts with this), just go QString str; qint32 a; in >> str >> a; So far for the expectation.
Now, let's talk about reality. Please change in your Foo example the type of data from QString to qint32.
This is what I am getting (and no, even throwing the computer out of the window and picking it up 4200 floors beyond didn't help):
-> Error: invalid conversion from 'qint32 {aka int}' to 'UnitStruct*' [-fpermissive] QDataStream& operator>>(QDataStream& in, UnitStruct*& foo) { foo = new UnitStruct(); in >> foo->data; return in; } ^
-> note: initializing argument 2 of 'QDataStream& operator>>(QDataStream&, UnitStruct*&)' QDataStream& operator>>(QDataStream& in, UnitStruct*& foo) { foo = new UnitStruct(); in >> foo->data; return in; } ^
-> Error: cannot bind rvalue '(UnitStruct*)((long int)foo->UnitStruct::data)' to 'UnitStruct*&' QDataStream& operator>>(QDataStream& in, UnitStruct*& foo) { foo = new UnitStruct(); in >> foo->data; return in; } ^
By experience, I know I can be veeeery slow to get things. So please pardon me if I, once again, don't see the huge bar right in front of my head, but I am completely hopeless about this right now.
Thanks for your patience...
Kalsan

[Chris Kawa]

Yup, works just as well:

struct UnitStruct {
    UnitStruct(qint32 foo = 0) : data(foo) {}
    qint32 data;
};

QDataStream& operator<<(QDataStream& out, const UnitStruct* foo) { out << foo->data; return out;  }
QDataStream& operator>>(QDataStream& in, UnitStruct*& foo) { foo = new UnitStruct(); in >> foo->data; return in;  }

QByteArray storage;

QList<UnitStruct*> orig { new UnitStruct(42), new UnitStruct(66) };
QDataStream storeStream(&storage, QIODevice::WriteOnly);
storeStream << orig;
qDeleteAll(orig);

QList<UnitStruct*> retr;
QDataStream retrStream(&storage, QIODevice::ReadOnly);
retrStream >> retr;
qDeleteAll(retr);

[A Former User]

Well I seriously wonder if I might have a corrupt Qt version or something... I copy-pasted the first 6 lines of your code above (up to an without QByteArray storate;) and I get the three compile errors I pasted above. I did clean and rebuild the entire project.
I'm using unpatched (direct from repo) Qt 5.4.1-9 with Qt Creator on Arch Linux, default gcc configuration.
Looks to me like the exact same code will compile on your machine but not on mine. Should I consider filing a bug report?
Cheers,
Kalsan

[Chris Kawa]

Should I consider filing a bug report?

You might, but I'm not sure to whom exactly. This doesn't look like a Qt bug. More like a compiler one. What version of gcc are you using? I don't have a linux to test this but it works fine on both VS2013 and MinGW 4.9.1(which is basically gcc).

Oh, and btw.
Error: invalid conversion from 'qint32 {aka int}' to 'UnitStruct*'
This looks like the wrong thing is passed to the operator>>. Are you sure you didn't by mistake try to read the data into QList<qint32> instead of QList<UnitStruct*>?

[A Former User]

I couldn't quite believe that this could be an actual bug, so I kept testing.
And the result... oh well... I'm truly ashamed... I have included QDataStream in every file... except the one I actually tested.
So the solution is: #include <QDataStream> Facepalm
Thanks for your tips, I'm now gonna implement what you told me.