Problem with gbinary files, and QVector

SGaist

You're truncating your file both when reading and writing. Then you have that data variable but it's unknown whether it's properly initialised.

Loc888

@SGaist Ok, i try again, but take in consideration in those tutorials i saw, they use that trunc.
I removed that trunc everywhere, now something is working, but the next problem i get, is it's crashing, because of the string "name" variable.
What happened, is i get the error, i run the debugger, and it points on the name variable, in the show_data function, if i just comment it, then it's working.

How to fix this? I guess it's the stream size fault, because last day i saw a weird error, and saw some weird symbols when i was trying to read the data, and the program crash few seconds later.

How to save all those QVectors, stings etc, when the size variable change?

SGaist

I do take that into consideration. You should also take into consideration that if there's something you are not sure about, you should look up the documentation regarding that element rather than blindingly adding things here and there.

As suggested before, since you are using Qt, use its facilities like QFile and QDataStream and be done with it. Streaming of QString, QVector etc, is already supported.

Loc888

@SGaist I don't know any documentation about files and arrays, if i was able to do it with that, i would not post anything, because there is no point.

aha_1980

@Loc888 The link to QDataStream that @SGaist gave you contains an almost reusable, yet small example for loading and saving. Have you looked at it?

Regards

mrjj

@Loc888
Hi
The real benefit of using QFile and QDataStream is that it already can handle many Qt types.
(including QString and QVector)

http://doc.qt.io/qt-5/datastreamformat.html

So you need not to do anything special besides how mini sample shows it

QFile file("file.dat");
file.open(QIODevice::WriteOnly);
QDataStream out(&file); // we will serialize the data into the file
QVector<double> list;
out << list; // serialize a list of doubles. handle size by it self.

However, if you QVector is a list of custom object ( your own class) you need to define
<< and >> for it so it know how to stream your types too.
Often that is just to stream the member variables if they in turn are plain Qt types.

Loc888

@mrjj Ok, i tried it with simple data, and it's working very good, it's even too easy to use, with comparision to that std stuff. So, about that standard method, if someone could help me learn how to do that, i mean how to read an array i will appreciate it, even if i am gonna use QFile because it's much more cleaner and faster too use.

mrjj

@Loc888
Hi
yes, its actually nicer than the std stuff.

Well, it can directly do it with QVector.
It actually write the size first then each item but its handled automatically.

When you say array, you mean QVector or what type ?

for any supported Qtype, a list or vector is the same as basic variables.
simply
outstream << thelist;
and
instream >> somelistvar;

Do you have a special case in in mind with the "array" ?
Can you show the declaration of your array ?
makes it easier to talk about.

Loc888

@mrjj Back to that QFile for one moment, i have to inherit that QDataStream, and overload the operators?

jsulm

@Loc888 No, you just implement << and >> operators for your own data types.
See documentation: http://doc.qt.io/qt-5/qdatastream.html#
You need:

QDataStream &operator<<(QDataStream &, const YOUR_TYPE &);
QDataStream &operator>>(QDataStream &, YOUR_TYPE &);

You don't have to do it for Qt types like QVector.

Loc888

@jsulm Ok, i took a look, and it says

"In addition to the overloaded stream operators documented here, any Qt classes that you might want to serialize to a QDataStream will have appropriate stream operators declared as non-member of the class:"

So i have to do that, outside the class, but how to use it then? If i crate a object, then how to use that overloaded operators?

mrjj

@Loc888 said in Problem with gbinary files, and QVector:

So i have to do that, outside the class, but how to use it then? If i crate a object, then how to use that overloaded operators?

Hi
That is the easy part.
Simply try it and see

QFile file("file.dat");
file.open(QIODevice::WriteOnly);
QDataStream out(&file);
out << *yourclassptr;
(note the *)
if Qt can find an << for a type, it will simply use it.

That way, you can << anything you like.

Loc888

@mrjj Ok, i try. SO i have to do a class, put there thise operators, and then use:

out << *new_class ?

mrjj

@Loc888
Yes and the reason for the * in from of object pointer is that the header says
(QDataStream &, const YOUR_TYPE &); << reference

so if you give it the pointer without *, it
will save/stream the address of the object as it then matches << for int instead.
so say always use * when you have a pointer to an object with <<

mrjj

Hi
Just to be sure
can you show your
QDataStream &operator<<(QDataStream &, const YOUR_Class &);
implementation.
(not the .h declaration) but the actual body of it.

Loc888

@mrjj In the QDataStream Class it looks like that, so i guess i am gonna use this template, except i am gonna use a custom data:

inline QDataStream &QDataStream::operator<<(quint16 i)
{ return *this << qint16(i); }

mrjj

Hi
Here is small sample.

--in the .H file--
class MyClass
{
public:
    int value = 6;
    QString name = "test";
};

QDataStream &operator<<(QDataStream &out, const MyClass &);
QDataStream &operator>>(QDataStream &in, MyClass &);

-----------
--in the .CPP file--
QDataStream &operator<<(QDataStream &out, const MyClass &classRef) {
return out << classRef.value <<  classRef.name; // save each member of your class
}

QDataStream &operator>>(QDataStream &in,  MyClass &classRef){
return in >> classRef.value >>  classRef.name; // read in IN SAME order
}

------------------------------
//--test of saving--
   {
        QFile file("e:/file.dat");
        file.open(QIODevice::WriteOnly);
        QDataStream out(&file);
        MyClass *ptr = new MyClass;
        out << *ptr;
    }
//--test of loading--
    {
        QFile file("e:/file.dat");
        file.open(QIODevice::ReadOnly);
        QDataStream in(&file);
        MyClass *otherptr = new MyClass;
        in >> *otherptr;      
    }

Loc888

@mrjj Something weird is happening, i declared those operators in a header file, then include it, and it's working, if i use that MyClass pointer, then i get the error..

mrjj

@Loc888
What error?

Loc888

@mrjj Nope, it's ok... I just typed the wrong object type. It's working finnaly. Thanks
everybody for help.