read binary data from QDataStream with feedback

DungeonLords

There is official example how to read binary data from a stream (from file). I want the same but with feedback: how to know that there was error while deserializing?

This is my example

void MainClass::read(){
    my_struct_t obj;
    QFile file("file.dat");
    file.open(QIODevice::ReadOnly);
    QDataStream in(&file); // read the data serialized from the file
    in.startTransaction();
    in >> obj.a; // extract 3 and true
    if(in.commitTransaction()){
        qDebug().noquote().nospace() << "read is ok";
    }else{
        qDebug().noquote().nospace() << "unknown error";
    }
    qDebug().noquote().nospace() << "obj.a=" << obj.a;
}

To know about deserializing errors, should I use in.commitTransaction() and in.commitTransaction()? Or it is for special interfaces like serial port/socket only?

Christian Ehrlicher

What other error do you expect than there was not enough data available for deserialization?

DungeonLords

@Christian-Ehrlicher said in read binary data from QDataStream with feedback:

What other error do you expect than there was not enough data available for deserialization?

I expect the only one type of error: if(data available % data of pack) {error();}. In human speech: not enough data in file or too much data in file is error. But how to now it happen?

I was thinking to check by

if(!(file.size() % __datasizeof(my_struct_t))) makeWarning();

But unfortunately I use GCC last version, but __datasizeof() is still unavailable... Am I right?

Well, I can calc struct size manually and check by

if(!(file.size() % magicConstant)) makeWarning();

But I want to avoid magicConstant..

Christian Ehrlicher

@DungeonLords said in read binary data from QDataStream with feedback:

But how to now it happen?

QDS::commitTransaction() will return false and QDataStream::status() will tell you why.

DungeonLords

Thanks. Can I drop startTransaction() and commitTransaction()? Use like this

void MainClass::read(){
    my_struct_t obj;
    QFile file("file.dat");
    file.open(QIODevice::ReadOnly);
    QDataStream in(&file); // read the data serialized from the file
    in >> obj.a;
    if(in.status() == QDataStream::Ok){
        qDebug().noquote().nospace() << "read is ok";
    }else{
        qDebug().noquote().nospace() << "error code " << in.status();
    }
    qDebug().noquote().nospace() << "obj.a=" << obj.a;
}

JonB

@DungeonLords
Yes, because you reading from regular file. You would need it if you were reading from a streaming device instead.

Using Read Transactions

When a data stream operates on an asynchronous device, the chunks of data can arrive at arbitrary points in time. The QDataStream class implements a transaction mechanism that provides the ability to read the data atomically with a series of stream operators