How to write entire QVector to a binary file?

[CJha]

@JonB Yeah I could use mutexes, but I prefer Queued Connection as there is just one QVector to be sent to a different thread. I do not use const QVector<double>& because I want to depend on Qt's implicit sharing i.e. if the QVector<double> is changed while I am still using the previous QVector to write data in my binary file then it would not affect my "Writer" thread. If I would use const QVector<double>& then it would refer to the original QVector in the main thread and then I would have to use QMutex to protect read and write operations. This is my function in the Writer class which inherits QObject and is run in a different thread:

void Writer::writeData(QVector<double> vec)
{
    ++sweepCount_; // Increament the count to keep track of number of times data vector is written

    if(isBin_){ // If user selects file type as .bin
        for(int ii = 0; ii < vec.length(); ++ii)
            binOut_ << vec[ii]; // binOut_ is a QDataStrem, assigned to a file when the user clicks on Start button
    }
    else{ // if the user selects file type as .csv
        if(vec.length() > 1){
            outStream_ << vec[0];
            for(int ii = 1; ii < vec.length(); ++ii)
                outStream_ << seperator_ << vec[ii]; // seperator_ = ',' or ';' depending on QLocale
            outStream_ << '\n';
        }
    }
}

[SGaist]

Hi,

In what format do you get the data in the callback ?

[J.Hilk]

@CJha said in How to write entire QVector to a binary file?:

I do not use const QVector<double>& because I want to depend on Qt's implicit sharing i.e. if the QVector<double> is changed while I am still using the previous QVector to write data in my binary file then it would not affect my "Writer" thread

So that you know, when passing your QVector through Qt::QueuedConnection - which is the default and correct one across threads - your QVector will be copied auto automatically, there will be no share until write. There will be a copy inside your thread

[CJha]

@SGaist Hi, It is double. The data is placed in a C Array from where I retrieve it.

[CJha]

@J-Hilk Thanks, one thing which I am not sure about: What would happen if I use const QVector<double>& instead of QVector<double>, will it still copy the data if the connection type is Qt:QueuedConnection or will it just copy the reference for the vector?

[KroMignon]

@CJha said in How to write entire QVector to a binary file?:

will it still copy the data if the connection type is Qt:QueuedConnection or will it just copy the reference for the vector?

In short: Yes they will be copied.
For more details take a look at this => https://www.embeddeduse.com/2013/06/29/copied-or-not-copied-arguments-signals-slots/

[CJha]

@KroMignon Thanks :)

[JonB]

@CJha
According to @KroMignon's link

The conclusion from the above results is that we should pass arguments to signals and slots by const reference and not by value. This advice is true for both direct and queued connections. Even if the sender of the signal and the receiver of the slot are in different threads, we should still pass arguments by const reference.

If you have both signal & slot declared with QVector<double> vec and you change both to have const QVector<double>&, you reduce 3 copies to 1.

Having said that: I do get lost as to what gets copied, aren't we only talking about the QVector structure and not the data it references??

[CJha]

@JonB If it's a copy then I assume it is always going to be the data because the structure is not that significant in terms of memory usage, but I am not sure about it either.

[jsulm]

@CJha said in How to write entire QVector to a binary file?:

If it's a copy then I assume it is always going to be the data

No, because Qt containers use copy-on-write. See https://doc.qt.io/qt-5/implicit-sharing.html