QtConcurrent question

[bair]

I am writing a program which performs some heavy calculations, so I'm exploring QtConcurrent. I've looked at the QtConcurrent Progress Dialog Example and tried to use QtConcurrent::map() in the same way. Here is my code:

void MainWindow::some_member_function()
{
    ...
    QVector<int> vector;
    int iterations=int(Ntotal/1000000)+1;
    for (int i = 0; i < iterations; ++i)
        vector.append(i);
    ...
    futureWatcher.setFuture(QtConcurrent::map(vector, cycle)); // <----- error
    ...
}
void MainWindow::cycle(int &start)
{
    //lots of calculations
...
}

void MainWindow::some_member_function()
{
    ...
    QVector<int> vector;
    int iterations=int(Ntotal/1000000)+1;
    for (int i = 0; i < iterations; ++i)
        vector.append(i);
    ...
    futureWatcher.setFuture(QtConcurrent::map(vector, cycle)); // <----- error
    ...
}
void MainWindow::cycle(int &start)
{
    //lots of calculations
...
}

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I'm getting this error:

mainwindow.cpp:345: error: argument of type `void (MainWindow::)(int&)' does not match `void (MainWindow::*)(int&)'

I'm confused. How should I use my member function cycle() in QtConcurrent::map() properly?

[bair]

Ok, now I see it was a stupid C++ question, sorry. Solved the problem by making cycle and member variables it uses static. Now everything seems to work, I've got 4x speedup.

[bair]

Things turn out to be not so easy. Yes, I've got 4x speedup, but cycle() function generates huge amount of data (hundreds of megabytes) which should be written to a file, but my attempts to write output data from that function have no success. In my MainWindow class I have static QDataStream member out, and I'm using it for output. During execution there is a lot of qDebug messages "QIODevice::write: ReadOnly device", and output file remains absolutely empty, though if I write something to out before QtConcurrent::map() is started, it is written to a file without problems.
As far as I understand, multiple threads generated by QtConcurrent::map() somehow automatically lock out and prevent each other from writing to it. Or may be I am wrong? Anyway, what could be done to avoid this?

Please give me a hint, I'm stuck and this thing is starting to drive me crazy...

[bair]

Any suggestion would be appreciated. Even something like "RTFM".

[wysota]

You can't access a QIODevice from multiple threads at once, that's for sure. Try returning the data from calculations to the main thread and then writing it sequentially from there.

[bair]

Thank you very much. I was thinking about returning data from the calculations to the main thread, but the problem is that there's too much data to handle, so I was afraid that memory consumption will be unacceptably high. Now I think I'll manage it.

Thanks again, you saved me from many hours of attempts to walk through a brick wall.