QtConcurrent Map Cannot Use Class Method

Question

Hello I am trying to pass QList to QtConcurrent::Map function, but it fails to start and I don't understand why, maybe some one knows what might be the problem?

This is the Class Method Code

void MainWindow::find_file(QStringList &lst){
    QString fl_name = ui->ln_FlName->text();
    QRegExp exp;
    exp.setPattern(".*/("+ui->ln_FlName->text()+").*");
    QRegExp exp_2;
    exp.setPattern(".*/("+ui->ln_FlName->text()+")");
     foreach(const QString &str,lst)
      {
        // Do some work...   
      }}

And this is the code when I try to execute this method

void MainWindow::on_btn_start_clicked(){
QDir start_path(ui->ln_Dir->text());
QList<QFuture<QStringList> > future_list;
QList<QStringList> dirs_lists;
QStringList temp_dir_list;
QList<QString> lst_str;
foreach(const QFileInfo fl_inf,start_path.entryInfoList(QDir::NoDotAndDotDot|QDir::Dirs))
{
    if(fl_inf.isDir())
    {
        //Here I use QtConcurrent::run with other method and it works fine

        QFuture<QStringList>ft;
        ft=QtConcurrent::run(this,&MainWindow::File_Search,fl_inf.filePath());
        future_list.append(ft);
        num_ft+=1;
        temp_dir_list.append(fl_inf.filePath());
        lst_str.append(fl_inf.filePath());
    }


}
dirs_lists.append(temp_dir_list);
long long int sum = 0;
for(int i=0;i<future_list.count();i++)
{

    sum += future_list.at(i).result().count();
    dirs_lists.append(future_list.at(i).result());
}


//But this doesn't want to work 

QFutureWatcher<void> ft_watcher;
ft_watcher.setFuture(QtConcurrent::map(dirs_lists,&MainWindow::find_file));
}

And the compiler outputs this errors:

error: no matching function for call to 'map(QList&, )' QtConcurrent::map(dirs_lists,MainWindow::find_file); ^

And

error: no match for call to '(QtConcurrent::MemberFunctionWrapper1) (QStringList&)' map(*it); ^

Answer 1

You're treating find_file as if it was a static method, not an instance method. There are two solutions:

1. Turn find_file into a static method.

2. Pass an instance to map: QtConcurrent::map(dirs_lists, &MainWindow::find_file, this).

In either case, you must design find_file to be thread-safe, of course.

Answer 2

You cannot use a member function in QtConcurrent::map like this. The docs state: QtConcurrent::map(), QtConcurrent::mapped(), and QtConcurrent::mappedReduced() accept pointers to member functions. The member function class type must match the type stored in the sequence. So in your case, you can only use QStringList member functions in QtConcurrent::map.