Question

My project is to create a small program which demonstrates the work of a search engine: indexing and returning result for arbitrary queries. I’ve done the work with the indexer part and now I want to improve it with indexing multiple files at once. The MainWindow class is here:

class MainWindow : public QMainWindow
{
    Q_OBJECT
    .....
private:
    Indexer * indexer;
    QStringList fileList;
    ....
    void index(QStringList list);
    void add(const QString &filename);
}

This is the implementation of add (add need to access fileList to avoid index the same files again, thus it can not be static method):

void MainWindow::add(const QString &filename)
{
    if (!fileList.contains(filename))
    {
        indexer->addDocument(filename.toStdString());
        fileList.append(filename);
        qDebug() << "Indexed" << filename;
        emit updatedList(fileList);
    }
}

The implement of index method is to receive a file lists and call add upon each file name:

void MainWindow::index(QStringList list)
{
    ....
    QtConcurrent::map(list, &MainWindow::add);
    ....
}

The error I receive when compiling these code is:

usr/include/qt4/QtCore/qtconcurrentmapkernel.h: In member function 'bool QtConcurrent::MapKernel<Iterator, MapFunctor>::runIteration(Iterator, int, void*) [with Iterator = QList<QString>::iterator, MapFunctor = QtConcurrent::MemberFunctionWrapper1<void, MainWindow, const QString&>]':
../search-engine/mainwindow.cpp:361:1:   instantiated from here
/usr/include/qt4/QtCore/qtconcurrentmapkernel.h:73:9: error: no match for call to '(QtConcurrent::MemberFunctionWrapper1<void, MainWindow, const QString&>) (QString&)'
/usr/include/qt4/QtCore/qtconcurrentfunctionwrappers.h:128:7: note: candidate is:
/usr/include/qt4/QtCore/qtconcurrentfunctionwrappers.h:138:14: note: T QtConcurrent::MemberFunctionWrapper1<T, C, U>::operator()(C&, U) [with T = void, C = MainWindow, U = const QString&]
/usr/include/qt4/QtCore/qtconcurrentfunctionwrappers.h:138:14: note:   candidate expects 2 arguments, 1 provided

I’m not really familiar with how QtConcurrent works, and the documentation doesn’t provide much details about it. I really hope that someone here can help. Thanks in advance.

Answer 1

To be able to call a pointer-to-member, you need, in addition to that functions formal arguments, an instance of that class (the this pointer that you get inside member functions).

There are two ways to handle this: create a simple functor to wrap the call, or use a lambda.

The functor would look like this:

struct AddWrapper {
  MainWindow *instance;
  AddWrapper(MainWindow *w): instance(w) {}
  void operator()(QString const& data) {
    instance->add(data);
  }
};

And you’d use it like:

AddWrapper wrap(this);
QtConcurrent::map(list, wrap);

(Careful with the lifetime of that wrapper though. You could make that more generic – you could also store a pointer-to-member in the wrapper for instance, and/or make it a template if you want to reuse that structure for other types.)

If you have a C++11 compiler with lambdas, you can avoid all that boilerpalte:

QtConcurrent::map(list, [this] (QString const& data) { add(data); });

---

Note: I’m not sure how QtConcurrent::MemberFunctionWrapper1 got involved in your example, I’m not seeing it here. So there might be a generic wrapper already in Qt for this situation, but I’m not aware of it.