QtConcurrent::map with QList<Item*>

[nepomuk]

Hello folks,

I want to use QtConcurrent::map for calling a member function (named extractFeatures) on every item of a QList. Unfortunately this doesn't work unless I change the list from QList<Item*> to QList<Item>. This is the code I wanted to use:

@
QFutureWatcher<void> watcher_ = new QFutureWatcher<void>;
QList<Item> itemList = ...;

watcher_->setFuture(QtConcurrent::map(itemList, &Item::extractFeatures));
watcher_->waitForFinished();
@

This is the error I get:
@c:/QtSDK/Desktop/Qt/4.7.4/mingw/include/QtCore/qtconcurrentmapkernel.h:73:9: error: no match for call to '(QtConcurrent::MemberFunctionWrapper<void, Item>) (Item*&)'
c:/QtSDK/Desktop/Qt/4.7.4/mingw/include/QtCore/qtconcurrentfunctionwrappers.h:119:14: note: candidate is: T QtConcurrent::MemberFunctionWrapper<T, C>::operator()(C&) [with T = void, C = Item]@

Is there a way to call QtConcurrent::map() properly while maintaining the current data structure, that is, a QList of Item-Pointers?
I would really appreciate any help. It's driving me mad.

[goetz]

The "documentation":/doc/qt-4.8/qtconcurrentmap.html#map says:

bq. Calls function once for each item in sequence. The function is passed a reference to the item, so that any modifications done to the item will appear in sequence.

So the map method calls your function with a refrerence-to-pointer argument. To make your function work, you must change its argument to and make it work with a reference-to-pointer of Item too:

@
void Item::extractFeatures(Item *& item);
@

http://www.codeguru.com/cpp/cpp/cpp_mfc/pointers/article.php/c4089 and http://markgodwin.blogspot.com/2009/08/c-reference-to-pointer.html have some explanations.

[nepomuk]

Thank you very much for this hint.
Actually the function does not take any arguments:
@void Item::extractFeatures();@
The idea of this function is to modify some instance members of the class.

In fact I want to do something like this (from the "documentation":http://developer.qt.nokia.com/doc/qt-4.8/qtconcurrentmap.html#using-member-functions):
@
// squeeze all strings in a QStringList
QStringList strings = ...;
QFuture<void> squeezedStrings = QtConcurrent::map(strings, &QString::squeeze);@
QString::squeeze is a non-static void method of QString and it doesn't take any arguments, too.

[goetz]

Oh, you're right. I don't know if it's possible to call a member function on a reference-to-pointer with QtConcurrent. I would try to implement a wrapper method that calls the other method on the pointer. But that's just a blind guess, it may fail too.

[nepomuk]

Indeed this didn't work either. It seems as if it's not possible to use container of pointers with QtConcurrent.
So I worked around this by iterating through the list and creating local copies of the items in a QList<Item>. This works though it's not very pretty. At least now I can take advantage of QtConcurrent and its automatic adjusting of the threads.
Thank you anyway for your hints.

[goetz]

You could try to use an external method for the concurrent map:

@
void extractFeaturesFromItem(Item *& item)
{
// do your work on item here
}

QFutureWatcher<void> watcher_ = new QFutureWatcher<void>;
QList<Item> itemList = ...;

// use the above proxy method for the map:
watcher_->setFuture(QtConcurrent::map(itemList, &extractFeaturesFromItem));
watcher_->waitForFinished();
@

It's not tested, just a wild guess again :)

[totem]

it worked for me, thank you very much