qt - Problems developing QFutureWatcher to improve image loading times -


i trying load multiple images using multithreading through qfuturewatcher class not being able properly. code read url of images , save them on qvector. pass these qstrings function load, resize , save images on qmap object. code is:

    void mainwindow::loadimages(){              mimagesloaderfuturewatcher = new qfuturewatcher<qstring>() ;         connect(mimagesloaderfuturewatcher, signal(finished()),this, slot(imageloadingfinished()));          qvector<qstring> imageslist = mproject->getimagesfilename();          // start computation.         mimagesloaderfuturewatcher->setfuture(qtconcurrent::map(imageslist,addimagethumnailtomap)); }      void mainwindow::addimagethumnailtomap(qstring imagename){         qsize desiredsize(100,100);         qimage orig(mproject->getbasepath()+"/"+imagename);         qimage scaled = orig.scaled(                     desiredsize,                     qt::keepaspectratio,                     qt::smoothtransformation);          mmapimages->insert(imagename,scaled);     }      void mainwindow::imageloadingfinished(){         qmessagebox msg;         msg.settext("images loaded");         msg.show();     }

the error receiving when try compile said list of arguments in call function "addimagethumnailtomap" not found think not neccesary specify on "qtconcurrent::map()" function. help

it's because function member of class. should call this:

mimagesloaderfuturewatcher->setfuture(    qtconcurrent::map(imageslist,&mainwindow::addimagethumnailtomap));

it looks qtconcurrent::map takes global functions or static functions, because there no pointer instance call addimagethumnailtomap. declear mainwindow::addimagethumnailtomap static.

i prefer use qtconcurrent::run, can handle class member functions. e.g:

void imagegroupview::setimages( qstringlist il ) {     //qfuture<int> _imageloader;         //qfuturewatcher<int> _watcher;     _imageloader = qtconcurrent::run(this, &imagegroupview::loadimages, il );        connect(&_watcher, signal(finished()), this, slot(processimagesready()));     _watcher.setfuture( _imageloader ); }

the template parameters happened int, because function loadimages returns number of loaded images. 

int imagegroupview::loadimages( qstringlist il ) {     int numloaded=0;     _images.clear();     foreach ( qstring img, il )     {         if( qfileinfo( img ).exists() )         {             _imagelist.append( qimage( img ).scaled(640,480) );             numloaded++;         }     }     return numloaded; }

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