QtConcurrent::mappedReduced - c++

Question

to improve my program, i want to use Qtconcurrent to parallelize my audio process.
Unfortunately, i have many errors and i don't know why ...
In my *.pro : QT += concurrent
I use minGW with Qt 5.3.1 and when i try to build, the compiler said "forming reference to void" (qvector.h).
Here my main code :
void Reduce(QString& result, const QString& txt){ result += txt;}
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
QFuture<QString> res = QtConcurrent::mappedReduced(wavSoundsList, &BenchFFT::process, Reduce, QtConcurrent::OrderedReduce);
res.waitForFinished();
qDebug("%s",qPrintable(res.result()));
return a.exec();
}
and my class :
class BenchFFT
{
public:
BenchFFT();
QString process(const QString &wavFile);
private:
QString soundsPath;
QStringList wavSoundsList;
//Audio file
bool OpenWav(const QString WavPath, SndfileHandle &file);
//FFT
bool ComputeFFT(SndfileHandle file);
double Blackman(int x, int iWindowSize);
//Intel IPP7
IppsFFTSpec_R_64f * pFFTSpec;
Ipp64f* data; // source FFT
Ipp64fc* fftResult; // dest FFT
};
if someone can explain me my mistakes.thx :)
Edit : now i don't have build error with this modification
QFuture<QString> res = QtConcurrent::mappedReduced(wavSoundsList, std::tr1::bind(&BenchFFT::process, BenchFFT(), std::tr1::placeholders::_1), &Reduce, QtConcurrent::OrderedReduce);
but my program crashed .....