multi threading or QConcurrent for the scenario

QtConcurrent
QtConcurrentMap

Both I have included.

Is what you posted a complete error message?

error: no matching function for call to 'mappedReduced'
QFuture<QString> decryptedContent = QtConcurrent::mappedReduced(fileList,DecryptMap('A '),ReduceS());

And there is nothing more? Nothing about candidates for the proper call or something like that?

The below warning are also found.

candidate template ignored: couldn't infer template argument 'ResultType'
QFuture<ResultType> mappedReduced(const Sequence &sequence,
^
candidate template ignored: deduced conflicting types for parameter 'Iterator' ('QList<QString>' vs. 'DecryptMap')
QFuture<ResultType> mappedReduced(Iterator begin,
^
candidate template ignored: deduced conflicting types for parameter 'Iterator' ('QList<QString>' vs. 'DecryptMap')
QFuture<typename QtPrivate::ReduceResultType<ReduceFunctor>::Result Type> mappedReduced(Iterator begin,
^
candidate template ignored: substitution failure [with Sequence = QList<QString>, MapFunctor = DecryptMap, ReduceFunctor = ReduceS]: implicit instantiation of undefined template 'QtPrivate::ReduceResultType<ReduceS>'
QFuture<typename QtPrivate::ReduceResultType<ReduceFunctor>::Result Type> mappedReduced(const Sequence &sequence,
~~~~~~~~~~~~~~~~ ^

So what are the exact signatures of fileList, DecryptA and ReduceS?

Code:

---

QList<QString> fileList;
 
struct DecryptMap {
    DecryptMap(const QChar &key) : m_key(key) {}
    typedef QString result_type;
    QString operator()(const QString &item)
    {
        Decrypt *decrypt = new Decrypt();
        qDebug()<<item;
        return decrypt->decryptFile(item,'A');
    }
    QChar m_key;
};
struct ReduceS
{
  typedef QString result_type;
  void operator()(QString & res, const QString & partial )
  {
    res.append(partial);
  }
};

---

And what is the exact call to mappedReduced?

Code:

---

QFuture<QString> decryptedContent =  QtConcurrent::mappedReduced(fileList,DecryptMap('A'),ReduceS());

---

Try:

Code:

---

QFuture<QString> decryptedContent = QtConcurrent::mappedReduced<QString>(fileList,DecryptMap('A'),ReduceS());

---

It worked.