Why are you complicating things so much?

Qt Code: 
  1. #include <QtConcurrentMap>
  2. #include <QStringList>
  3. #include <QByteArray>
  4. #include <QFile>
  5.  
  6. struct Decrypter {
  7.   Decrypter(const QByteArray &key) : m_key(key) {}
  8.  
  9.   typedef QByteArray result_type;
  10.  
  11.   QByteArray operator()(const QString &fileName) {
  12.     QFile f(fileName);
  13.     if(!f.open(QIODevice::ReadOnly)) return QByteArray();
  14.     QByteArray result;
  15.     result.reserve(f.size());
  16.     int keyIndex = 0;
  17.     char input;  
  18.     char keyByte;
  19.     while(!f.atEnd()) { 
  20.       f.getChar(&input);
  21.       keyByte = m_key.at(keyIndex);
  22.       keyIndex = (keyIndex+1) % m_key.size();
  23.       result.append( input ^ keyByte );
  24.     }
  25.     return result;
  26.   }
  27. private:
  28.   QByteArray m_key;
  29. };
  30.  
  31. int main() {
  32.   QStringList filesToDecrypt = { "file1", "file2", "file3" };
  33.   QFuture<QByteArray> future = QtConcurrent::mapped(filesToDecrypt, Decrypter("abcdef"));
  34.   return 0;
  35. }
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