QString -> unsigned char* :
      QString  str = "ABCD";
      int length = str.length();
      unsigned char* sequence = NULL;
sequence =(unsigned char*)qstrdup(str.toAscii().constData());
      delete[] sequence;
      - sequence length = 5 --> [‘A‘] [‘B‘] [‘C‘] [‘D‘] [‘/0‘]
      - sequence is now "independant" from str
      - sequence has to be deleted with -> delete [] sequence
QString -> char:
      const QByteArray ba = string.toAscii(); // make ba const, because modifying this array might otherwise invalidate the pointer
  const char* sequence = ba.constData(); // now sequence will remain valid within the current scope.    
  

  The call to toAscii() creates a temporary QByteArray which goes out of scope when used like this: 
    char *sequence = string.toAscii().constData();
    // sequence is now a dangling pointer!

http://wxpjiujiang.blog.163.com/blog/static/203994030201292661134137/

QString unsigned char* 的转换