QString unsigned char* 的转换

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!