how to conver QString to const char*

[high_flyer]

Just to be clear: my point was NOT about using a temp objects.

Back to our discussion:

The data buffer in the temporary QByteArray is a transformed copy of the data buffer that is in the string's buffer.

I just checked and you are right:
toAscii() (via toLatin1()) indeed transforms the data:

QByteArray QString::toLatin1() const
{
    QByteArray ba;
    if (d->size) {
        ba.resize(d->size);
        const ushort *i = d->data;
        const ushort *e = d->data + d->size;
        uchar *s = (uchar*) ba.data();
        while (i != e) {
            *s++ = (*i>0xff) ? '?' : (uchar) *i;
            ++i;
        }
    }
    return ba;
}

QByteArray QString::toLatin1() const
{
    QByteArray ba;
    if (d->size) {
        ba.resize(d->size);
        const ushort *i = d->data;
        const ushort *e = d->data + d->size;
        uchar *s = (uchar*) ba.data();
        while (i != e) {
            *s++ = (*i>0xff) ? '?' : (uchar) *i;
            ++i;
        }
    }
    return ba;
}

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One of the main disadvantages answering such post during work is that I can often just skim the posts. :-\