change the positions of characters in a string

[wysota]

However, the first method suggested by Wysota works perfectly.

So this will work as well:

QString str="173562";
QChar *dat = str.data();
qSwap(dat[1], dat[2]);

QString str="173562";
QChar *dat = str.data();
qSwap(dat[1], dat[2]);

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[spud]

The following code will in most likely cause your application to crash:

char * str1 = Q_date.toAscii().constData();
swap( str1[0], str1[2] );

char * str1 = Q_date.toAscii().constData();
swap( str1[0], str1[2] );

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and even if it doesn't it won't work.
You shouldn't save a pointer to the temporary object returned by toAscii(), it goes out of scope immediately. Go with Wysota's suggestion.

@jpn + wysota: It's an interesting side effect of Qt's trick with QCharRef that swap doesn't work as expected. I think they should mention QCharRef in the documentation and that it's sometimes more efficient(in this case necessary) to work on the raw QChar array returned by data()