QString::append() weirdness

[mattc]

Thank you for your efforts Wysota.

It turns out the code does not work "because the standard says so". In other words, while this code is ok:

// bind a temporary *expression* to a const reference
const QString& s = QString("hello");

// bind a temporary *expression* to a const reference
const QString& s = QString("hello");

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this one leaves a dangling reference:

// bind a temporary *object* to a const reference
const QString& s = QString("hello").append("");

// bind a temporary *object* to a const reference
const QString& s = QString("hello").append("");

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I believe that from a purely "technical" point of view there is nothing wrong with the second snippet (a compiler could easily generate working code), but in fact it is not supposed to work.

[wysota]

Thankfully with the "imfamous implicit sharing" Qt offers you can happily do this:

QString s = QString("hello").append("");

QString s = QString("hello").append("");

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[mattc]

LOL

You know, I am a bit nervous about "implicit sharing" since I reported this bug (read: feature):
http://bugreports.qt.nokia.com/browse/QTBUG-12941

In short, though QLinkedList offers the same "iteration" interface of a std::list (begin(), end()), the behaviour of the iterators it provides is not standard conforming.

For example, the standard says that a std::list::end() iterator should remain constant through the lifetime of a list. But calling QLinkedList::clear() can in fact change the value returned by QLinkedList::end().

So, when in my code I blindly replaced some std::lists with QLinkedLists I got crashes everywhere.

Nothing to pull your hair about, of course, but still annoying... and guess who's to blame for that? :-)

[wysota]

I fail to see how this can make you nervous about implicit sharing itself. As long as you don't call clear() on the list the behaviour will be as you expect it. And the behaviour of clear() is not related to implicit sharing itself but rather to limiting memory usage of empty lists (which happens to be implemented using implicit sharing).

[tbscope]

Besides that, changing the items while iterating over them is not safe.