HOWTO make your app work with Qt 4.5

Hi wow another helpful tip in Qt 4.5 thanks!
I had the same problem before and it surely is helpful.
I was looking for a proper solution for a few days.
I am a bit disappointed with the other sites.
They gave too much complexity and thanks for sharing this.
I am really having a great time reading tips from this site.

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Okay, I was curious and digged around a bit to find the reason of all of this.

Original problem/bug in Qt 4.4.x
================================

First, there is a bug in Qt 4.4.3 in that a parameter passed as a const reference is changed. Look at this contrived example:

-------- cut here --------------

#include <QString>
#include <QRegExp>
#include <iostream>
using namespace std;
int main(int argc, char **argv)
{
QString s("Hello world!");
const QRegExp re("world");
cout << "re.matchedLength(): " << re.matchedLength() << endl;
const int ml = re.matchedLength();
s.indexOf(re); // Note that re is declared as constant!!
cout << "re.matchedLength(): " << re.matchedLength() << endl;

if (ml != re.matchedLength()) {
cout << "Bug in QString::indexOf: modifies const parameter." << endl;
}
return 0;
}

------ cut here -------

Running this program on my system (Debian unstable, Qt 4.4.3-2), I get:


torsten@transit:~/qtbug-modifies-const$ ./demo
re.matchedLength(): -1
re.matchedLength(): 5
Bug in QString::indexOf: modifies const parameter.


Which is obviously a bug if you ask me - passing an object as const reference should not make any visible changes to that object. It seems like Qt 4.5 fixed that, and in fact, the documentation at http://doc.trolltech.com/4.5/qstring.html#indexOf-4 shows that there is now a const- and a non-const variant of indexOf.

Why this broke Quassel
======================

Now, let's look at Quassels code or rather the diff that fixed the hang. The bug of the original code is in the line marked with !!BUG!!:


X-Git-Url: http://git.quassel-irc.org/?p=quassel.git;a=blobdiff_plain;f=src%2Fqtui%...

diff --git a/src/qtui/chatitem.cpp b/src/qtui/chatitem.cpp
index de96b2d..934fadf 100644
--- a/src/qtui/chatitem.cpp
+++ b/src/qtui/chatitem.cpp
@@ -445,7 +445,7 @@ QList ContentsChatItem::findClickables() const {
for(int i = 0; i < regExpCount; i++) {
if(matches[i] < 0 || matchEnd[i] > str.length()) continue;
if(idx >= matchEnd[i]) {
- matches[i] = str.indexOf(regExp[i], qMax(matchEnd[i], idx));
+ matches[i] = regExp[i].indexIn(str, qMax(matchEnd[i], idx));
if(matches[i] >= 0) matchEnd[i] = matches[i] + regExp[i].cap(1).length(); // !!BUG!!
}
if(matches[i] >= 0 && matches[i] < minidx) {


The problem: The marked line of code uses the state of regExp[i] to access the result of the str.indexOf operation. However, str.indexOf takes the regExp[i] argument as a const reference, so regExp[i].cap(1) can't have that result. If it actually not documented what QRegExp::cap() will return when no match data was acquired, on my system it is the empty string.

If any match is found, this will lead to the loop to find the same match again and again. This is because it continues searching at

457 idx = matchEnd[type];

which unfortunately is assigned

449 if(matches[i] >= 0) matchEnd[i] = matches[i] + regExp[i].cap(1).length();

or, in other words: matchEnd[i] = matches[i]. So the next search will find the same match again, ...

Calling regExp[i].indexIn() looks just as bad, as it is a const method. But even the Qt 4.5 documentation states that this updates the match information anyway (yuck!).

Why recompiling helps
=====================

This problem magically goes away when recompiling with Qt 4.5. Why?

Actually it is quite simple. The relevant source line is

448 matches[i] = str.indexOf(regExp[i], qMax(matchEnd[i], idx));

Building against Qt 4.4.x, the method QString::indexOf in this call will get resolved to the overload

int QString::indexOf(const QRegExp & rx, int from = 0) const;

This method will change rx in Qt 4.4.x and got fixed in 4.5 to not mess with the state of rx. Now, rebuilding against Qt 4.5, there are two candidates of indexOf with a QRegExp as first argument:

(1) int QString::indexOf(const QRegExp & rx, int from = 0) const;
(2) int QString::indexOf(QRegExp & rx, int from = 0) const;

This time the compiler will chose the second implementation, as in the context of line 448 of chatitem.cpp, the QRegExp argument is writable. The only difference of overload (2) vs. (1) is (quoting the Qt 4.5 docs):

"If there is a match, the rx regular expression will contain the matched captures (see QRegExp::matchedLength, QRegExp::cap)."

So the code originally worked "by accident" due to a bug in Qt.