QValueVector in Qt 3(.3.8) is broken

[fear]

I have put a lot of effort to rewrite huge part of my app to work with QValueVector but all in vain. Below I present proof of this broken component. Correct me if I'm wrong and I'll be happy.

My code:

qDebug(">");
		playlistsList::iterator i, j;
		i = playlists.begin();
		j = i;
		i += where;
 
		//qDebug(playlists.at(which).name);
		//qDebug((*i).name);
 
		for (int i = 0 ; i != playlists.count() ; i++, j++)
		{
			qDebug(playlists.at(i).name + " " + (*j).name);
		}
 
		qDebug("i'll insert " + playlists.at(which).name + " before " + (*i).name);
 
		playlists.insert(i, playlists.at(which));
 
		for (int i = 0 ; i != playlists.count() ; i++)
			qDebug(playlists.at(i).name);
 
		qDebug("after redundant removed");
 
		i = playlists.begin();
		j = i;
		i += (which + 1);
 
		playlists.erase(i);
 
		for (int i = 0 ; i != playlists.count() ; i++, j++)
			qDebug(playlists.at(i).name + " " + (*j).name);
 
		qDebug("");
		qDebug("");

qDebug(">");
		playlistsList::iterator i, j;
		i = playlists.begin();
		j = i;
		i += where;
		
		//qDebug(playlists.at(which).name);
		//qDebug((*i).name);

		for (int i = 0 ; i != playlists.count() ; i++, j++)
		{
			qDebug(playlists.at(i).name + " " + (*j).name);
		}

		qDebug("i'll insert " + playlists.at(which).name + " before " + (*i).name);

		playlists.insert(i, playlists.at(which));

		for (int i = 0 ; i != playlists.count() ; i++)
			qDebug(playlists.at(i).name);

		qDebug("after redundant removed");
		
		i = playlists.begin();
		j = i;
		i += (which + 1);

		playlists.erase(i);

		for (int i = 0 ; i != playlists.count() ; i++, j++)
			qDebug(playlists.at(i).name + " " + (*j).name);

		qDebug("");
		qDebug("");

To copy to clipboard, switch view to plain text mode 

Results after first execution of above code are correct:

>
aa aa
bb bb
i'll insert bb before aa
bb
aa
bb
after redundant removed
bb bb
aa aa

>
aa aa
bb bb
i'll insert bb before aa
bb
aa
bb
after redundant removed
bb bb
aa aa

To copy to clipboard, switch view to plain text mode 

Second execution gives INCORRECT result:

>
bb bb
aa aa
i'll insert aa before bb
bb
bb
aa
after redundant removed
bb bb
bb bb

>
bb bb
aa aa
i'll insert aa before bb
bb
bb
aa
after redundant removed
bb bb
bb bb

To copy to clipboard, switch view to plain text mode 

Can anyone help me?

[jacek]

Could you post a minimal compilable example?

[fear]

/**
	Broken QValueVector demo (Qt 3.3.8, GCC 4.1.1)
	@author amdfanatyk
 
	compilation: c++ qvalvecshit.cpp -o valvec -lqt-mt
**/
 
#include <qstring.h>
#include <qvaluevector.h>
 
struct elem
{
	elem() {}
	elem(const QString & _s1, const QString & _s2) { s1 = _s1; s2 = _s2; }
 
	QString s1;
	QString s2;
};
 
typedef QValueVector<elem> elemsList;
 
void printList(const elemsList & list)
{
	qDebug("---");
 
	for (int i = 0 ; i != list.count() ; i++)
		qDebug(list[i].s1 + " " + list[i].s2);
 
	qDebug("---");
}
 
void insertElem(elemsList & e)
{
	if (e.count() < 2)
		return;
 
	elemsList::iterator i = e.begin();
 
	printList(e);
 
	QString e1s1 = e.at(1).s1;
	QString e0s1 = (*i).s1;
 
	qDebug("*- i'll insert " + e1s1 + " before " + e0s1);
 
	e.insert(i, e.at(1));
 
	printList(e);
 
	if ((e.at(0).s1 != e1s1) || (e.at(1).s1 != e0s1))
		qDebug("Ooops!");
	else
		qDebug("OK");
}
 
int main()
{
	elemsList elems;
 
	elem e1("a", "aa");
	elem e2("b", "bb");
 
	elems.append(e1);
	elems.append(e2);
 
	insertElem(elems);
	insertElem(elems);
 
	return 0;
}

/**
	Broken QValueVector demo (Qt 3.3.8, GCC 4.1.1)
	@author amdfanatyk

	compilation: c++ qvalvecshit.cpp -o valvec -lqt-mt
**/

#include <qstring.h>
#include <qvaluevector.h>

struct elem
{
	elem() {}
	elem(const QString & _s1, const QString & _s2) { s1 = _s1; s2 = _s2; }

	QString s1;
	QString s2;
};

typedef QValueVector<elem> elemsList;

void printList(const elemsList & list)
{
	qDebug("---");

	for (int i = 0 ; i != list.count() ; i++)
		qDebug(list[i].s1 + " " + list[i].s2);

	qDebug("---");
}

void insertElem(elemsList & e)
{
	if (e.count() < 2)
		return;

	elemsList::iterator i = e.begin();

	printList(e);

	QString e1s1 = e.at(1).s1;
	QString e0s1 = (*i).s1;

	qDebug("*- i'll insert " + e1s1 + " before " + e0s1);

	e.insert(i, e.at(1));

	printList(e);

	if ((e.at(0).s1 != e1s1) || (e.at(1).s1 != e0s1))
		qDebug("Ooops!");
	else
		qDebug("OK");
}

int main()
{
	elemsList elems;

	elem e1("a", "aa");
	elem e2("b", "bb");

	elems.append(e1);
	elems.append(e2);

	insertElem(elems);
	insertElem(elems);

	return 0;
}

To copy to clipboard, switch view to plain text mode 

---
a aa
b bb
---
*- i'll insert b before a
---
b bb
a aa
b bb
---
OK
---
b bb
a aa
b bb
---
*- i'll insert a before b
---
b bb
b bb
a aa
b bb
---
Ooops!

---
a aa
b bb
---
*- i'll insert b before a
---
b bb
a aa
b bb
---
OK
---
b bb
a aa
b bb
---
*- i'll insert a before b
---
b bb
b bb
a aa
b bb
---
Ooops!

To copy to clipboard, switch view to plain text mode 

[jacek]

I've changed

e.insert(i, e.at(1));

e.insert(i, e.at(1));

To copy to clipboard, switch view to plain text mode 

to

elem el = e.at(1);
e.insert(i, el);

elem el = e.at(1);
e.insert(i, el);

To copy to clipboard, switch view to plain text mode 

and it seems to work OK now.

IMO the problem is that at() returns a reference, which is valid only while the vector remains unchanged. But insert() has to make space for the new item, so it moves existing items and the reference becomes invalid.

[fear]

Thanks. Now it works but I don't think I'll use QValueVector and QTL. For me that's a bug. insert() should insert a copy of, so first it should make a copy and then insert it. To get it work I must make one copy and then vector makes another copy and I waste a lot of memory, it sucks. When I use my own engine I only change values of pointers - there is no need to make copies and remove redundant objects. With QPtrVector I think I'll have similar problems.

[jacek]

For me that's a bug. insert() should insert a copy of, so first it should make a copy and then insert it.

insert() itself is OK. The problem is that the item is passed by reference.

When I use my own engine I only change values of pointers - there is no need to make copies and remove redundant objects.

That's how QPtrVector works. QValueVector, just like std::vector, stores items, not pointers, so it has to copy them. But if you have your own collection that suits you better, just use it.