[Qt-interest] Problem with qSort and QObjects

[Carl Snellman]

Hey,

I did not have Q_OBJECT at the beginning, but adding that does no
difference. Same error... :( The code I pasted below is all code in my
test project that fails to compile.

Thanks,
Carl

On Sat, Feb 13, 2010 at 4:51 PM, Tibor Kiss <tibor at wiesen.at> wrote:
> Hi!
>
> Well, first do you have the Q_OBJECT makro at the beginning of the class
> definition, or is it only missing from your code snippet?
>
> 2010/2/14 Carl Snellman <carl.snellman at gmail.com>
>>
>> Hey all,
>>
>> I've been fighting to get qSort working with following code: (adopted
>> from http://lists.trolltech.com/qt-interest/2008-07/thread00695-0.html)
>> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>> #include <QtCore/QCoreApplication>
>> #include <iostream>
>> #include <QtAlgorithms>
>> #include <QDebug>
>> #include <QObject>
>>
>> class Account : QObject {
>> public:
>>    Account() {};
>>    Account(QString aItem, QString bItem)
>>             : aItem(aItem), bItem(bItem) {};
>>    QString aItem;
>>    QString bItem;
>>    QString toString(){
>>        QString result;
>>        return result.append("aItem = ").append(aItem).append("\t")
>>                .append("bItem = ").append(bItem).append("\n");
>>    };
>>    bool operator<(const Account & other ) const {
>>        qDebug() << "Comparing: " << aItem << ":" << other.aItem;
>>        return aItem < other.aItem;
>>    };
>> };
>>
>> int main(int argc, char *argv[]) {
>>    QCoreApplication a(argc, argv);
>>    QList<Account> list;
>>    list << Account("item 5", "Dummy");
>>    list << Account("item 1", "Dummy");
>>    list << Account("item 3", "Dummy");
>>    list << Account("item 4", "Dummy");
>>    list << Account("item 2", "Dummy");
>>
>>    for (int i=0; i<5; ++i)
>>        std::cout << list.value(i).toString().toStdString();
>>    std::cout << "==================\n";
>>
>>    qSort(list);
>>
>>    for (int i=0; i<5; ++i)
>>        std::cout << list.value(i).toString().toStdString();
>>    return 0;
>> }
>> <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
>>
>> Above gives me the following compilation errors:
>>
>> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>> Running build steps for project qSortAccountTest...
>> Configuration unchanged, skipping QMake step.
>> Starting: /usr/bin/make -w
>> make: Entering directory
>> `/scratchbox/users/maemo/home/maemo/qSortAccountTest/qSortAccountTest'
>> g++ -c -pipe -g -Wall -W -D_REENTRANT -DQT_CORE_LIB -DQT_SHARED
>> -I/home/maemo/qtsdk-2010.01/qt/mkspecs/linux-g++ -I.
>> -I/home/maemo/qtsdk-2010.01/qt/include/QtCore
>> -I/home/maemo/qtsdk-2010.01/qt/include -I. -o main.o main.cpp
>> /home/maemo/qtsdk-2010.01/qt/include/QtCore/qobject.h: In copy
>> constructor 'Account::Account(const Account&)':
>> main.cpp:7: instantiated from 'T QList<T>::value(int) const [with T =
>> Account]'
>> main.cpp:35: instantiated from here
>> /home/maemo/qtsdk-2010.01/qt/include/QtCore/qobject.h:309: error:
>> 'QObject::QObject(const QObject&)' is private
>> main.cpp:7: error: within this context
>> /home/maemo/qtsdk-2010.01/qt/include/QtCore/qlist.h: In member
>> function 'T QList<T>::value(int) const [with T = Account]':
>> /home/maemo/qtsdk-2010.01/qt/include/QtCore/qlist.h:589: note:
>> synthesized method 'Account::Account(const Account&)' first required
>> here
>> /home/maemo/qtsdk-2010.01/qt/include/QtCore/qobject.h: In member
>> function 'Account& Account::operator=(const Account&)':
>> main.cpp:7: instantiated from 'void
>> QList<T>::node_construct(QList<T>::Node*, const T&) [with T =
>> Account]'
>> /home/maemo/qtsdk-2010.01/qt/include/QtCore/qlist.h:472: instantiated
>> from 'void QList<T>::append(const T&) [with T = Account]'
>> /home/maemo/qtsdk-2010.01/qt/include/QtCore/qlist.h:315: instantiated
>> from 'QList<T>& QList<T>::operator<<(const T&) [with T = Account]'
>> main.cpp:28: instantiated from here
>> /home/maemo/qtsdk-2010.01/qt/include/QtCore/qobject.h:309: error:
>> 'QObject& QObject::operator=(const QObject&)' is private
>> main.cpp:7: error: within this context
>> /home/maemo/qtsdk-2010.01/qt/include/QtCore/qlist.h: In member
>> function 'void QList<T>::node_construct(QList<T>::Node*, const T&)
>> [with T = Account]':
>> /home/maemo/qtsdk-2010.01/qt/include/QtCore/qlist.h:353: note:
>> synthesized method 'Account& Account::operator=(const Account&)' first
>> required here
>> make: Leaving directory
>> `/scratchbox/users/maemo/home/maemo/qSortAccountTest/qSortAccountTest'
>> make: *** [main.o] Error 1
>> Exited with code 2.
>> Error while building project qSortAccountTest
>> When executing build step 'Make'
>> <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
>>
>> Now, if I change the Account class not to extend QObject, everything
>> works fine, and output is sorted.
>> So I would have two questions:
>> 1) What I need to do to get the QObject version working? qSort docs
>> say that the items need to implement operator<(), but clearly that is
>> not enough.
>>
>> 2) Is it possible to overload the operator<() this way (using
>> pointers) and use it with qSort?
>>    bool operator<(const Account * other ) const;
>> My app actually stores pointers to objects (extending QObject) in the
>> QList that I want to sort, so will it be possible at all?
>>
>> Any help is greatly appreciated!
>> Thanks,
>> Carl
>> _______________________________________________
>> Qt-interest mailing list
>> Qt-interest at trolltech.com
>> http://lists.trolltech.com/mailman/listinfo/qt-interest
>
>