Pass value by reference to a thread and modify - c++

Question

I want to modify a Qlist by reference in a thread because the sorting could take a few seconds.
But it seems like that I can't modify the List. Outside of the thread the List has not beend changed.
In QMainwindow:
QThread *thread = new QThread();
FarmSortWorker *farmsort_worker = new FarmSortWorker();
farmsort_worker->setFarmData(farm_list);
farmsort_worker->moveToThread(thread);
connect(farmsort_worker, &FarmSortWorker::error, [=](QString error_msg){
qDebug() << error_msg;
logger->logEvent(error_msg, Logger::ERROR);
});
connect(thread, &QThread::started, farmsort_worker, &FarmSortWorker::processSort);
connect(farmsort_worker, &FarmSortWorker::finished, thread, &QThread::quit);
connect(farmsort_worker, &FarmSortWorker::finished, farmsort_worker, &QThread::deleteLater);
connect(thread, &QThread::finished, thread, &QThread::deleteLater);
thread->start();
My Thread (.h)
#ifndef FARMSORTWORKER_H
#define FARMSORTWORKER_H
#include <QObject>
#include "../framcontainer.h"
#include <QList>
#include <qDebug>
#include <QString>
class FarmSortWorker : public QObject
{
Q_OBJECT
public:
explicit FarmSortWorker(QObject *parent = 0);
~FarmSortWorker();
void setFarmData(QList<FramContainer> &farm_container);
private:
QList<FramContainer> farm_container;
signals:
void error(QString error);
void finished();
public slots:
void processSort();
};
#endif // FARMSORTWORKER_H
My Thread (.cpp)
#include "farmsortworker.h"
FarmSortWorker::FarmSortWorker(QObject *parent) :
QObject(parent)
{
}
FarmSortWorker::~FarmSortWorker()
{
}
void FarmSortWorker::setFarmData(QList<FramContainer> &farm_container)
{
this->farm_container = farm_container;
}
void FarmSortWorker::processSort()
{
qDebug() << "count:" << farm_container.size();
for(int i = 0; i < farm_container.size(); i++)
{
FramContainer park = farm_container.at(i);
qDebug() << "original:" << park.getFarmName();
}
for(int i = 0; i < farm_container.size(); i++)
{
FramContainer *park =& farm_container[i];
park->setFarmName("test");
}
for(int i = 0; i < farm_container.size(); i++)
{
FramContainer park = farm_container.at(i);
qDebug() << "neu:" << park.getFarmName();
}
emit finished();
}
Thank you all in advance.

Answer 1

It's because you save a copy of the list in your thread class, not a reference.
Instead of having a separate function to set the list, pass it (by reference) to the constructor, and have it set the reference variable:
class FarmSortWorker : public QObject
{
Q_OBJECT
public:
explicit FarmSortWorker(QList<FramContainer>& list, QObject *parent = 0);
...
private:
QList<FramContainer>& farm_container; // Store as a reference
...
};
...
FarmSortWorker::FarmSortWorker(QList<FramContainer>& list, QObject *parent) :
QObject(parent), farm_container(list)
{
}
...
QThread *thread = new QThread();
FarmSortWorker *farmsort_worker = new FarmSortWorker(farm_list);
...