How to use QThreadStorage class?

[joeQ]

hi, I saw the QThreadStorage in manual. I don't know how to use it. I didn't find anything about QThreadStorage in internet.

the manual give me the code

  QThreadStorage<QCache<QString, SomeClass> > caches; /// is caches a member of subclass QThread? do i must use some class ? 

  void cacheObject(const QString &key, SomeClass *object)
  {
      caches.localData().insert(key, object);
  }

  void removeFromCache(const QString &key)
  {
      if (!caches.hasLocalData())
          return;

      caches.localData().remove(key);
  }

1. Can you show me a demo about it?
2. Can you tell me when i should to use the QThreadStorage in thread?
3. What are the advantages use the QThreadStorages?

I tried some way, not work.

#ifndef SUBQTHREAD_H
#define SUBQTHREAD_H

#include <QThread>

#include <QPoint>
#include <QCache>
#include <QThreadStorage>

/** I tried struct, but failed at first. so, i used the QPoint to tried, also failed. */
struct Data{
    bool bFlag = false;
    int  num = 0;
    double dVal = 0.0;
    float  fVal = 0.0;
};

class SubQThread : public QThread
{
    Q_OBJECT
public:
    explicit SubQThread(QObject *parent = 0);
    //explicit SubQThread(Data &dt,QObject *parent = 0);
    explicit SubQThread(QPoint *pt,QObject *parent = 0);

private:
    void run() Q_DECL_OVERRIDE;

private:
    //QThreadStorage<QCache<int,Data>> _thStorage;
    QThreadStorage<QCache<int,QPoint>> _thStorage;
};

#endif // SUBQTHREAD_H

#include "subqthread.h"

#include <QThread>
#include <QDebug>

SubQThread::SubQThread(QObject *parent):QThread(parent)
{
}
/*
SubQThread::SubQThread(Data &dt, QObject *parent):QThread(parent)
{
    //_thStorage.localData().insert(1,&dt);
}
*/
SubQThread::SubQThread(QPoint *pt, QObject *parent):QThread(parent)
{
    _thStorage.localData().insert(1,pt);
}

void SubQThread::run()
{
    qDebug() << "SubThread Id" << QThread::currentThreadId() <<
                "has the Data" << _thStorage.hasLocalData();
}

#include <QCoreApplication>
#include "subqthread.h"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    SubQThread th;
    th.start();

    /*Data dt;
    SubQThread th2(dt);
    th2.start();*/

    QPoint pt;
    SubQThread th3(&pt);
    th3.start();

    th.wait();
    //th2.wait();
    th3.wait();

    return a.exec();
}

[SGaist]

Hi,

Except that you are mixing pointer to QPoint and QPoint object which is wrong.

1. Look in Qt's sources
2. When you need to store and retrieve data on a per thread basis
3. You don't need to do the data/thread matching yourself both when storing and retrieving data.

[joeQ]

@SGaist Thank u, i get it. but, i have some trouble.

like the code

SubQThread.h

class SubQThread : public QThread
{
    Q_OBJECT
public:
    explicit SubQThread(QPoint *pt,QObject *parent = 0);

private:
    void run() Q_DECL_OVERRIDE;

private:
    QThreadStorage<QPoint*> _thStorage;
};

SubQThread.cpp

#include "subqthread.h"

#include <QThread>
#include <QDebug>

SubQThread::SubQThread(QPoint *pt, QObject *parent):QThread(parent)
{
    _thStorage.setLocalData(pt);
    qDebug() << "has the Data:" << _thStorage.hasLocalData(); ///here is true
}

void SubQThread::run()
{
    qDebug() << "SubThread Id" << QThread::currentThreadId() <<
                "has the Data" << _thStorage.hasLocalData(); /// here is false
}

main.cpp

#include <QCoreApplication>
#include "subqthread.h"

/** QtCore */
#include <QDebug>
#include <QThread>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    qDebug() << "Main Id:" << QThread::currentThreadId();

    QPoint* pt = new QPoint;
    SubQThread th3(pt);
    th3.start();

    return a.exec();
}

Why done QThreadStorage qDebug false in run function?

[SGaist]

Because the constructor runs in the thread of the object that creates your SubQThread instance while the run method will be called in the thread that is handled by QThread.

[joeQ]

@SGaist Can you help me ? Show me some code about use the QThreadStorage in thread. just a little demo is OK. i tried failed. I only want to write a blog about how to use the QThreadStorage. but, i didn't find any demo about QThreadStorage.

[jsulm]

@joeQ As @SGaist said: you're storing in QThreadStorage in the constructor of SubQThread. This constructor is executed in DIFFERENT thread than run() method - that's why you get false. QThreadStorage stores thread local data - data stored in one thread is not available in another. Move this line

_thStorage.setLocalData(pt);

to run() then it should print true.
There is already a demo in documentation: http://doc.qt.io/qt-5.8/qthreadstorage.html

[joeQ]

@jsulm @SGaist Thank u, I get what i want. i am sorry to bother you.

[jsulm]

@joeQ Well, the forum is there to ask questions :-)