static QTimer

[PauloF91]

I want to get a single timer to run for several objects.
If I put this in the header file of my class:
static QTimer* timer = new QTimer(this);

It says: « 'this' cannot appear in a constant-expression ». I understand i cannot use 'this', since I want a class member, not an object member (I guess that makes sense), but I don't know how to solve it. How do I put it then?

Thanks in advance.

PF91

[Santosh Reddy]

Here is a static class member example

//a.h
Class A
{
private:
    static QTimer timer1;
    static QTimer *timer2;
}
 
//a.cpp
#include "a.h"
QTimer A::timer1;
QTimer * A::timer2 = new QTimer(0); //EDIT: Added *

//a.h
Class A
{
private:
    static QTimer timer1;
    static QTimer *timer2;
}

//a.cpp
#include "a.h"
QTimer A::timer1;
QTimer * A::timer2 = new QTimer(0); //EDIT: Added *

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[PauloF91]

Lets say I have class B that inherits from class A:

//a.h
Class A
{
public:
    static QTimer timer1;
    static QTimer *timer2;
}
 
//b.h
Class B : public A
{
}
 
//b.cpp
#include "b.h"
B::B(){
QTimer A::timer1;
QTimer A::timer2 = new QTimer(0);
}

//a.h
Class A
{
public:
    static QTimer timer1;
    static QTimer *timer2;
}

//b.h
Class B : public A
{
}

//b.cpp
#include "b.h"
B::B(){
QTimer A::timer1;
QTimer A::timer2 = new QTimer(0);
}

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And I'm getting this error:
invalid use of qualified-name 'A::timer1'
invalid use of qualified-name 'A::timer2'

Where is this wrong?

PS: I don't think I'm getting this straight. If I'm gonna have a single (static) timer, I guess I should only start it once. In this code, that will be done each time I instantiate an object from class B, I guess that's not correct.
All I wanna do is a QObject::connect function from my timer1's timeout() SLOT to a SIGNAL, and that signal is different from object to object (since I have others classes that inherit from class A, besides class B) that's why I'm running it in B's (or other subclass) constructor. I guess that's doable.

[Santosh Reddy]

Oh yes you got it wrong.

If I understand your problem, you have a base class say A, and there are different derived classes from it each wanting to connect to a common signal from base object but do derived class specific slot.

Will this example work?

//a.h
Class A
{
public:
    static QTimer *timer;
}
 
//a.cpp
#include "a.h"
QTimer * A::timer = new QTimer(0);
 
//b.h
Class B : public A
{
}
 
//b.cpp
#include "b.h"
B::B() : A() {
QObject::connect(timer, SIGNAL(timeout()), object, SLOT(object_slot()));
}

//a.h
Class A
{
public:
    static QTimer *timer;
}

//a.cpp
#include "a.h"
QTimer * A::timer = new QTimer(0);

//b.h
Class B : public A
{
}

//b.cpp
#include "b.h"
B::B() : A() {
QObject::connect(timer, SIGNAL(timeout()), object, SLOT(object_slot()));
}

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Anyway this is just a basic C++ problem, nothing to do with Qt, you better read more about C++ static class members.

[PauloF91]

If I understand your problem, you have a base class say A, and there are different derived classes from it each wanting to connect to a common signal from base object but do derived class specific slot.

Yes that's it.
Ye.. this is my very first project in C++ or anything related to OOP, I should read some stuff about this, but I gotta get this done asap.

Just a couple things I still don't get.
What do you mean by "object" in:

QObject::connect(timer, SIGNAL(timeout()), object, SLOT(object_slot()));

QObject::connect(timer, SIGNAL(timeout()), object, SLOT(object_slot()));

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Should I put '0' or 'this'? Cause neither or them will work. I got this (obvious) error:

QObject::connect: Cannot connect QTimer::timeout() to (null)::MySlot()

Sorry if this is too obvious, I wouldn't ask if I didn't really had to.
Thanks in advance.

[wysota]

I would recommend against using a static timer.

[Santosh Reddy]

I am sure even if showed you how to use QTimer or SIGNAL/SLOTS in general that will still not solve you problem. Instead can you tell use what actually you have to do, and how you are doing it?

[PauloF91]

Instead can you tell use what actually you have to do, and how you are doing it?

Im plotting some data in a few graphics I've made myself.
Each graph represents an object. And I have 2 kinds of graphs (therefore, 2 classes):
- line graphs (Class GraphLine);
- histograms (Class GraphHistogram).

And they both inherit from a base class Graph.
So I thought I could use a Timer, so each time it timed out, I could update all graphs with new data.
I had it working, but I was creating one timer per graph. That was not a problem until I realized I wanted to stop and re-start all timers at once each time I wanted (while the program was running).

That's when I thought of a static Timer.

//graph.h
#ifndef GRAPH_H
#define GRAPH_H
#include <QTimer>
 
class Graph{
public:
    Graph(/*stuff*/);
    static QTimer *timer;
 
protected:
    /*stuff*/
};
 
#endif // GRAPH_H

//graph.h
#ifndef GRAPH_H
#define GRAPH_H
#include <QTimer>

class Graph{
public:
    Graph(/*stuff*/);
    static QTimer *timer;

protected:
    /*stuff*/
};

#endif // GRAPH_H

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//graph.cpp
#include "graph.h"
 
QTimer *Graph::timer = new QTimer(0);
 
Graph::Graph(/*stuff*/){
    /*stuff*/
}

//graph.cpp
#include "graph.h"

QTimer *Graph::timer = new QTimer(0);

Graph::Graph(/*stuff*/){
    /*stuff*/
}

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//graphline.h (same goes for graphhistogram.h)
#ifndef GRAPHLINE_H
#define GRAPHLINE_H
#include "graph.h"
class GraphLine : public Graph{
public:
    GraphLine(/*stuff*/);
    void updateGraph(int rand = 0);
 
private:
    int graphArray[100];
 
public slots:
    void MySlot();
};
#endif // GRAPHLINE_H

//graphline.h (same goes for graphhistogram.h)
#ifndef GRAPHLINE_H
#define GRAPHLINE_H
#include "graph.h"
class GraphLine : public Graph{
public:
    GraphLine(/*stuff*/);
    void updateGraph(int rand = 0);

private:
    int graphArray[100];

public slots:
    void MySlot();
};
#endif // GRAPHLINE_H

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//graphline.cpp
#include "graphline.h"
 
GraphLine::GraphLine(/*stuff*/) : Graph(/*stuff*/){
    updateGraph(0);
    QObject::connect(timer, SIGNAL(timeout()), this, SLOT(MySlot()));
    timer->start(40);
}
 
void GraphLine::updateGraphic(int rand){ 
    /*graphic draw process*/ 
}
 
void GraphLine::MySlot(){
    updateGraph(rand() % 100); // now i'm just giving rand values instead of true data
}

//graphline.cpp
#include "graphline.h"

GraphLine::GraphLine(/*stuff*/) : Graph(/*stuff*/){
    updateGraph(0);
    QObject::connect(timer, SIGNAL(timeout()), this, SLOT(MySlot()));
    timer->start(40);
}

void GraphLine::updateGraphic(int rand){ 
    /*graphic draw process*/ 
}

void GraphLine::MySlot(){
    updateGraph(rand() % 100); // now i'm just giving rand values instead of true data
}

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[Santosh Reddy]

See if this example helps

#include <QtGui>
#include <QtWidgets>
#include <QApplication>
 
class Graph : public QWidget
{
    Q_OBJECT
public:
    explicit Graph(QWidget * parent = 0)
        : QWidget(parent)
        , mGridLayout(new QGridLayout(this))
        , mLabel(new QLabel("Graph"))
        , mCount(0)
    {
 
        if(mTimer.interval() != mInterval)
            mTimer.setInterval(mInterval);
 
        QPushButton * startButton = new QPushButton("Start Update", this);
        QPushButton * stopButton = new QPushButton("Stop Update", this);
 
        connect(startButton, SIGNAL(clicked()), this, SLOT(startUpdateTimer()));
        connect(stopButton, SIGNAL(clicked()), this, SLOT(stopUpdateTimer()));
        connect(&mTimer, SIGNAL(timeout()), this, SLOT(updateGraph()));
 
        mGridLayout->addWidget(mLabel,      0, 0, 1, 2);
        mGridLayout->addWidget(startButton, 1, 0, 1, 1);
        mGridLayout->addWidget(stopButton,  1, 1, 1, 1);
    }
 
public slots:
    void startUpdateTimer(int interval = mInterval)
    {
        if(mTimer.interval() != interval)
            mTimer.setInterval(interval);
        mCount = 0;
        mTimer.start();
    }
 
    void stopUpdateTimer(void)
    {
        mTimer.stop();
    }
 
    virtual void updateGraph(void) = 0;
 
protected:
    QGridLayout * mGridLayout;
    QLabel * mLabel;
    int mCount;
 
private:
    static QTimer mTimer;
    static const int mInterval = 1000;
};
 
QTimer Graph::mTimer;
 
class GraphLine : public Graph
{
    Q_OBJECT
public:
    explicit GraphLine(QWidget * parent = 0)
        : Graph(parent)
    {
        setWindowTitle("GraphLine");
    }
 
public slots:
    virtual void updateGraph(void)
    {
        QString str = QString("GraphLine::updateGraph(): %1").arg(mCount++);
        qDebug() << str;
        mLabel->setText(str);
    }
};
 
class GraphHistogram : public Graph
{
    Q_OBJECT
public:
    explicit GraphHistogram(QWidget * parent = 0)
        : Graph(parent)
    {
        setWindowTitle("GraphHistogram");
    }
 
public slots:
    virtual void updateGraph(void)
    {
        QString str = QString("GraphHistogram::updateGraph(): %1").arg(mCount++);
        qDebug() << str;
        mLabel->setText(str);
    }
};
 
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
 
    GraphLine g1;
    g1.show();
 
    GraphHistogram g2;
    g2.show();
 
    return app.exec();
}
 
#include "main.moc"

#include <QtGui>
#include <QtWidgets>
#include <QApplication>

class Graph : public QWidget
{
    Q_OBJECT
public:
    explicit Graph(QWidget * parent = 0)
        : QWidget(parent)
        , mGridLayout(new QGridLayout(this))
        , mLabel(new QLabel("Graph"))
        , mCount(0)
    {

        if(mTimer.interval() != mInterval)
            mTimer.setInterval(mInterval);

        QPushButton * startButton = new QPushButton("Start Update", this);
        QPushButton * stopButton = new QPushButton("Stop Update", this);

        connect(startButton, SIGNAL(clicked()), this, SLOT(startUpdateTimer()));
        connect(stopButton, SIGNAL(clicked()), this, SLOT(stopUpdateTimer()));
        connect(&mTimer, SIGNAL(timeout()), this, SLOT(updateGraph()));

        mGridLayout->addWidget(mLabel,      0, 0, 1, 2);
        mGridLayout->addWidget(startButton, 1, 0, 1, 1);
        mGridLayout->addWidget(stopButton,  1, 1, 1, 1);
    }

public slots:
    void startUpdateTimer(int interval = mInterval)
    {
        if(mTimer.interval() != interval)
            mTimer.setInterval(interval);
        mCount = 0;
        mTimer.start();
    }

    void stopUpdateTimer(void)
    {
        mTimer.stop();
    }

    virtual void updateGraph(void) = 0;

protected:
    QGridLayout * mGridLayout;
    QLabel * mLabel;
    int mCount;

private:
    static QTimer mTimer;
    static const int mInterval = 1000;
};

QTimer Graph::mTimer;

class GraphLine : public Graph
{
    Q_OBJECT
public:
    explicit GraphLine(QWidget * parent = 0)
        : Graph(parent)
    {
        setWindowTitle("GraphLine");
    }

public slots:
    virtual void updateGraph(void)
    {
        QString str = QString("GraphLine::updateGraph(): %1").arg(mCount++);
        qDebug() << str;
        mLabel->setText(str);
    }
};

class GraphHistogram : public Graph
{
    Q_OBJECT
public:
    explicit GraphHistogram(QWidget * parent = 0)
        : Graph(parent)
    {
        setWindowTitle("GraphHistogram");
    }

public slots:
    virtual void updateGraph(void)
    {
        QString str = QString("GraphHistogram::updateGraph(): %1").arg(mCount++);
        qDebug() << str;
        mLabel->setText(str);
    }
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    GraphLine g1;
    g1.show();

    GraphHistogram g2;
    g2.show();

    return app.exec();
}

#include "main.moc"

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[PauloF91]

Looking at your example did help me! I got it working... a single timer to run all my graphs.
I had to redo my base class to inherits from QWidget, and now my QObject::connect function is working just fine.

Thanks!