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 *

[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);
}


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()));
}


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()));
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.cpp
#include "graph.h"

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

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

//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.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
}

[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"

[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!