Function pointers in Qt

giesbert

If you always connect the same types, it is absolutly no problem:

@
LinkedSliderDoubleSpinBox::LinkedSliderDoubleSpinBox(double minValue, double maxValue, long int bins, QSlider* pSlider, QWidget *parent) :
QWidget(parent)
{
connect(pSlider, SIGNAL(valueChanged(int), this, SLOT(setValue(int));
connect(this, SIGNAL(valueChanged(int), pSlider, SLOT(setValue(int));
setValue(pSlider->value());
}
@

TheDestroyer

Here is the thing. setValue(...) is a member function of another class's object, where I want to call the function INSIDE that object after to set a value INSIDE that object.

Since this operation is not specific and isn't supposed to control a single situation, or in other words one object's variable but rather apply the same slider with the spinbox to many other variables (this is the idea in the first place), I want to pass the "setValue(...)" function to the class, so that the class could use it as it needs.

Doesn't this look difficult?

giesbert

If you want to work with any type of objects, and those objects are QObject derived classes AND the value to set is a property, you could also use Qt's meta object system so set and get the property. :-) Is that elegant enough?

TheDestroyer

ahhhhh, man! this is veryyyyyyyyyyyyyyyy diasppointing. The class isn't Qt derived :D

but I think I'm starting to get the solution. but I need your help!

I'm reading a book called "Professional C++" by Solter and Kleper.

An example solution to my problem is the following (under Pointers to Methods):

@
SpreadsheetCell myCell;
double (SpreadsheetCell::*methodPtr) () const = &SpreadsheetCell::getValue;
cout << myCell.*methodPtr) () << endl;
@

I'm trying to do like this code as follows:

@
double (Blochsim::*getConstantFieldDirPtr) () const = &Blochsim::getConstantFieldDirection();
@
but this line is giving me the following error:

cannot call member function 'double Blochsim::getConstantFieldDirection()' without an object.

I tried to define and object from the class Blochsim (which I know doesn't make sense because it's in another line):

@
Blochsim bs;
double (Blochsim::*getConstantFieldDirPtr) () const = &Blochsim::getConstantFieldDirection();
@
but still gives the same error!

any idea how to solve this??? :D I'm happy I'm getting close to the solution!!!

giesbert

remove the brakets :-)

@
double (Blochsim::*getConstantFieldDirPtr) () const = &Blochsim::getConstantFieldDirection;
@

But where do you get the object instance from to call the method on?

TioRoy

It would be:

@
(bs.*getConstantFieldDirPtr)();
@

Example:
@
class TObj {
public:
double funcA() {
return 0;
}

};

int main(int argc, char *argv[])
{
TObj tobj;

double (TObj::*funcAPtr)() = &TObj::funcA;

(tobj.*funcAPtr)();

}
@

I've write some years an similar code on PalmOS.

giesbert

But then again, you need the object where you call it. If I understood correctly, he does not want to have the object in present. otherwise, he would not need that.

I would go with "boost::bind":http://www.boost.org/doc/libs/1_46_1/libs/bind/bind.html#with_boost_function

TioRoy

I agree.

giesbert

So, I have a working example for your needs together with boost::bind and boost::function.
here we go:

@
#include <QtGui/QWidget>
#include <boost/function.hpp>

class MyWidget : public QWidget
{
Q_OBJECT
public:
explicit MyWidget(QWidget *parent = 0);

protected slots:
void changed1(int);
void changed2(int);

private:
boost::function<void(int)> setValue1;
boost::function<void(int)> setValue2;
};
@

I have a widget with two children, a slider and a spin box. The change of one of the triggers the slots. There I will (without using the class) change the value of the other object:

@
#include <QtGui/QSlider>
#include <QtGui/QSpinBox>
#include <QtGui/QVBoxLayout>
#include <boost/bind/bind.hpp>

MyWidget::MyWidget(QWidget parent) :
QWidget(parent)
{
QSlider pSlider = new QSlider(Qt::Horizontal, this);
pSlider->setRange(0,100);
pSlider->setValue(20);
connect(pSlider, SIGNAL(valueChanged(int)), this, SLOT(changed1(int)));

QSpinBox* pSpin = new QSpinBox(this);
pSpin->setRange(0,100);
pSpin->setValue(20);
connect(pSpin, SIGNAL(valueChanged(int)), this, SLOT(changed2(int)));

QVBoxLayout* pLayout = new QVBoxLayout(this);
pLayout->addWidget(pSlider);
pLayout->addWidget(pSpin);
setLayout(pLayout);
setFixedSize(sizeHint());

setValue1 = boost::bind(&QSlider::setValue, pSlider, _1);
setValue2 = boost::bind(&QSpinBox::setValue, pSpin, _1);

}

void MyWidget::changed1(int value)
{
setValue2(value);
}

void MyWidget::changed2(int value)
{
setValue1(value);
}
@

the trick here is the usage of

boost::function<void(int)> setValue1;
together with:
setValue1 = boost::bind(&QSlider::setValue, pSlider, _1);

The only disadvantage is: you need boost (at least partially...)

you need (at least on boost 1.44) all headers in boost, and the following folders in boost:

bind
config
detail
exception
function
function_types
mpl
preprocessor
type_traits
utility

TheDestroyer

I have the object under the following hierarchy, which is in another window!

((MainWindow*)parentWidget())->openGLApp->blochsim->setConstantFieldDirection(double value)

I'll give it a shot and try!! thank you all!

do you see it a problem anyway calling the object like that? I have access to the object that way!!

giesbert

it is the same logic:

@
boost::function<void(double)> setValue1;

setValue1 = boost::bind(&CBlochism::setConstantFieldDirection, ((MainWindow*)parentWidget())->openGLApp->blochsim, _1);

@

make the boost::function the parameter, like here:

@
LinkedSliderDoubleSpinBox::LinkedSliderDoubleSpinBox(double minValue, double maxValue, long int bins,
boost::function<void(double)>,
boost::function<double()>, QWidget *parent) :
QWidget(parent)
{...}
@

and call it like this;

@
boost::function<void(double)> setValue = boost::bind(&CBlochism::setConstantFieldDirection,
((MainWindow*)parentWidget())->openGLApp->blochsim, _1);
boost::function<double()> getValue = boost::bind(&CBlochism::getConstantFieldDirection,
((MainWindow*)parentWidget())->openGLApp->blochsim, _1);

constantFieldDirChanger = new LinkedSliderDoubleSpinBox(minConstantDir, maxConstantDir, sliderLength, 
                                                        setValue, getValue, this);

@

EDIT: updated the examples, Gerolf

TheDestroyer

Thanks! I'll tell you what I get as soon as it's tested!

Unfortunately, I'll have to go get some sleep now because I'm traveling tomorrow early to Berlin... so I'll try everything I can in the train :D

Good night everyone, and thanks for the help!

see you tomorrow, with success hopefully :-)

giesbert

Good night :-)
Please regard the updates I made to the code