Function pointers in Qt

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