Function pointers in Qt

giesbert

The problem is, that normal "C" functions only need their parameters. A class member also needs the this pointer. These functor ideas (also function adapters) bind both together.

It's sometimes not the trivialst thing, but you should google for it.
"Like here":http://stackoverflow.com/questions/1849571/calling-pointer-to-member-function-in-call-for-a-function-passed-to-a-template-fu

yshurik

Kind of:

@
class A{
void setX(int value);
};

template<typename Set, typename Value>
void change_obj_by_func(A * obj, Set set, Value value) {
set(obj, value);
}

...
A * a = new A;
change_obj_by_func(a, std::mem_fun(&A::x), 35);
@

Benefit of this to have possibility of applying different member functions in same code and this member function can be passed as parameter.

TheDestroyer

The problem is that I'm not allowed to edit Blochsim. So I need to use function adaptors. I tried the following (as it's done in the link you gave me, Gerolf) but it doesn't compile

@
constantFieldDirChanger = new LinkedSliderDoubleSpinBox(minConstantDir,maxConstantDir,sliderLength,std::bind1st(mem_fun(&(((MainWindow*)parentWidget())->openGLApp->blochsim->setConstantFieldDirection)),this),std::bind1st(mem_fun(&(((MainWindow*)parentWidget())->openGLApp->blochsim->getConstantFieldDirection)),this),this);

@

Error is: ISO C++ forbids taking the address of a bound member function to form a pointer to member function. Say '&Blochsim::setConstantFieldDirection'

Any ideas?

TheDestroyer

Thank you for your example. Unfortunately, your example doesn't imitate my problem. The function I want to call is supposed to edit the object's member variables of the class... not only use it within the members of the current stack!

Let's rephrase the problem. Is there anyway (not necessarily function pointers) to pass blochsim object's member function (that affect the object's member variables of blochsim) to my class to use it later when needed?

yshurik

[quote author="TheDestroyer" date="1305202824"]
Let's rephrase the problem. Is there anyway (not necessarily function pointers) to pass blochsim object's member function (that affect the object's member variables of blochsim) to my class to use it later when needed?[/quote]

Why not? Just keep the functor + pointer to object to invoke them later

HuXiKa

You could also try the following method to call a class member:

@#define CALL_MEMBER_FN(object,ptrToMember) ((object).*(ptrToMember))
typedef < return_type > (MyClass::*pt2Member)(< parameters >);
...
// somewhere where you want to call a member function
pt2Member p = &Myclass::memberfunction;
CALL_MEMBER_FN(< pointer_to_MyClass_object >, p)(< parameters >)@

But this is still based on the fact Gerolf said: you need a this parameter to call a member function.

andre

This is basic C++ stuff. Perhaps the "C++ FAQ":http://www.parashift.com/c++-faq-lite/pointers-to-members.html would be a good guide here.

TheDestroyer

But I'm not allowed to edit Blochsim that has the function that is going to edit the variable I need! I can't "create" a functor for Blochsim! the thing I'm doing is passing a set and get functions. Can I convert these functions to functors?

Could you please elaborate more on that if I misunderstood you?

TheDestroyer

HuXiKa, thank you for your efforts. But I don't want to call a member function. I want to pass it to a function.

SteveKing

Not sure if I'm missing something, but if you want to link a QSlider to a QDoubleSpinBox why not just use signals and slots?

TheDestroyer

I want to have them automatically linked in this class, because I have like 20 instances of this.

SteveKing

In that case create a QSlider and a QDoubleSpinBox and link them in the constructor of the LinkedSliderDoubleSpinBox. You can also do all the laying out of the slider and the spin box within the LinkedSliderDoubleSpinBox at this point. You could always derive from QFrame rather than QWidget to allow borders etc.

When you want to use the LiLinkedSliderDoubleSpinBox just create one. If you use layouts etc. it should give you a nice widget you can use anywhere.

Steve

TheDestroyer

The problem is not creating and linking them. The problem is linking them together to the "set" function that is going to do the changes in the simulation. The set function is located in another class (blochsim). I don't have variables access in that class, just set and get functions to access the variables.

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