Function pointers in Qt

  • Hello guys, :)

    I'm trying to load a function pointer to a class of mine. But I'm getting a "non-matching function or call" when compiling.

    my classes constructor looks like this:
    LinkedSliderDoubleSpinBox::LinkedSliderDoubleSpinBox(double minValue, double maxValue, long int bins, void (*setValueFunction)(double), double (*getValueFunction)(), QWidget *parent) :

    my call looks like this:

    LinkedSliderDoubleSpinBox* constantFieldDirChanger = new LinkedSliderDoubleSpinBox(minConstantDir,maxConstantDir,sliderLength,((MainWindow*)parentWidget())->openGLApp->blochsim->setConstantFieldDirection,((MainWindow*)parentWidget())->openGLApp->blochsim->getConstantFieldDirection,this);

    min and maxConstantDir are doubles, sliderLength is an integer
    and the functions setConstantFieldDirection and getConstantFieldDirection look have the following prototypes:
    void setConstantFieldDirection(double value);
    double getConstantFieldDirection();

    The class I created in order to link a QSlider to a QDoubleSpinBox.

    I'm not so familiar with function pointers. So I don't really know whether I did a good mistake with them. Could you guys please help? why doesn't the call match the prototype?

    Thank you. Any efforts are highly apprecaited! :-)

  • You are trying to pass function - member of class.
    You have to use function adaptors in such case like std::mem_fun

  • Hi,

    first moved this to C++, as the problem is a generic C++ problem, not a Qt problem.

    Function pointers MUST use C - functions, not member functions of classes. A function pointer might never be a C++ member function pointer. To achieve such things, you would need functors, like you get from boost.

  • Thank you for the answers, guys!

    Interesting! I thought I could simply use the pointer of any function!

    I've put this here because I thought it has to do with Qt class hierarchy.

    Could you guys please post an example of how I could use function adaptors to handle this situation? I don't really get the idea whether I have to use it in the constructors parameters or call or so on.

  • 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":

  • 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.

  • 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?

  • 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?

  • [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

  • 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.

  • This is basic C++ stuff. Perhaps the "C++ FAQ": would be a good guide here.

  • 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?

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

  • 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?

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

  • 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.


  • 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.

  • 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) :
    connect(pSlider, SIGNAL(valueChanged(int), this, SLOT(setValue(int));
    connect(this, SIGNAL(valueChanged(int), pSlider, SLOT(setValue(int));

  • 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?

  • 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?

  • 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!!!

  • remove the brakets :-)

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

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

  • It would be:


    class TObj {
    double funcA() {
    return 0;


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

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


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

  • 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":

  • I agree.

  • 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
    explicit MyWidget(QWidget *parent = 0);

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

    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) :
    pSlider = new QSlider(Qt::Horizontal, this);
    connect(pSlider, SIGNAL(valueChanged(int)), this, SLOT(changed1(int)));

    QSpinBox* pSpin = new QSpinBox(this);
    connect(pSpin, SIGNAL(valueChanged(int)), this, SLOT(changed2(int)));
    QVBoxLayout* pLayout = new QVBoxLayout(this);
    setValue1 = boost::bind(&QSlider::setValue, pSlider, _1);
    setValue2 = boost::bind(&QSpinBox::setValue, pSpin, _1);


    void MyWidget::changed1(int value)

    void MyWidget::changed2(int 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

  • 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!!

  • 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<double()>, 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

  • 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 :-)

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

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