Function pointers in Qt
Page |
1 |
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! :-)
29 replies
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 [stackoverflow.com]
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?
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 [parashift.com] 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?
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
You must log in to post a reply. Not a member yet? Register here!
