C++: Store pointer to a member function of an object in another object

Question

I have a class which shall invoke a function specified by the user on certain occasions. Therefore the class has a method void setExternalPostPaintFunction(void(*function)(QPainter&)); that can be used to "register" a function. This function then will be called on that occasion:

class A {
    public:
        void setExternalPostPaintFunction(void(*function)(QPainter&));
    private:
        void (*_externalPostPaint)(QPainter&);
        bool _externalPostPaintFunctionAssigned;
};

The function pointer is saved in the member variable _externalPostPaint. The implementation of setExternalPostPaintFunction looks like this:

void A::setExternalPostPaintFunction(void(*function)(QPainter&)) {
    _externalPostPaint = function;
    _externalPostPaintFunctionAssigned = true;
}

Now, this works with normal functions. However, I want to be able to also pass pointers to member functions of objects. From what I know I also have to pass and store the pointer to the object in this case. However, I don't know which type the other object will have. So I guess I'm forced to use templates. I already thought of something like this:

class A {
    public:
        template <typename T>
        void setExternalPostPaintFunction(void(T::*function)(QPainter&), T* object);
    private:
        void (T::*_externalPostPaint)(QPainter&);       //<- This can't work!
        bool _externalPostPaintFunctionAssigned;
};

This way I can pass a function pointer and an object pointer to setExternalPostPaintFunction and would probably be able to call the function on the object inside that function. But I'm not able to store it in the variable _externalPostPaint because the type T is only deduced when the function setExternalPostPaintFunction is called, thus I can't have a member variable that depends on this type, since the type of my member variable has to be known when the object is created and apart from that it cannot change, but it would have to in the case when a new function is assigned which possibly could be a member function of an object of different type.

So what is the proper way to do this, or is there any? I'm not super fit with templates and function pointers, so I might have overlooked something.

Anoter option would certainly be to create a functor class with a virtual member function which can be overwritten in a derived class and then pass + store an object pointer of that type instead of the function pointer. But I somehow would prefer my approach if it is somehow possible.

EDIT: SOLUTION

TartanLlama brought me on the right track by suggesting the use of std::function. Here is how I solved it:

class A {
    public:
        template <typename T>
        void setExternalPostPaintFunction(T* object, void(T::*function)(QPainter&)) {
            _externalPostPaint = std::bind(function, object, std::placeholders::_1);
            _externalPostPaintFunctionAssigned = true;
        }
        void setExternalPostPaintFunction(std::function<void(QPainter&)> const& function);
    private:
        std::function<void(QPainter&)> _externalPostPaint;
        bool _externalPostPaintFunctionAssigned;
};

As you see, the pointer to the function/member function is stored in an std::function<void(QPainter&)> object now. The advantage is, that an std::function can basically store any callable target. Then there are two overloads: one that can be used for any std::function object that also accepts e.g. a normal function pointer (because the std::function that is expected then is implicitly constructed from that) and one for member functions that have to be called on an object (more for convenience). The latter is implemented as a template. This uses std::bind to create a std::function object of the call of that member function (the user passed) on the object (the user passed).

The overload that takes an std::function is implemented in the source file like this:

void ImageView::setExternalPostPaintFunction(std::function<void(QPainter&)> const& function) {
    _externalPostPaint = function;
    _externalPostPaintFunctionAssigned = true;
}

Invoking that stored function in the code of class A is now as simple as that:

//canvas is a QPainter instance    
if (_externalPostPaintFunctionAssigned) _externalPostPaint(canvas);

The user who wants to register a member function as callback function just has to do the following:

//_imageView is an instance of "A"
//"MainInterface" is the type of "this"
_imageView->setExternalPostPaintFunction(this, &MainInterface::infoPaintFunction);

Or if it's not a member function but just a normal function:

void someFunction(QPainter& painter) {
    //do stuff
}

_imageView->setExternalPostPaintFunction(&someFunction);

Or he can explicitly create a std::function object and pass it:

std::function<void(QPainter&)> function = [&](QPainter& painter){ this->infoPaintFunction(painter); };
_imageView->setExternalPostPaintFunction(function);

Works like a charm.

Answer 1

You could use std::function:

class A {
    public:
        //PostPaintFun can be anything which acts as a function taking a QPainter&
        //Could be a lambda, function pointer, functor, etc.
        using PostPaintFun = std::function<void(QPainter&)>;
        void setExternalPostPaintFunction(PostPaintFun fun);
    private:
        //Names beginning with an underscore are reserved, don't use them
        //Ending with an underscore is fine
        PostPaintFun fun_;
        bool externalPostPaintFunctionAssigned_;
};

Now you can use member functions like so:

struct B
{
    void exec(QPainter&) const;
};

void foo() {
    B b;
    a.setExternalPostPaintFunction(
        [b] (QPainter& p) {b.exec(p);}
    );
}

//or inside B
void B::foo() {
    a.setExternalPostPaintFunction(
        [this] (QPainter&p) {this->exec(p);}
    );
 }

Answer 2

I have to say I prefer TartanLlama's answer, but here you have something it could work for you.

This might to need some work, but I'm sure you'll get the idea.

struct IFunctionHolder {};                      // Used for pointing to any FunctionHolder
typedef IFunctionHolder* functionHolder_ptr;    // Alias for IFunctionHolder* .

template<typename Function>                     // The template for the actual function holders.
struct FunctionHolder:  public IFunctionHolder
{
    Function function;
};


class A {
    public:
        template <typename T>
        void setExternalPostPaintFunction(void(T::*function)(QPainter&), T* object);
    private:
        functionHolder_ptr *function_holder;            // This memeber can hold eny instantiation of template<> FunctionHolder.
                                                        // Instantiate this member wen calling setExternalPostPaintFunction
        bool _externalPostPaintFunctionAssigned;
};

You could have some code like this:

A some_a;
void some_a.setExternalPostPaintFunction(&SomeInstance::some_fnunction);    // Here take place the instantiation of FunctionHolder.
some_a.function_holder.function(some_painter);