Class member function as function pointer

Question

I have a class and one of its member functions is actually a function pointer. That way the user can overwrite what does this function do. I unfortunately have some difficulties

Answer 1

What you're trying to do isn't going to work. A pointer to a non-static member function is not the same as a pointer to a free function, because unlike the latter, the former must be invoked on an instance of the object. So you cannot declare a variable of one type and assign a pointer to a function of the other type to it.

First, let's fix half your code:

Since fcn_ptr is a pointer to member function, it's definition needs to be:

double (Object::*fcn_ptr)(const double &) = NULL;

Then, the cast in your constructor is invalid. You're attempting to cast a pointer to a member function to a pointer to a free function. Get rid of the cast.

Object() : fcn_ptr(&Object::fcn_default)
{}

Finally, when you invoke fcn_ptr, you can't simply conjure it out of thin air. It is a data member of Object and so you need an instance of the class to access fcn_ptr. So call it as:

(test1.*(test1.fcn_ptr))(2)

Make all these changes, and half your code will compile and produce the correct result. Live demo

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The other half, where you try to assign a pointer to a free function to fcn_ptr still won't work because of the reasons stated earlier. The way to fix this is to use std::function instead of a function pointer.

class Object{
    public:
        Object() : fcn_ptr(std::bind(&Object::fcn_default, this, std::placeholders::_1))
        {}
        std::function<double(double const&)> fcn_ptr;

    private:
        double fcn_default(const double &phit){
            return 3.2+phit;
        }
};

And then use it as:

cout << (test1.fcn_ptr)(2) << endl;

test2.fcn_ptr = &fcn_mod;
cout << (test2.fcn_ptr)(2) << endl;

Live demo

Answer 2

You do not have a class with a "function which is a function pointer", but rather a class with a data member which is a function pointer. This is an important distinction as the function won't receive a this pointer, nor will it have access to protected or private members.

You've declared your function pointer correctly, but you need to use the .fcn_ptr rather than .*fcn_ptr. I believe this is sufficient to solve your problem.

Do note that you're using old C style function pointer syntax. You may want to learn the C++ function pointer alternatives including std::function.