pass a function pointer between classes - c++

Question

I have two claces.
here is the first one
class one{
one(){ }
public:
void change(double a){
//print a
}
void run(){
two tw;
tw->functionpointer=&change;
}
};
and here is the two class
public two{
two();
static void progresschange(double v){
functionpointer(v);
}
public:
void (*functionpointer)(double);
};
as you see in example I have a function in class one, I want to call it from an static function in class two As progresschange is static I can only call change function in class one uisng function pointers. but It does not work and compiles says
error: cannot convert ‘void (one::*)(double)’ to ‘void (*)(double)’ in assignment
it happens in this line
tw->functionpointer=&change;
how can I make it work. it is even possible to pass a function using its function pointer to another class using this method?
thanks

Answer 1

The error message is quite straightforward:
error: cannot convert void (one::*)(double) to void (*)(double) in assignment
void one::change(double) is a member function of the class one, not just a function. Therefore, you can't assign a pointer to this member function (i.e.: void (one::*)(double)) to a pointer to a function with the same signature (i.e.: to a void (*)(double)).
Besides, a non-static member function (like one::change() above) has to be called on an object, so you also need to provide an one object to call that pointed-to non-static member function.
Basically, you can achieve what you want by adding the following data members in two:
void (one::*mem_func_ptr)(double);
one *obj_ptr;
That is, a pointer to the member function (mem_func_ptr) and a pointer to the object to call this member function on (obj_ptr).
To assign to the member function pointer:
mem_func_ptr = &one::change;
Then, to call the member function pointed by mem_func_ptr on the object pointed by obj_ptr with 0.0 as argument:
(obj_ptr->*mem_func_ptr)(0.0);
It can be done analogously by keeping a copy of an object one as data member instead of a ponter. In that case you should use the operator .* instead of ->*.

Answer 2

The problem is that one::change() is a class member so you need to pass a pointer to a class instance as well.
The modern way to do it is to use std::function, std::bind and std::mem_fn:
class two {
....
std::function<void(double)> functionpointer;
}
tw->functionpointer = std::bind(std::mem_fn(&one::change), _1, one_instance);
Alternatively you can use a lambda function:
tw->functionpointer = [&one_instance](double x) { one_instance->change(x); }