Misunderstanding function pointer - passing it as an argument

Question

I want to pass a member function of class A to class B via a function pointer as argument. Please advise whether this road is leading somewhere and help me fill the pothole.

#include <iostream>


using namespace std;

class A{
public:

int dosomeA(int x){
    cout<< "doing some A to "<<x <<endl;
    return(0);
}
};

class B{
public:

B(int (*ptr)(int)){ptr(0);};
};

int main()
{
A a;
int (*APtr)(int)=&A::dosomeA;

B b(APtr);
return 0;
}

This brilliant piece of code leaves me with the compiler error:

cannot convert int (A::*)(int)' toint (*)(int)' in initialization

Firstly I want it to compile.
Secondly I don't want dosomeA to be STATIC.

Answer 1

Pointers to members are different from normal function pointers. As the compiler error indicates the type of &A::dosomeA is actually int (A::*)(int) and not int (*)(int).

Inside B's constructor you need an instance of A to call the member on using one the .* or ->* operators.

E.g'

B(int(A::*ptr)(int))
{
    A atmp;
    (atmp.*ptr)(int);
}

Answer 2

http://www.parashift.com/c++-faq-lite/pointers-to-members.html

You don't have a pointer to a function that returns an int and takes an int. You have a pointer to a member-function that returns an int and takes an A* and an int. Not only do you need to fix the type of your pointer, B needs to provide an A* to serve as dosomeA's this parameter.

Boost::bind has some functionality meant to simplify using function pointers in general, and they provide support for pointers to member functions. See here.